3*79 aie/J Mo. Uif

STUDIES OF THE MECHANISMS OF REACTIONS

OF BINARY METAL CARBONYLS

DISSERTATION

Presented to the Graduate Council of the

North Texas State University in Partial

Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

By

Jerry E. Pardue, B.S

Denton, Texas

May, 1977

Pardue, Jerry E., Studies of the Mechanisms of Reactions

of Binary Metal Carbonyls. Doctor of Philosophy (Chemistry),

May, 1977, 11 pp., 11 tables, 20 illustrations, bibliography,

119 titles.

A kinetic study of the reactions of Group VI-B hexa-

carbonyls with primary amine and halide ligands was undertaken

in order to determine the possible mechanisms of these reactions.

As well as the expected dissociative pathway, the reactions with

the primary amines were seen to proceed by a concurrent pathway

which was dependent upon the ligand concentration. Since

nitrogen donor ligands are expected to be poor donor ligands,

the mechanism proposed was a "dissociative interchange"

mechanism which should not be too dependent upon the nucleo-

philicity of the ligand. Comparison of the rate constants for

the amines studied as well as those of the previously

investigated Lewis base ligands indicated all such reactions

may proceed through the same mechanism. The similarity in

rate constants for the ligand-independent and ligand-dependent

pathways supports this mechanism.

Since reactions with these ligands were seen to proceed

by a ligand-dependent pathway it is speculated that i T - b o n d i n g

does not significantly stabalize the transition state for such

reactions since amines were incapable of ir-acceptance. Also,

fr-bonding is thought not to direct the attack at the metal

atom for these reactions. Comparison of reactivities as a

function of the metal atom with other systems involving attack

at the metal atom and at the carbonyl carbon indicated that the

metal atom is the attack site. This is substantiated by the

observation that reactions between halide ligands (which are

also incapable of 7r-acceptance) and tungsten and molybdenum

hexacarbonyl proceeded by attack at the metal atom. However,

the reactivities of the halides with chromium hexacarbonyl were

less than expected. Comparison of the relative reactivities

of the halides with tungsten and chromium hexacarbonyl indicated

tungsten hexacarbonyl to be more sensitive than chromium to the

steric requirements of the ligand. Reactions of the halides

thus proceed via attack at the carbonyl carbon for chromium

hexacarbohyl.

The presence of a ligand-independent term in reactions

of chromium hexacarbonyl with the halides is consistent with

the commonly observed "dissociative" mechanism. Since the rate

constants for the ligand-dependent pathway are much larger than

those for the ligand-independent pathway, an "associative"

mechanism was proposed for the ligand-dependent pathway. Also,

the rate constants for the ligand-dependent term were very

dependent upon the nucleophilicities of the ligands. The

reactivities of the halide ligands with a given metal carbonyl

were seen to be consistent with their steric demands.

The reactions of iron pentacarbonyl with mercuric halides

were also investigated since the products obtained seem to

indicate attack both at the metal atom and carbonyl carbon.

The reactions are oxidative elimination reactions which involve

an increase by one in the coordination number of the metal.

The investigation of the reaction with mercuric chloride

indicated the formation of the previously unobserved product,

Fe(C0) (H^Clg) . The rate of disappearance of Fe(CO)g varied

directly with the square of the ligand concentration, a fact

consistent with mechanisms proposed for other oxidative

elimination reactions. Such a mechanism involves the successive

formation of 1:1 and 1:2 "adducts" of substrate and ligands.

The 1:2 adduct then decomposes to form the product, Fe(CO)^-

(HgCl)25 and phosgene gas. The structure of a related complex

suggests for the 1:2 adduct a structure which would allow for a

carbonyl ligand to abstract two chlorine groups from the compound

to lead to phosgene. This mechanism involving attack at the

carbonyl carbon is plausible since the mercuric halide

substituents on iron would be expected to cause an electron

deficiency in the carbon atom.

The rate of formation of the final product was seen to

be dependent upon the square of the mercuric halide concentration.

Therefore, the conversion of Fe(CO)^(HgX)^ "to the final product

was proposed to proceed by the successive abstraction by each

HgX group of two molecules of mercuric halide. These oxidative

elimination reactions are related to a chemical model for

the intermediate step in the reduction of dinitrogen to ammonia

and their similarities and differences are discussed.

TABLE OF CONTENTS

LIST OF TABLES iv

LIST OF ILLUSTRATIONS.

INTRODUCTION x

Chapter

I. STUDIES OF THE KINETICS AND MECHANISM OF THE REACTION OF GROUP VI-B METAL CARBONYLS WITH PRIMARY AMINES IN DECALIN

Problem Materials Determination of Reaction Rates Identification of Reaction Products Treatment of Data Results Discussion

II. STUDIES OF THE KINETICS AND MECHANISM OF THE REACTION OF GROUP VI-B METAL CARBONYLS WITH TETRABUTYLAMMONIUM HALIDES 41

Problem Materials Determination of Reaction Rates Identification of Reaction Products Results Discussion

III. STUDIES OF THE KINETICS AND MECHANISM OF THE REACTION OF IRON PENTACARBONYL WITH MERCURIC CHLORIDE IN ACETONE 6 6

Introduction Problem Materials Determination of Reaction Rates Identification of Reaction Products Results Discussion

APPENDIX 10?

BIBLIOGRAPHY

11X

LIST OF TABLES

Table Page

I. Rates of Reaction of Metal Hexacarbonyls with Cyclohexylamine, Benzylamine, and Triisopropyl Phosphite in Decalin at Various Temperatures 21

II. First and Second Order Rate Constants and Activation Parameters for the Reactions of the Metal Hexacarbonyls with Benzylamine and Molybdenum Hexacarbonyl with Cyclo-hexylamine and Triisopropyl Phosphite in Decalin 2 8

III. Enthalpies of Activation for Reactions of Group VI-B Metal Hexacarbonyls Believed to be Occuring by Two Different Associative Pathways . 31

IV. First and Second Order Rate Constants for the Reaction of Group VI-B Metal Hexacarbonyls with Various Lewis Bases in Decalin 34

V. Rates of Reaction of Metal Hexacarbonyls with Tetrabutylammonium Halides in Chlorobenzene at Various Temperatures. . . . 48

VI. Calculated Rate Constants and Activation Parameters for the Reaction of the Group VI-B Metal Hexacarbonyls with Tetrabutyl-ammonium Halides 54

VII. Steric Requirements of Various Phosphorus Ligands 61

VIII. Carbonyl Stretching Frequencies of Products Obtained from the Reaction of Iron Penta-carbonyl with Mercuric Chloride 8 3

IX. Rates of Reaction of Iron Pentacarbonyl with Mercuric Chloride in Acetone at Various Temperatures Obtained by Monitoring the Disappearance of the Substrate 8 5

IV

LIST OF TABLES--Continued

Table Page

X. Rates of Reaction of Iron Pentacarbonyl with Mercuric Chloride in Acetone at Various Temperatures Obtained by Monitoring the Appearance of the Product 8 7

XI. Rate Constants and Activation Parameters for the Reaction of Iron Pentacarbonyl with Mercuric Chloride in Acetone 9 0

v

LIST OF ILLUSTRATIONS

Figure Page

1. Cross Section of Molecular Orbitals Involved in a) sigma and b) pi Bonding Between Transition Metal Atoms and Carbonyl Ligands. . 2

2. Unimolecular Dissociative Mechanism of Octahedral Metal Hexacarbonyls. . 5

3. Possible Pathways for an Associative Mechanism. . . 6

4. Plots of k0bsd vs- CL] f o v t h e Reactions of the

Metal Hexacarbonyls with Benzylamine in Decalin Solvent at Various Temperatures. . . . 26

5. Plots of k0bsd vs- Camine] for the Reaction of

Molybdenum Hexacarbonyl with Benzylamine in Decalin at Three Temperatures 2 7

6. Plots of vs. [amine] for the Reaction of Molybdenum Hexacarbonyl with Pure and Impure Cyclohexylamine in Decalin at 112°C 38

7. Plots of k h d vs. [Br~] for the Reaction of_ a) Molybdenum Hexacarbonyl and b) Chromium Hexacarbonyl with Tetrabutylammonium Bromide in Chlorobenzene . . . . . 52

8. Plots of k o b s d vs. [Br-] for the Reaction of Tungsten Hexacarbonyl with Tetrabutyl-ammonium Bromide in Chlorobenzene at Three Temperatures 53

9. Direct Ligand to Ligand Donation Effect Proposed by Fenske and DeKock 57

10. Metal-halide Bond Formation Synchronous with Ion-Pair Separation 59

11. Infrared Cell Constructed to Allow Solution Contact only with Unreactive Surfaces 76

12. Reaction Vessel which Allows Sample Withdrawal with No Metal Contact 78

V I

LIST OF ILLUSTRATIONS—Continued

Figure Page

13. Carbonyl Stretching Spectrum of a) Fe(CO(HgCl)^ and b) FeCCO).(Hg9Clo)2 Obtained from Nujol Mulls 8 2

14. Plots of vs. [HgC^]2 for the Reaction of Iron Pentacarbonyl with Mercuric Chloride in Acetone Obtained by Monitoring the Rate of Disappearance of the Pentacarbonyl 91

15. Plots of k^gd vs. [HgCl^]2 for the Reaction _of Iron Pentacarbonyl with Mercuric Chloride in Acetone at 55°C Obtained by Observing the Appearance of the Product 92

16. Possible Structures of Fe(C0)1+(Hg2Clg ^3

17. Arrhenius Plot of the Rate Constants of the Reaction of Iron Pentacarbonyl with Mercuric Chloride in Acetone at Different Temperatures (Indicated Temperatures are in °C) . . . . . . 9 6

18. Proposed Mechanism for the Reaction of Fe(C0)g with HgClg • • • 98

19. Possible Structure of the 1:2 Adduct (Intermediate 12 in the Text) Obtained in the Reaction of Iron Pentacarbonyl with Mercuric Chloride in Acetone 10 0

20. Proposed Mechanism for the Reaction of Fe(CO),.-(HgCl)2 with Mercuric Chloride 10 3

Vll

INTRODUCTION

Metal carbonyl complexes are a unique class of complexes

in that the central low oxidation state metal atom is tightly

bound to relatively unreactive carbon monoxide ligands. This

stability is accounted for by the CO acceptance of the charge

generated by sigma bonding through the empty antibonding

pi orbitals. The donation and acceptance of this charge has

a mutually reinforcing effect; the removal of the charge by

pi bonding results in the metal becoming electron deficient

and thus more susceptable to charge acceptance through the

sigma bond. This mutual reinforcement has been termed

"synergistic" bonding (1). The sigma bonds are formed by

the primarily sp-hybrid orbital of CO which is situated

principally on the carbon atom and is slightly antibonding with

respect to the CO bond (2). This orbital is filled, and the

donation of electron density would increase the C-0 bond

strength. This orbital is energetically available to interact

with the octahedrally coordinated empty orbitals of the metal

atom. The orbitals of the metal atom may be thought of as

hybrid orbitals composed of d 2 and d 2 2 orbitals and the £1 2C y

s,px,py a n^ P z orbitals of the next higher shell. The dative

overlap of the CO orbital with one of these hybrid orbitals of

the metal is shown in Figure 1-a.

V J

G>

bG C

•H

£ 0

rQ

•H ft

a a*

rd s bO

•H 03

/--v rd

£ •H

a) > rH. o > »

£ 03 *rH nj

£ 03 rd i—1 bO fti •H -P H *H rQ rH

>> O £

0 ?4 rQ rd i—1 nj 3 a o a) T3 i—i a o fd e

03 MH £ o 0

•P £ rd O

•H i—1 -P rd a •P CD cu 03 s

03 £ 03 O O •H $4 -P O 1 03 1 £

i—i rd • -P

bO •H £ PM CD

a) £ -p CD rQ

Figure 1-b shows the overlap of the empty antibond-

ing orbitals of the CO with the energetically accessible

metal d orbitals of proper symmetry (the degenerate dXy, d^z

and d orbitals). This view shows only bonding with one X Z J

d orbital. However, each CO will be of proper symmetry to

overlap with two d orbitals. The CO groups trans to one

another will mutually share two d orbitals and CO groups

cis to one another will share only one. These d orbitals

are filled and form dative pi bonds with the CO or other

donor groups which possess vacant orbitals of the appropri-

ate energy and symmetry.

Metal carbonyls and their derivatives undergo a wide

variety of substitution reactions under favorable conditions

at rates which are usually easily monitored by standard

instrumental techniques such as visible and infrared

spectroscopy. These complexes are considered "inert" (3),

whereas "labile" systems are those in which the reactions

are complete within the time of mixing (one minute at room

temperature). These reactions can be studied by a method

which monitors absorptions in the visible region called the

stopped-flow technique (H). The stopped-flow technique has

not been used extensively to follow the reactions of metal

carbonyls since it is relatively new, but studies employing

this technique are becoming more common (5,6). In general,

the labile complexes are those which contain at least one

vacant lower d orbital in their electron configuration

(from valence bond theory) (7). A lower d orbital would be

a d , d or d orbital of the same shell as the d 2 and xy' yz xz z

d 2 2 orbitals involved in formation of the d2sp3 hybrid x-y

orbitals. From this consideration, the Group VI-B metal

carbonyls have completely filled lower d orbitals and would

therefore be expected to be inert.

Substitution reactions of the Group VI-B metal hexacar-

bonyls usually follow rate laws of three general types:

a) first order, b) second order, and c) a combination of

both (8). The rate law that is first order overall is

independent of the ligand concentration employed and has

been attributed to a mechanism involving the rate determin-

ing dissociation of CO, as illustrated in Figure 2, to form

a five coordinated intermediate or transition state followed

by a faster step involving attack at the five coordinated

complex by the nucleophile, L, to give the monosubstituted

hexacarbonyl. This simple mechanism is called a dissoci-

ative mechanism, D, and results in an intermediate of

reduced coordination number as opposed to a dissociative

interchange mechanism, 1^, which involves a concerted

process (9).

The second order term, which is dependent upon the

concentration of the ligand, is consistent with a mechanism

involving nucleophilic attack at the metal atom (path a,

Fig. 3) or initial attack at the carbonyl carbon (path b,

Fig. 3), followed by a fast step or steps to yield the

o

o o

u

CO r—i >>

£ o

rQ & rd O rd X CD

r C -

I—{ rd

• P (D a

s U

• u o

o u

u

o u - • u o

o u

rd & T J CD

r C rd

- P O G

<4H o

CO • H a rd & O a)

CD > • H • P rd

• H a o CO CO

• H

54 rd

i—I 3 O CD

r—I 0 e

• H a

P 1 I

CM

bO • H Pm

Path (a) path (b)

O

°cC

O c TW! UC. j . v o .c L^iJ c.

c o

o

-co fas t -co

O, X e

O C w

O

!e< i \ 1 C C O

O

Fig. 3—Possible pathways for an associative mechanism

monosubstituted product. This mechanism is termed an

associative process, A, and involves an intermediate of

increased coordination number while an associative inter-

change mechanism, 1^, involves a concerted process (9).

Initial attack at the metal atom should result in a

seven-coordinated complex; indeed, seven-coordinated Group

VI-B complexes in which the metal exhibits a +2 oxidation

state have been isolated and characterized (10-13). The

stereochemistry of seven-coordinated species can possibly

be the pentagonal bipyramid structure as proposed by

Stiddard (10) or a capped octahedral (14,15) or capped

trigonal prismatic (16,17) structure known to exist for

certain molybdenum and tungsten complexes.

Attack at the carbonyl carbon would not require the for-

mation of a seven-coordinate complex. All molecular orbital

calculations on Cr(C0)g indicate a positive net atomic

charge on the carbon atom which should make nucleophilic

attack at that site possible (18-20), and two of the three

indicate this site is more likely than the metal (18,20).

The reactions between ketone carbonyl groups? and phosphites

are also known (21) and indicate the possibility of attack

at the carbonyl carbon for metal carbonyl complexes also.

Mechanisms can be inferred not only from the rate law,

but also from the type and stereochemistry of the reaction

products, and activation parameters which are obtained from

rate data at three or more temperatures. Enthalpies of

activation are not very helpful, but entropies of activation

can indicate the degree of total bond making or breaking in

the transition state, from which an associative or dissoci-

ative path can be inferred. Entropies of activation would

not distinguish between attack at the metal atom or at the

carbonyl carbon since both should involve a more ordered

system in the transition state and thus should afford a

negative entropy of activation. A dissociative process is

favored from entropy considerations since it involves

progression to a system of more disorder and should show

a positive value for the entropy of activation.

Dissociative processes are more prevalent than assoc-

iative processes, presumably because of steric crowding and

van der Waals and electrostatic ligand repulsions which

would hinder the approach of an incoming ligand (22). More-

over, in metal carbonyls, the metal atom is expected to have

a very low positive charge which should not be conducive

to nucleophilic attack (23).

In 1966, the kinetics of reactions of the Group VI-B

metal hexacarbonyls with various Lewis bases, L, were studied

by Werner and Prinz (24,25),

M(C0) 0 + L > MCC0)5L + CO CI)

M=Mo,W,Cr PR3,AsR3 SbR3,RNH2 RNC

These studies showed a rate law that was independent of

the concentration of the ligand used.

d[M(CO)6] k,[M(C0)„] (2)

dt 1 6

This is consistent with the proposal of a dissociative

mechanism. However, the ligand concentration range employed

was very narrow and may have been too narrow to observe a

second order dependence. The following year Graham and

Angelici (26) studied the kinetics of the reaction of the

hexacarbonyls with phosphine, phosphite and arsine ligands

over a wider concentration range and obtained a two term

rate law,

d[M(CO)g]

dt k1[M(CO)g] + k2[L][M(CO)g] (3)

which was dependent upon the concentration and nature of

the ligand used. This would seem to indicate concurrent

dissociative and associative pathways, both of which are

rate determining. The entropies of activation calculated

for these processes are in agreement with the associative

pathway for the ligand dependent term and a dissociative path-

way for the ligand independent term. The associative path-

way was believed to involve initial ligand attack at the

metal atom (path a, Fig. 3).

10

The reactions of the hexacarbonyls with tetraethyl-

ammonium azide yield the isocyanate products, and the

kinetics of this reaction indicated an associative type

mechanism which was believed to involve initial attack of

the azide ion at the carbonyl carbon with a Curtius—type

degradation to yield the final product (27).

6 + 6 ~ (CO) M-C = 0 + N" > [(C0)5M-q=0]

_ —> [ (CO) r M — C = 0]' 6 0 Nt V ' 3 / "N

(4)

N2

[(C0)5M-N=C=0]~ + N2

The attack was believed to be at the carbonyl carbon from

the reaction products obtained, and the fact that the reac-

tion conditions were very similar to those for reactions of

the hexacarbonyls with alkyl and aryl lithium compounds,

which yielded carbonylation products (28).

M(C0)6 + RLi > (C0)5M(Cx=0)~ (5) R

Grignard reagents also give acyl products, and the kinetics

of the reactions of the hexacarbonyls with benzylmagnesium

bromide (29) and also with methyllithium (30) indicated

associative mechanisms.

CHAPTER I

STUDIES IN THE KINETICS AND MECHANISM OF THE

REACTION OF GROUP VI-B METAL CARBONYLS

WITH PRIMARY AMINES IN DECALIN

Problem

Lewis bases coordinated through nitrogen have been

observed to be poor nucleophiles in reactions with metal

carbonyls, and the D mechanism was believed to be the only

pathway operative for such poor nucleophiles (8). However,

a two-term rate law was observed in the kinetic studies of

the reactions of acetonitrile with Group VI-B metal hexa-

carbonyls,

M(CO) 0 + CH3CN — > M(C0) g (CHgCN) + CO gij|5gxt;U-t;ecl5 (6) products

indicating the involvement of an associative component as

well as a dissociative pathway (31). An associative component

for such a weak nucleophile as CHgCN might lead to the

proposal of a mechanism other than the simple "associative"

mechanism. One mechanism which should give a ligand-depend-

ent term for a wide range of nucleophiles would be the

"dissociative interchange" (Id) mechanism (9). This

mechanism involves an incoming ligand, favorably positioned

for interchange, bonding as a group on the substrate leaves.

11

12

Thus, there would be relatively little bond making in the

transition state. This process involves a solvent-encased

substrate and entering ligand and has been used by Covey and

Brown to explain the results of the reaction of (amine)Mo(CO)

complexes with various nucleophiles (32).

To test the possibility of this process for reactions

of the hexacarbonyls with Lewis bases in general, the study

of the reactions of the hexacarbonyls with primary amines,

which are also poor nucleophiles, was undertaken. These

reactions had been investigated by Werner and Prinz (24,25)

and had shown only a ligand independent term. However, the

reactions of the hexacarbonyls with phosphine ligands had

shown only a ligand independent term when investigated by

Werner and Prinz (24), but a ligand dependent term was

observed when these reactions were reinvestigated by Graham

and Angelici (26). The failure to observe this term was

evidently due to the low ligand concentration range employed

by Werner and Prinz; the amine concentration range was also

low.

It is also generally thought, however, that ir-bonding

of the incoming ligands to the metal atom should stabilize

the transition state in the associative process, and for

ligands incapable of ir-bonding, such as amines, the ligand-

dependent term would not be expected.

Also, if a ligand-dependent term is observed, determi-

nation of the site of the ligand attack would be informative.

13

Attack may be directed at the metal atom by the filled d

orbitals of the metal atom having the proper symmetry to

overlap with the rr orbitals of ligands such as acetonitrile

or the energetically-accessible, vacant d orbitals of ligands

such as phosphines and phosphites. The reactions of the

hexacarbonyls with these ligands are indeed believed to

occur by initial nucleophilic attack at the metal atom. The

amines, which possess no energetically-accessible Tr-bonding

orbitals, therefore, might not be expected to attack at the

metal atom. The analogous reactions of the primary amines

with manganese cations and their substituted derivatives

gave carbamoyl products (33), e.g.,

Mn(CO)* + 2RNH„ > Mn(CO)c(C-NHR) + RNH, . (6a) O Z O O

The formation of these products strongly suggest initial

attack of the amine at the carbonyl carbon. The Group VI-B

metal carbonyls, however, gave simple substitution products

in their reactions with primary amines, and the

M(CO)g + amine > (amine)M(CO)5 + GO > su§stxtuted products

question of the site of attack is an open one.

11+

Materials

The reactions of the hexacarbonyls with the amines were

found to be extremely sensitive to traces of impurities so

the reagents were rigorously purified. The hexacarbonyls

were obtained from Pressure Chemical Company C(Cr(CO)g and

W(CO)g] and Climax Molybdenum Company [Mo(CO)g] and were

purified by sublimation under high vacuum (ca. 0.05 torr)

at room temperature [Cr(C0)g and Mo(C0)g3 and at 40°C

[W(C0)g]. They were stored in foil wrapped vials in a

desiccator and occasionally placed under high vacuum briefly

to insure their dryness.

Aniline and cyclohexylamine were obtained from Matheson,

Coleman, and Bell Company and benzylamine from the J. T.

Baker Chemical Company. Cyclohexylamine was purified by

refluxing with ten per cent acetone overnight to remove

sulfur impurities (34-). Diethyl ether was then added and the

solution was acidified with 12M HC1 until two phases were

present. The amine hydrochloride was then repeatedly extracted

with diethyl ether until the last two ether extractions were

colorless. The aqueous layer was then further acidified,

and the precipitated [CgH.^NHg]+Cl~ was filtered, washed

with ether, and recrystaliized from 95 per cent ethanol

five times. The purified salt was then added to a saturated

aqueous KOH solution and the liberated amine, containing

15

water, was distilled at atmospheric pressure (91°C). The

amine was separated, water again was added, and the distil-

lation repeated (35). The amine was then separated, dried

over anhydrous CaCl^ and distilled (36). It was then dried

over sodium under nitrogen and redistilled (six times,

133-4°C) under an atmosphere of nitrogen which had been

passed over anhydrous calcium chloride.

Benzylamine was purified in much the same manner; the

final distillations were carried out under reduced pressure

(69°C, 10 torr, nitrogen bleed). Potassium hydroxide was

used as a drying agent rather than calcium chloride (37).

Aniline was purified similarly with an additional puri-

fication step added. This consisted of adding the aniline

in small portions to a saturated solution of recrystallized

oxalic acid. The resultant aniline oxalate salt was re-

crystallized from water three times,washing it with ethanol

each time. The aniline was then liberated by adding the

salt to an aqueous solution saturated with recrystallized

sodium carbonate. The aniline was separated from the water

by addition of KOH, distilled over barium oxide three times,

distilled from powdered zinc twice (38), and redistilled

three more times under reduced pressure (70°C, 11 torr,

nitrogen bleed) rejecting the first and last portions.

Aniline and benzylamine were kept under vacuum between

distillations. During the necessary handling in preparing

for the kinetic runs the amines were kept under a nitrogen

16

atmosphere. Cyclohexylamine was at all times kept under a

nitrogen atmosphere. These precautions were taken to avoid

contact with air which would result in uptake of water and

probable formation of amine oxides.

Triisopropyl phosphite was obtained from Alrich Chemi-

cal Company and was purified as previously described (39).

Decalin (decahydronaphthalene) was vacuum distilled

over sodium (68°C, 12 torr) three times and stored under

nitrogen atmosphere.

Determination of Reactions Rates

Reactions rates were determined on a Perkin-Elmer 621

grating infrared spectrophotometer by monitoring the de-

crease in intensity of the carbonyl stretching mode of

the hexacarbonyl at the appropriate frequency (1985-1990cm~l)

C+0). The reactions were run under pseudo-first-order

conditions (at least a 12-fold excess of the ligand).

Pseudo first-order conditions are those in which the

ligand concentration is in such a large excess of the sub-

strate concentration that it remains essentially constant

during the course of the reaction. This will allow the rate

expression to involve only the variables of substrate con-

centration and time. The ligand concentration will be

incorporated into the rate constant and will give an over-

all apparent rate constant, k obsd

17

The reaction vessels (25 ml volumetric flasks) were

cleaned first with chromic acid and then saturated KOH

solution. A subsequent rinsing with acetone was followed

by a drying period in an oven after which the flasks were

brought to room temperature in a desiccator. The flasks

remained in the desiccator until they were weighed.

Immediately prior to each run, the amines were distilled

twice and conveyed under nitrogen to a nitrogen filled

glove bag. The amines were transferred in the glove bag

to the cleaned, weighed volumetric flasks. The flasks

containing the ligand were closed and removed from the

glove bag for weighing. They were then placed in a water

bath containing a Haake constant temperature circulator

which maintained the temperature to ^0.05°C. The solvent

was then brought to volume under a nitrogen purge at the

reaction temperature. The substrate (ca. 5xlO~L|'molar) was

placed in a 50 ml volumetric flask and the system was then

flushed with nitrogen and sealed with a rubber septum. The

equilibrated solvent-ligand solution was then injected into

the reaction flask with a large syringe, after removal of

an equal volume of nitrogen.

The reaction mixture was allowed to re-equilibrate

for at least twenty minutes, after which time samples were

withdrawn and injected into the cells (NaCl plates, 0.5mm

pathlength).

18

The ligands and reaction products were found not to

absorb significantly at the wavelength monitored, so absorb-

ance readings at t^ were not taken; hence zero and one-

hundred per cent transmittance values were set on the

spectrophotometer with solvent in each cell. Plots of In

absorbance vs. time were linear to at least four half-lives.

Several runs, for each system studied, were carried out

with gas-tight syringes as the reaction vessels to eliminate

the gas phase (24), but results showed that possible sub-

limation of the hexacarbonyl and/or gas phase reactions

were of negl'igable influence on the observed reaction rates.

Identification of Reaction Products

The scans of the carbonyl stretching region (2200-1900

cm"1) of the infrared spectra of the reaction solutions as

a function of time revealed the initial formation of

(amine)M(CO)r products, followed by further substitution

to di- and tri-substituted products (41).

Most of the (amine)M(CO)complexes are known (41-43),

and they exhibit well-documented and characteristic carbonyl

stretching spectra in hydrocarbon solvent (43,44). The

initial product of the reaction of triisopropyl phosphite

and Mo(CO)r was identified through comparison of its carbonyl b

stretching spectrum to those of the closely- related

P(OR)3Mo(C0)5 complexes (45).

19

Treatment of Data

Values of the pseudo first-order rate constants,

k , and the rate constants were calculated employing obsd'

a linear least-squares computer program executed on an

IBM 360 Model 50 computer of the North Texas State

University computing center. The limits of error (cited

in parenthesis in Tables I, II, and IV) are one standard

deviation. Values which differ from the mean by more

than two standard deviations were rejected.

Results

The rate data obtained from the reactions in decalin

solvent ( ko b s d values) are presented in Table I. Plots

of k , , vs. [CcHcCH0NH0] for the reaction of benzylamine obsd 6 5 2 2

with the hexacarbonyls are shown in Fig. 4. Fig. 5 shows

plots of kQksd vs. [amine] for the reaction of benzylamine

with Mo(C0)g at three temperatures. The computer calcu-

lated first- and second-order rate constants, k^ and ^2,

which are obtained from plots of vs. [amine] from

the equation,

kobsd = kl + ^2[amine] , (8)

are given in Table II. Activation parameters for the

reaction of Mo(C0)g with benzylamine at three temperatures

are also shown in Table II.

20

Discussion

The kinetic results support the rate law,

-d[M(C0)g]/dt = k1[M(C0)g] + k-2 CM( CO) g ] [RNH2 ] , (9)

which involves two reaction paths occurring at the

same time: a pathway independent of the ligand concen-

tration and a competing, ligand-dependent pathway. Both

paths lead, initially to the simple substitution

products,

M(C0)6 + RNH2 > (RNH2)M(CO)5 + CO . (10)

The ligand-independent pathway governed by ^ is consistent

with a dissociative mechanism as illustrated in Fig. 2.

This involves rate-determining loss of a CO ligand to yield

a five-coordinated intermediate, followed by rapid attack

of the amine ligand. This is the commonly accepted

mechanism for this kinetic behavior observed for related

reactions (8,4-6). The rates and activation parameters

for this path (Table II) are in reasonable agreement with

the rates and activation parameters obtained from the

studies of the reactions of the hexacarbonyls with other

Lewis bases under similar conditions (24-26).

There has been considerable spectroscopic evidence

for the detection of the above mentioned intermediates,

fM(C0)t], when they are produced by ultraviolet photolysis O

21

TABLE I

RATES OF REACTION OF METAL HEXACARBONYLS WITH CYCLOHEXYLAMINE, BENZYLAMINE, AND

TRIISOPROPYL PHOSPHITE IN DECALIN AT VARIOUS TEMPERATURES

Metal Ligand HI

VJ 0

O CL], M " X b s d ' s e c _ 1

Cr C H CH NH 1 3 0 . 0 0 ( 5 ) 0 . 0 1 4 1 1 . 2 2 ( 2 ) Cr 6 5 2 2

0 . 0 1 4 5 1 . 3 0 ( 1 )

0 . 0 1 7 0 1 . 2 4 ( 1 )

0 . 0 4 6 8 1 . 3 4 ( 1 )

0 . 0 5 0 3 1 . 2 4 ( 3 )

0 . 0 6 1 2 1 . 2 5 ( 1 )

0 . 0 7 8 4 1 . 2 4 ( 2 )

0 . 1 2 6 1 1 . 3 4 ( 2 )

0 . 1 2 6 4 1 . 3 3 ( 1 )

0 . 1 3 0 4 1 . 4 1 ( 1 )

0 . 1 3 4 1 1 . 4 1 ( 2 )

0 . 1 5 3 4 1 . 3 6 ( 1 )

0 . 1 6 2 8 1 . 4 3 ( 1 )

0 . 1 7 5 4 1 . 4 1 ( 2 )

0 . 1 9 7 3 1 . 3 7 ( 1 )

0 . 2 1 5 6 1 . 3 9 ( 1 )

0 . 2 4 6 8 1 . 3 9 ( 1 )

0 . 2 6 3 6 1 . 4 6 ( 1 )

TABLE I Continued

22

Metal Ligand T, [L], M 10 xk -1

obsd:

W

Mo

C H CH NH 150.00(5) 6 5 2 2

C H NH 6 11 2

112.00(5)

0.0173

0.0213

0.0484

0.0800

0.1282

0.1514

0.1729

0.1844

0.2182

0.2257

0.2371

0.2841

0.0053

0.0332

0 .0367

0.0524

0 . 0596

0.0786

0.0 851

0.1119

0.1283

0.1313

0.1352

0.247(3)

0.262(2)

0.263(6)

0.277(5)

0.290(2)

0.301(2)

0.299(3)

0.306(4)

0.314(5)

0.324(1)

0.330(4)

0.341(4)

2.36(2)

2.54(1)

2.62(2)

2.66(1)

2.55(1)

2.78(1)

2.65(5)

2.78(2)

2.90(2)

3.00(3)

2.96(3)

23

TABLE I Continued

Metal Ligand T, °C [L], M lO^xk , sec obsd

Mo C„HrCH0NH 102.00(5) 6 5 2 2

107.00(5)

0.1 +64 3.05(3)

0.1478 3.06(2)

0.1479 3.00(1)

0.1609 3.05(3)

0.1837 3.16(3)

0.2209 3.21(2)

0.2332 3. 27(2)

0.2605 3.61(2)

0.2655 3.48(2)

0.2803 3.61(2)

0.0226 0.708(8)

0.0512 0.763(4)

0.0901 0.826(7)

0.1065 0.867(7)

0.1651 0.960(9)

0.1888 0.992(4)

0.2323 1.07(1)

0.2536 1.09(1)

0.2949 1.18(1)

0.2959 1.16(1)

0.3184 1.21(1)

0.0370 1.2 9(3)

0.0619 1.41(1)

24

TABLE I Continued

Metal Ligand T, °C M . . 1 0 xkobsd' S 6 G

0.0626 1.36(2)

0 . 0738 1.1+5(1)

0.0774 1.44(2)

0.0909 1.51(3)

0.129 7 1.6 2(1)

0.1467 1.66(2)

0.2110 1.85(1)

0.2220 1.80(1)

0.2222 1.85(1)

112.00(5) 0.0118 2.61(12)

0.0179 2.45(1)

0.0183 2.48(2)

0.0410 2.53(4)

0.0548 2.85C2)

0.0 56 3 2.61(2)

0.0696 2.73(1)

0.0909 2.78(1)

0.1044 2.95(1)

0.1165 3.01(5)

0.1289 3.00(2)

0.1795 3.21(3)

0.1901 3.40(5)

0.2139 3.28C3)

25

TABLE I Continued

Me~t sil Ligand T, °C [L], M lO^xk , ,, sec 1 obsd

Mo 3 2

0. 2169 3 . 34(4)

0. 2299 3 .40(5)

0. 2717 3 .74(6)

0. 00 66 2 .52(3)

0. 0417 2 .58(4)

0. 0588 2 .67(4)

0. 0886 2 .60(3)

0. 1166 2 .88(3)

0. 1415 2 .81(3)

26

o X

u CD 0)

~o

JO O

Mo® 112

Cr@ 130

<•>©-©— 150'

0.1 [L] M 0.2

-o-

Fig. 4—Plots of k0bsd vs• ^L] for the reactions of the

metal hexacarbonyls with benzylamine in decalin solvent at various temperatures.

27

3-

O ^

X

u 9 (D (/) T3 tO J2 O

j*:

o

° 112°

0- [amine] 0-^ m 0.3

Fig. ,5—Plots of k0t>sd vs. [amine] for the reaction of molybdenum hexacarbonyl with benzylamine in decalin at three temperatures.

28

TABLE II

FIRST AND SECOND ORDER RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE REACTIONS OF THE METAL HEXACARBONYLS WITH

BENZYLAMINE AND MOLYBDENUM HEXACARBONYL WITH CYCLOHEXYLAMINE AND TRIISOPROPYL

PHOSPHITE IN DECALIN

Metal Ligand •pft

•zh o i—1 X I—1

k2 x 104

(°C) (sec •*") CM" sec"-*-)

W C6H5CH2NH2 150.00 0. 248(3) 0. 327(15)

Cr 130.00 1.24(2) 0. 79(14)

Mo** C6H5CH2NH2** 102.00 0.677(5) 1. 67(2)

107.00 1.22(2) 2. 88(15)

112.00 2.44(4) 4. 43(26)

C6H11NH2 112.00 2.38(3) 4. 23(19)

((CH3)2CHO)3P 112.00 2.49(6) 2 . 4(7)

ft +0.05°C

ft ft AH* = 3 6.0(29) kcal/mole; ASF = 19 .1(92) cal/deg•mole

AH* = 27.2(24) kcal/mole; AS* = -3 .7(62) cal/deg*mole

29

of the M(C0)g complexes. This detection was made possible

by trapping the photochemically generated intermediates

in hydrocarbon glasses (4-7,48) and various inert matrix

(49) by means of low temperatures. The flash photolysis

of chromium hexacarbonyl in cyclohexane solvent (50)

also produced a highly reactive species suggested to

be [Cr(C0)5].

The term governed by k£ in the observed rate law

suggests that the rate-determining step involves the

amine ligand. The fact that these reactions occur by

a ligand-dependent pathway even though the amines are

expected to be weak nucleophiles may indicate the

"dissociative interchange" mechanism (9) instead of an

associative mechanism.

Since the same mechanism should give the same

relative order of reactivities, the site of the nucleo-

philic attack may be inferred through comparison of the

second-order rates as a function of the identity of the

metal atom. Kinetic investigations of the reactions of

the hexacarbonyls with the phosphine, phosphite, and

arsine ligands (26) and with acetonitrile (31), where

attack is believed to occur at the metal atom, showed

the rates to vary, Mo>Cr>W. In contrast, the reactions

with the azide ion (27), Grignard reagents (29) and

organolithium reagents (30), where attack is believed

to occur initially at the carbonyl carbon, were seen to

30

vary W>Mo>Cr. The rates (or activation parameters) for

the azide system and the tributylphosphine system were

compared. The enthalpies of activation together with

their differences for the corresponding systems are shown

in Table III. The relative enthalpies of activation for

reactions of Cr(C0)g and Mo(C0)g by both paths are

similar, but those for W(C0)g differ appreciably.

TABLE III

ENTHALPIES OF ACTIVATION FOR REACTIONS OF GROUP VI-B METAL HEXCARBONYLS BELIEVED TO BE OCCURING

BY TWO DIFFERENT ASSOCIATIVE PATHWAYS

Substrate P(n-C^Hg)3* [(C2H5)1+N]N3** Difference

Mo(CO)6

Cr(CO) c b

W(C0)g

21.7(13)***

25.5(29)

29.2(16)

15. 3(8)

18.2(6)

12.8(10)

6.4(21)

7.3(35)

16.4(26)

* Reference 2 6

** Reference 2 7

AAA A H values in kcal/mole

A normal coordinate analysis of the hexacarbonyls

based on the use of a general quadratic valence force

field was carried out by L. H. Jones, et al. (51), and

the magnitude of the carbonyl and metal-carbon stretching

31

force constants indicated stronger W-CO than Mo-CO or

Cr-CO sigma bonding. Electron impact (52) and thermo-

chemical (53) data also support a greater W-C bond

strength which should transfer more charge from the

caronbyl carbon to the metal. This should inhibit

nucleophilic attack at the metal atom and increase the

relative rate of attack at the carbonyl carbon, as is

observed.

For the reactions of benzylamine with the hexa-

carbonyls the rate constants for the associative pathway

(Table II) are seen to vary with the metal atom, Mo>Cr>W,

which is the order observed for reactions which are

thought to involve attack at the metal atom. This order,

and the fact that only simple substitution products

are observed, indicates that for these reactions attack

probably occurs at the metal atom (path a, Fig. 3)

instead of a carbonyl carbon (path b, Fig. 3). This

would indicate that u-bonding is not the effect that

directs attack at the metal atom.

The calculated entropies of activations (Table II)

indicate that the ligand-independent pathway is a

dissociative process (yields a positive value for AS^).

However, the value calculated for the ligand-dependent

pathway (AS^) is zero within experimental error indicating

a process involving very little bond making in the

transition state.

32

The reaction of Mo(C0)g with benzylamine and with

cyclohexylamine show k2 values that are equal within

experimental error. From electronic consideration

cyclohexylamine would be expected to be more reactive

but from steric consideration benzylamine should be.

The aniline was never sufficiently purified for good

kinetic results, but a ligand-dependence was observed in

plots of vs. [CgHj-NE^]. The essential equality

of the k2 values of benzyl and cyclohexylamine indicates

a low dependence of nucleophilicities on k2, and comparison

of the amines with the other Lewis base ligands (Table IV)

indicates that the k2 terms vary little with nucleo-

philicities .

The low dependence of the k2 terms on the nucleophile

indicates bond making is not a dominant influence in

the transition state. From Hammond's postulate (54),

if the transition state involves considerable bond making

it would possess many properties of the product molecule.

Since the M(CO)gL products should possess different

stabilities the transition states would also possess

different free energies depending on the amount of

bond formation. Since the k2 terms obtained from reactions

of the hexacarbonyls with various Lewis bases are of

comparable magnitude the transition state vary little

in free energy. The bond formation is accomplished,

therefore, from the transition state to the product and

33

is an exothermic step. Since the transition state does

not involve bond formation there will be no intermediate

of significant stability and the process will be

concerted. Such a pathway is an "interchange" (9)

process.

An associative interchange mechanism differs

from a dissociative interchange mechanism in that the

former involves participation of the entering group

in the transition state whereas the latter does not (9).

The rate of the reaction by the associative interchange

process will have a small dependence upon the nature of

the entering group. The effect of the ligand on the

rate by the dissociative process will only be the

result of change in the environment upon the encasement

of the entering group with the substrate by the solvent. 4= 4-

The lower value of AH^ as compared to AH^ (from Table II)

is due to the influence of the entering ligand, although

much of the difference is probably due to residual

bonding between the metal and the departing ligand.

From the limited amount of data on this subject it is

still not clear how sensitive both interchange processes

should be to the nature of the entering group (32).

The process operative for the reactions of the hexcar-

bonyls with Lewis bases is concluded to be an I, d

mechanism by comparison with the results of Covey and

34

TABLE IV

FIRST AND SECOND ORDER RATE CONSTANTS FOR THE REACTION OF GROUP VI-B METAL HEXACARBONYLS WITH

VARIOUS LEWIS BASES IN DECALIN

Metal Ligand T

(°C)

k x lo4**

(sec""'")

k x 10 2 (sec'-'-M-!)

Cr

Mo

W

a C6 H5 C H2™2 130. 0 0* 1. 24(2) 0. 79(14)

b p < C 6 V 3 130. 7 1. 40(3) 0. 36(14)

b p ( O C2 V 3

130. 7 1. 33(3) 0. 53(7)

bP(n-C1+Hg) 3 130. 7 1. 39(4) 0. 84(12)

g c h 3 c n 64. 5 0. 0 0 6 * * * 0. 023

c c 6 h 5 c n 64. 5 0. 00 5*** 0. 029

c C 2 H 5 C N 64. 5 0. 005*** 0. 034

bAs(C0H5)3 112. 0 2. 22(9) 0. 89(24)

b p ( ° c 6H

5 ) 3 112. 0 2. 15(3) 1. 40(6)

aP(OCH(CH3)2)3 112. 00* 2. 49(6) 2. 4(7)

bP(OCH2)3CC2H5 112. 0 2. 18(12) 3. 43(18)

aC6HllNH2 112. 0 0* 2. 38(3) 4. 23(19)

a C6 H5 C H2 N H2 112. 0 0* 2. 44(4) 4. 43(26)

bP(°C2H5)3 112 . 0 2. 34(5) 5 . 76(10)

b p < » - ° H H 9 ) 3 112 . 0 2. 26(10) 20. 1(2)

a C6 H5 C H2 H H2 150.

* o o 0. 248(3) 0. 327(15)

bpCC6H5'3 165. 7 1. 08(2) 1. 14(11)

35

TABLE IV Continued

Metal Ligand

/—N

I—h|

0

0

vy

kx x 104

(sec--'-)

k2 x 104

(sec-1M-1)

bP(OCH2)3CC2H5

bP(OC2H5)3

bP(n-C4H9)3

165. 7

165.7

165. 7

1.07(2)

1.19(7)

1.21(4)

1.60(5)

1.61(21)

6.99(9)

*" ±0. 05°C.

** Numbers in parenthesis are limits of error to one standard deviation.

*** Calculated from given k o b s d, [L] and k2 by the relationship, k0bsd = k1+k2[L].

a This work.

b Ref. 26; All cited rate data are values recalcu-lated by a least-squares computer program from their krtK_ A and [L] values.

c Ref. 31; Kinetics performed in C2H2CI2 solvent.

36

Brown (32). The rates for the ligand-dependent pathway

for the reactions of Mo(C0)g(pip) (pip = piperidine)

with various Lewis bases varied over approximately the

same range as the hexacarbonyls with various Lewis

bases (Table IV).

The magnitude of the terms closely parallel

those for the k£ terms for the systems in Table IV,

indicating that the factors that affect the reactivity

by the dissociative path also influence reactivity by

the associative pathway. This supports an I .

mechanism.

As mentioned earlier in the chapter, rigorous

purification of the amines had to be performed to remove

impurities. Also, special precautions had to be taken

with the purified amines to avoid the reintroduction

of impurities. The presence of impurities was found

to give erroneous results in the rate data. Data

obtained from the reaction of molybdenum hexacarbonyl

with cyclohexylamine which had been insufficiently

purified are presented in Fig. 6. Fig. 6 also shows

the data obtained from "the reaction of molybdenum

hexcarbonyl with cyclohexylamine which was rigorously

purified. Both reactions were at the same temperature

(112°C). It can be seen that the slope of k , , * obsd

vs. [CgH^NI^] for the system employing pure amine

37

is parallel to the slope of k Q b s d vs. [CgH^NHj] for

the system employing; impure amine. The impurity

therefore hinders the dissociative component (indicated

by a lower k-j value), while the associative component

is unaltered. It is unexpected that impurities

would affect the dissociative component of the

reaction instead of the associative term since k^

should be independent of the ligand used. This same

effect has been noted, however, for the reaction of

W(CO)g(aniline) with P(CgH5)3 (55), except that the

dissociative component is accelerated and not hindered

as in our substrate-ligand systems. This alteration

of the k^ value for the reaction of triphenylphosphine

with W(CO)5(aniline) was shown to be the result of

phosphine oxide impurities and was verified by purposely

adding triphenylphosphine oxide. The reaction of

HRe(C0)g with triphenylphosphine also was seen to

give irreproducible kinetic data (56), and the

dissociative component of the reaction of (arene)M(CO)-o

(M=Tfo,W) with trimethylphosphite (57) was found to

be accelerated by impurities formed by oxygen and

moisture contact. On the basis of the above, it is

not unlikely that amine oxides were affecting the

k^ terms in our systems.

38

O Q

3 "

O

u

9 2 CO

"U cf) n o

1-

0.1 0.2 [amine] m

fig* 6--Plots of kobgci vs. [amine] for the reaction of molybdenum hexacarbonyl with pure and impure cyclohexylamine in decalin at 112°C.

39

To test; the purity, of the amine ligands, a ligand

which is easy to purify can be reacted with the hexa-

carbonyls under identical conditions employed in reactions

with the amine ligand. Equality of the values of

obtained from investigations employing this easily

purified ligand the amine ligand should indicate

purity of the amines. The results of gas chromatography

and mass spectroscopy analysis (58) indicate that the

purity of triisopropyl phosphite is relatively easy

to achieve. Pure triisopropyl phosphite was reacted

with Mo(C0)g in the same solvent and at the same

temperature as the amine ligands. From Table II it

can be seen that the ^ values for reactions involving

triisopropyl phosphite, benzylamine and cyclo-

hexylamine are essentially equal indicating purity of

the amines.

In conclusion, the kinetic investigations of the

reactions of the hexacarbonyls with primary amines

supported a two-term rate law containing a ligand-

dependent pathway is consistent with the proposal of

an 1^ mechanism. An 1^ mechanism was proposed because

of the insensitivity of the second-order rate constant

to changes in the nucleophile. The 1^ mechanism may

be the mechanism operative in reactions of hexacarbonyls

with Lewis bases in general. Since the amines, which

40

are not capable of u-bonding, reacted by a ligand-

dependent pathway it may be thought that u-bonding

may not significantly stabilize the transition state

for ligand attack as has previously been held. Pi-

bonding also seems not to direct attack at the metal

atom since the amines are believed to attack at that

site. This position of attack was inferred by

comparison of the values as a function of the metal

atom with the relative reactivities of other nucleophiles

with the hexacarbonyls. The relative reactivities of

phosphine and acetonitrile ligands with the hexcarbonyls

as a function of the metal atom were the same as the

amines indicating attack at the metal atom. To gain

further insight into the factors determining the

mechanism and site of ligand attack on the carbonyl

complex for the ligand-dependent pathway a kinetic

study of the reactions of the hexcarbonyls with the halide

ions was undertaken and is discussed in Chapter II.

CHAPTER II

STUDIES OF THE KINETICS AND MECHANISM OF THE

REACTION OF GROUP VI-B METAL CARBONYLS

WITH TETRABUTYLAMMONIUM HALIDES

Problem

The Group VI-B hexacarbonyls are known to react with

N-methylpyridinium iodide (59) and with tetraalkylammonium

halides (55,56) to give the M(C0)gX~ anions (eqn. 11).

M(C0) g + R^NX > [R1+N][M(C0)5X] + CO (11)

X=Cl,Br,I

The fluoride reaction either did not occur or the product

was too unstable for isolation. Alkali-metal halides also

react with the hexacarbonyls to yield these anions in

diglyme solvent with the metal salt existing as an etherate,

[N(digly)nl[M(CO)5X] (N=Li,Na,K,Rb,Cs; X=Cl,Br,I; digly=

diethylene glycol dimethyl ether; n=l,2,or3=the degree of

etheration) (60,61). The isoelectronic manganese complexes,

XMn(C0)j- (X = Cl,Br,I), react with tetraalkylammonium halides

to give cis-[Mn(C0)^X2]~ products and the kinetic investi-

gation of the reactions indicated a dissociative mechanism

(62) only.

-d[XMn(CO)g]/dt = k1[XMn(C0)53 (12)

41

42

The conditions employed for the reactions of the

hexacarbonyls with the halide ions were much too mild for

the reactions to be proceeding by a dissociative mechansim

only. King (63) found the reactions of Mo(C0)g with the

halides to be: complete in sixteen hours using refluxing tetra-

hydrofuran (THF) solvent (b.p. 6 5°C) and the reactions of

W(CO) and Cr(CO) to be complete in the same time limit 6 6

using refluxing dioxane solvent (b.p. 101°C). If only a

dissociative mechanism is operative, the reactions of W(C0)g

with the halides would be less than one per cent complete,

and the reactions of Mo(C0)„ and Cr(CO)„ should be less than 6 6

ten per cent complete, in this time limit. This was

determined by extrapolation of the Arrhenius plot of the

first order rate data for the reactions of the hexacarbonyls

with tributylphosphine (26) to the temperature employed by

King.

Kinetics of the reactions of tetraalkylammonium haldies

with the group VI-B hexacarbonyls thus should show a path-

way other than a dissociative pathway. This other route

must predominate (vide supra) which seems to indicate the

mechanism to be different than the 1^ mechanism proposed

for the reactions of the hexacarbonyls with the Lewis bases

(Chapter I). For these reactions the concurrent 1^ and D

pathways were seen to be of comparable magnitude. It may

be speculated that the reactions of the hexacarbonyls with

the halides proceed predominately by an associative mechanism.

43

Kinetic studies of the reactions of the hexacarbonyls

with the halides should also indicate the site of the ligand

attack Cat the carbonyl carbon (path b, Fig. 3) or the metal

atom (path a, Fig. 3)). It was concluded from Chapter I

that T T - b o n d i n g does not direct attack at the metal atom.

Determination of the site for halide attack should help

substantiate this proposition. Halides are not expected to

function as pi-accepter ligands since the p orbitals which

are of appropriate symmetry to interact with the metal

d^-orbitals are filled. The halides would therefore be

expected to act as ir-donor ligands. However, molecular

orbital (MO) calculations for XMn(CO>5 (X=Cl,Br,I) indicate

there is no transfer of charge from the halogen atom to the

metal atom through the pi orbitals (2).

The halides would be expected to be bulky ligands and

thus to attack at the carbonyl carbon from this consideration,

It is also noted that the reaction of the hexacarbonyls with

ligands containing a negative charge such as the azide ion

(27), Grignard reagents (29) and organolithium reagents (30)

were believed to occur, through the ligand-dependent path-

way, by attack at the carbonyl carbon. On the other hand,

the reactions with uncharged ligands (phosphines, phosphites,

and arsines (26), acetonitrile (31) and primary amines) are

believed to occur through this pathway by attack at the

metal atom. It may be speculated that the carbonyl carbon

is the preferred site of attack for charged ligands, whereas

44

uncharged ligands prefer attack at the metal atom. The

site of halide attack should test this hypothesis as

well as provide additional information in the continuing

investigation of the factors that determine rates and

mechanism of reactions of metal carbonyls.

Materials

The tetrabutylammonium halides (X=Cl,Br,I) were

obtained from Eastman Organic Chemicals (Eastman Kodak

Company), and were twice recrystallized from chlorobenzene

and washed with hexane and ether. The chloride salt

retained solvent of crystallization which was removed by

melting the salt under vacuum (0.05 torr) and heating it

at 100°C for four hours. The halides were stored in dark

bottles in a desiccator since the salts were seen to

absorb moisture. The hexacarbonyls were obtained from

Pressure Chemical Company [Cr(C0)g and W(C0)g] and Climax

Molybdenum Company [Mo(C0)g] and were purified by subli-

mation under high vaccum at 4 0°C for Mo(C0)g and W(C0)g

and at room temperature for Cr(C0)g. Chlorobenzene was

distilled twice from P2^5 an<^ scored over Linde 4A

molecular sieves.

Determination of Reaction Rates

Reaction rates were determined in much the same way

as the reactions with the amine ligands (Chapter I).

45

Less rigorous precautions had to be taken since these

reactions were less sensitive to impurities or either

the reagents were easier to purify. An oil bath was

used instead of a water bath because of the higher

temperatures necessary in these investigations. The

equilibrated solvent-ligand mixture was not injected

into the equilibrated flask containing the substrate

(ca. 2X10"3M of W(CO)c and 1X10-3M for Mo(CO)c and

6 6

Cr(CO)g) but was added under nitrogen purge. The

reaction vessel was sealed with a teflon stopcock

fitted with a rubber septum; the stopcock was used to

avoid contact of the rubber with the solution and safe-

guard the solution from possible leaks in the septum.

The reaction solution was re-equilibrated for about

one half-life. The samples were then withdrawn (after

addition of an equal volume of nitrogen) and iniected

into the cells (NaCl; 1mm spacers) for determinations.

For the faster reactions the samples were with-

drawn and injected into evacuated test tubes sealed

with rubber septa and stored at lower temperatures

(ca. 10°C) for later absorbance determinations. The

reactions of Cr(CO)g were monitored employing gas-

tight syringes (Hamilton Company) as reaction vessels

(26) to eliminate the gas phase and thus to prevent

sublimation of the substrate on the cooler parts of

the reaction vessel during the course of the reaction.

46

Several runs at 120.00°C and low Cr(C0)~ concentration b

employing the conventional reaction vessels demonstrated

that evolved CO trapped in the gas-tight syringe did

not affect the rate of the reaction. The reaction

products were seen not to absorb significantly at the

wavelength monitored and therefore solvent-halide

absorbance readings were used instead of Aro

values. The plots of vs* time were linear

from 1.5-3 half-lives depending on the system studied

and the concentration of the halide employed. A

logarithmic least-squares computer program was used for

determination of values, and a linear least-squares

program was used to determine values of k- and k£ • The

cited limits of error (uncertainty of last digit(s))

are one standard deviation.

Identification of Reaction Products

The substrate-halide combinations studied kinetically

were allowed to react in chlorobenzene under conditions

similar to those employed in the kinetic runs. A comparison

of the carbonyl stretching spectra of these solutions

(after reaction completion) with the previously reported

spectra of M(CO)^X~ complexes indicated these anions to be

the reaction products. The previously reported spectra

were obtained from the complexes in KBr discs (60). The

spectrum of Mo(C0)^I~ was obtained from the complex in

dichloromethane solvent (61).

47

Results

The rate data are presented, in Table V. Plots of

tc , , vs. EBu,,NBrH for the reaction of the bromide ion with ' obsd 4

CrCCOK and MoCCOl are shown in Fig. 7, Figure 8 shows 6 6

plots of ^ o b s d vs. iBu^NBrl for the reaction of the bromide

ion with WCCO) at three different temperatures, The 6

computer calculated firsts and second-order rate constants

are given in Table VT as are the activation parameters for

the reaction of the bromide ion with WCCQl.g,

Discussion

The results support the rate law,

•-dlMCCQlgil . r _ —2- = kotMCC0Ic3lBu]iNXj , (131

dt ' 1

for the reactions of the halide ions with MoGCQlg and WCCOlg

while a two-term rate law,

• _ d [ C^ C Q ) 6-. = k1CCrCC0l63 + k^CrCCOlgJlBu^X.] , (14)

is. observed for their reactions with GrCCOl^, The. first-

order term in Eqn. 14 may well indicate the rate—determining

dissociation of CO to form iCrCCOlg'U as was discussed earlier,

There is a possibility of several geometries for a five-

coordinated complex.

48

TABLE V

RATES OF REACTION OF METAL HEXACARBONYLS WITH TETRABUTYLAMMONIUM HALIDES IN CHLOROBENZENE

AT VARIOUS TEMPERATURES

Metal Halide T, °C [X~], M lO^xk^^, sec 1

Cr Br 120.00(5) 0.0300 1.30(5)

0.0600 2.57(9)

0.0850 2.99(3)

0.1100 3. 78(4-)

0.1403 4.20(3)

0.1503 4.87(5)

0.1700 5.82(10)

0.1700 5.89(4)

0.2000 6.55(6)

0.2300 6.89(13)

Cr I 125.00(5) 0.0201 1.60(1)

0.0503 2.29(2)

0.0903 3.08(4)

0.1304 4.16(4)

0.1614 4.92(4)

0.2091 5.62(5)

Mo Br 55.00(5) 0.00985 0.444(3)

0.0202 0.869(7)

0.0303 0.974(13)

TABLE V—Continued

49

Metal Halide T, °C [X-], M 10 xkQbsd, sec'

W CI 95 . 00(5 )

W Br 100.00(5)

0.0400 1.57(1)

0.0800 2.73(2)

0.1200 4.03(5)

0.1600 5.33(6)

0.2000* 6.44(8)

0.2400* 7.41(6)

0.2400* 7.41(3)

0.2 8 00* 9.15(1)

0.0314 1.49(2)

0.0622 2.82(3)

0.0816 3.39(2)

0.1166 4.67(4)

0.1388 6.15(7)

0.1543 6.53(5)

0.0400 0.920(23)

0.0800 1.65(5)

0.1024 2.06(2)

0.1100 2.20(5)

0.1400 2.75(4)

0.1700 3.51(3)

0.1900 3.81(2)

0.2100 4.32(3)

TABLE V — C o n t i n u e d

50

Metal H a l i d e T , [X~]> M 1 0 x k0 b s d ' s e c - 1

1 1 0 . 0 0 ( 5 )

1 2 0 . 0 0 ( 5 )

W 1 2 0 . 0 0 ( 5 )

0 . 0 2 8 9 1 . 4 8 ( 2 )

0 . 0 5 0 0 2 . 6 7 ( 3 )

0 . 0 6 7 2 3 . 3 9 ( 3 )

0 . 1 0 1 8 5 . 1 9 ( 3 )

0 . 1 1 5 0 6 . 3 9 ( 5 )

0 . 1 3 0 0 6 . 9 7 ( 3 )

0 . 1 8 0 0 9 . 0 0 ( 1 4 )

0 . 2 4 0 0 1 2 . 2 ( 1 )

0 . 3 0 0 0 1 5 . 5 ( 2 )

0 . 0 2 4 1 3 . 3 0 ( 3 )

0 . 0 6 2 5 8 . 2 8 ( 4 )

0 . 0 9 8 7 1 3 . 3 ( 1 )

0 . 1 3 0 0 1 7 . 2 ( 1 )

0 . 1 5 0 0 1 9 . 5 ( 1 )

0 . 1 7 0 0 2 2 . 6 ( 2 )

0 . 1 7 0 0 2 2 . 7 ( 1 )

0 . 2 0 0 0 * 2 4 . 4 ( 1 )

0 . 2 3 5 2 * 2 7 . 6 ( 6 )

0 . 24-00* 2 7 . 4 ( 2 )

0 . 2 7 0 0 * 3 1 . 6 ( 3 )

0 . 3 0 0 0 * 3 6 . 2 ( 3 )

0 . 0 2 0 0 0 . 6 8 8 ( 1 8 )

5 1

TABLE V—Continued

Metal Halide T, °C [X~], M l O ^ x k ^ ^ , sec 1

0 . 0 4 0 0 1 . 4 3 ( 2 )

0 . 0 6 0 0 2 . 0 0 ( 3 )

0 . 0 8 0 0 2 . 4 1 ( 1 )

0 . 1 0 0 3 3 . 4 3 ( 3 )

0 . 1 2 0 0 3 . 8 5 ( 6 )

0 . 1 3 0 0 4 . 2 5 ( 3 )

0 . 1 6 0 0 * 4 . 9 9 ( 6 )

0 . 1 7 0 0 * 5 . 1 6 ( 8 )

0 . 2 0 0 0 * 6 . 0 1 ( 4 )

0 . 2 2 0 0 * 6 . 4 9 ( 9 )

0 . 2 4 0 0 * 6 . 8 3 ( 1 0 )

*Not employed in determination of rate constants, see text

52

a)

If) 6 ' "

b) O 6

M , m Fig. 7--Plots of k0bsd

vs• [Br]"" for the reaction of a) molybdenum hexacarbonyl and b) chromium hexacarbonyl with tetrabutylammonium bromide in chlorobenzene.

5 3

0.1 f. =, 0 - 2 ( B r ^ M

Fig. 8--Plots of k Q b s d vs. [Br]- for the reaction^of

tungsten hexacarbonyl with tetrabutylammonium bromide in chlorobenzene at three temperatures.

54

TABLE 71

CALCULATED RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE REACTION OF THE GROUP VI'r-B METAL HEXACARBQNYLS

WITH TETRABUTYLAMMONIUM HALTDES

Metal Halide

coc5

kl X 1 q 4

CseG"1) -

k x 104 2 Cmol"1, sec-1).

Mo Br 55.00 0.15(6) 32.4(7)

Cr Br 120.00 0.60(22) 28.9(16)

Cr I 125.00 1.19(13) 22.0(9)

W CI 95.00 0.14(22) 41.4(20)

W"5': Br* " 100.00 0.05(6) 20.0(5)

110.00 0.10(15) 51.0(9)

120.00 0.09(20) 131.9(16)

w I 120.00 0.07(11) 32.0(13)

* ±0.05°C

** AH* = 26.5(4) kcal/raole; AS* = -0 .3(10) cal/deg'mole

55

It has been shown theoretically that these penta-

coordinated Group VI-B metal carbonyl complexes should

exhibit square pyramidal, rather than trigonal bipyramidal

geometry (64,49). Substantial evidence indicates a square

pyramidal geometry for the [M(CO)5] complexes both in solu-

tion (50) and in inert matrices (65-69).

The first-order rate constants were estimated to be some

fifty per cent higher than the corresponding values for the

reaction of Cr(C0)„ with the Lewis base ligand, tributyl-b

phosphine, in decalin solvent as determined through extra-

polation of the Arrhenius plot of their data at three

temperatures. Substitution reactions of the Group VI-B

hexacarbonyls are usually relatively insensitive to the

nature of the solvent employed since the ligands are usually

neutral and the metal carbonyls non-ionic. The reaction of Mo(CO),. (dipy) with P(0CH,_) CCH in various solvents

^ l 3 3

showed the solvents to have a relatively minor role in

affecting the reaction rates (70). The reaction of Ni(CO)^

and P(CgHg)g in a wide variety of solvents also showed

very little effect of the dielectric constant of the solvent

on the reaction rate (71). Chlorobenzene is more polar than

the decalin solvent used for the reaction with the Lewis

bases, and therefore, a greater solvent interaction would be

expected in the transition state, lowering the activation

energy. For the reaction of Ni(CO)^ and P (CgH,-) a similar

increase in rate of CO dissociation was noted in chlorobenzene

as compared to n-heptane (71).

56

The reactions of all three metal carbonyls with each

halide show a halide-dependent term which can be contrasted

to the results of XMn(C0)g reacting with X". For the latter

reactions the strongly-labilizing halide substituent makes

the path involving rate-determining dissociation of CO more

accessible. Consequently, these reactions proceed under

much milder conditions (ca. 30°C) than do reactions of M(CO)g

with X" (62, 72). This increased lability is also noted for

the Group VI-B halopentacarbonyls in their reactions with

phosphines (73). These reactions proceed by initial loss of

CO at 20-30°C as compared to 100-150°C for reactions of un-

substituted hexacarbonyls with phosphine ligands (26).

If it is assumed that rate of CO dissociation is

determined by the M-CO bond strengths, iT-bonding arguments

would not explain the above results. From ir-bonding argu-

ments, replacement of a carbonyl group with a ligand in-

capable of n-acceptance (such as a halide ion) would be

expected to strengthen the metal to CO bond. The inability

of a halide ligand to accept charge from the d orbitals

would allow more charge to be accepted by the ir* orbitals of

the remaining CO ligands. However, the facilitation of CO

displacement can be explained in terms of the strength of

the M-CO a-bond. The replacement of CO with a poorer it-

acceptor ligand would increase the charge on the metal atom

causing it to act as a poorer election acceptor for the CO

57

e>

Fig. 9--Di.rect ligand to ligand donation effect proposed by Fenske and DeKock.

groups. It is also possible that a stabilization of activated

complex by the halides determines a faster rate of CO dis-

sociation with the bond strengths playing a minor role (8).

Dobson (46) has stated recently that both transition state

stabilization and ground-state bonding energies may be im-

portant in influencing reactivity.

58

Another bonding mechanism believed to be operative in

XMnCCO)^ complexes is the "direct donation" mechanism pro-

posed by Fenske et al. (2). This donation (illustrated in

Figure 9) involves interaction of the sigma bonding Pg

orbital of the halogen with the it* orbitals of equatorial

carbonyls. This overlap of orbitals allows for direct

transfer of charge from the halogen to the carbonyls in-

stead of indirect transfer through the d^ orbitals. The

amount of direct donation increases with the halogen Cl<Br<I.

The recent theoretical work by Hall and Fenske (74) has in-

dicated that direct donation may also be possible in halogen

substituted chromium and iron carbonyl complexes and the

degree of direct donation increases Cr (CO)gX<:Mn(CO) ,-X< ,

Fe(C0)5X+.

For several systems studied (of W(CO)g with Br and I

at 120.00°C and Mo(C0)r with Br") there were deviations from b

linearity in plots of ^ohsd vs- [X-] at high ligand concen-

trations (Figs. 7 and 8). These data, which are marked in

Table V with asterisks, were not employed in the determina-

tion of the rate constants. There seems to be no consistency

from system to system, the only correlation being that the

deviations always occurred at high halide concentration.

Therefore there is no evident explanation for the behavior.

The calculated entropy of activation (Table VI) for the

reaction of tungsten hexacarbonyl with tetrabutylammonium

59

bromide is zero within experimental error. This might be ex-

pected since the chlorobenzene is a relatively non-polar

solvent in which the tetrabutylammonium halide would be ex-

pected to exist as a contact ion-pair. Substrate-halide

bond formation would be expected to occur in conjunction with

the ion-pair separation, as is indicated in Figure 10. The

6

Fig. 10--Metal-halide bond formation synchronous with ion-pair separation.

60

concomitant bond formation and bond breaking should lead to

a AS* value of about zero.

It can be seen from Table VI that the relative rates as

a function of the metal atom (Mo>W) are similar to those for

systems believed to involve attack at the metal atom but are

opposite to results for systems involving attack at the car-

bonyl carbon (W>Mo). Steric considerations could not explain

this discrepancy since these metal atoms have essentially the

same covalent radii (15). Attack of the halide at the metal

atom is, therefore, favored for the Mo and W systems.

The Cr(C0)g relative reactivity is, however, substantial-

ly less than would be expected for metal attack based on

kinetic data for attack of ligands such as the phosphines

(26). This might be expected since the covalent radius of

Cr is less than for Mo and W. It would therefore be less

likely to form an activated complex of higher coordination

number from steric considerations. The first row octahedral

metal carbonyls have been noted to react less by an associative

pathway than the second and third row complexes because of

steric influences (7G, 75). The halide ions would be ex-

pected to pose greater steric demands than the phosphines

since the non-polar covalent radius of P is 1.10A0 and the

apparent ionic radii of CI , Br , and I are 1.806, 1.951,

and 2.168A°, respectively (76). However, a comparison of

the steric requirements of the phosphorus donor ligands and

61

the halide ligands, employing a method devised by Tolman

(77), seems to indicate comparable steric demands. Tolman

has ranked the various phosphine and phosphite ligands

according to their steric requirements in zerovalent nickel

complexes, NiL^, where L is a phosphite or phosphine ligand.

The ligands were ranked according to cone angles, or the

angle of a cone, centered on the metal, which encloses the

van der Waals radii of the outermost atoms of the ligand.

The angles were very large (>100°), and Table VII shows the

cone angles of the phosphorus donor ligands employed in

reactions with the hexacarbonyls (26).

TABLE VII

STERIC REQUIREMENTS OF VARIOUS PHOSPHORUS LIGANDS

Ligand Minimum Cone Angle deg

P(OCH2)3CCH3 101

pcoc 2h5) 3 109

P(0CgHg)3 121

p < w 3 130

p < w 3 145

Group VI-B metal hexacarbonyls should be more sensitive

to the steric nature of the ligands than nickel complexes

62

because of the presence of six ligands instead of four.

Approximating the bond distances for Ni-X as the sum of

the metallic radii of Hi (78) and the apparent ionic radii

of X (76) and calculating the cone angles for the halides,

using their van der Waals radii (79), shows their cone

angles to be 75°, 78° and 81° for Cl~, Br~ and I~, respec-

tively. It can be seen that steric requirements, predicted

from Tolman cone angles, are actually less for the halides.

However, comparison of the relative molecular geometries of

the halides and the phosphorus donor ligands shows that

the electron cloud of the halides would be much closer to

the center of the metal atom than the substituents on phos-

phorus. These groups on phosphorus may be far enough out

from the center of the octahedral complexes not to be

significantly repelled by the relatively non-bulky CO

ligands. Their steric requirements may become influential

only when a bulkier ligand, such as another phosphine or

phosphite ligand, is coordinated to the metal. This steric

effect may be the explanation to the observation that di-

substituted products are obtained when Group VI-B metal

carbonyls are reacted with P(CgH5)3, whereas trisubstituted

products are obtained with P(OCgH5)3 and tetrasubstituted

products are obtained with P(OCH3)3 (45).

If attack on Cr(CO)g occurs at the metal atom, then

there should be a greater discrimination in rate between Br"

63

and I- for Cr(CO)g than W(C0)g. However, from Table VI it

can be seen that W(C0)c rather than Cr(CO),, is more sensi-b 6

tive to the size of the ligand. Steric effects may, there-

fore, cause the halides to attack Cr(C0)g at the carbonyl

carbon instead of at the metal atom, as was observed for

reactions with Mo(CO)r and W(CO)„. 6 6

The rates of reaction were seen to vary among the

halides, CI >Br >1 , with a given metal atom. In a low

polarity solvent the reactivities of the halide should paral-

lel substrate-halide bond strengths from electronic con-

siderations. However, this information is not available.

The reactions of cis-[PR3M(CO)1|X]~ (M=Cr,Mo,W) with PRg

yields (PR^JgMCCO)^, and the kinetic investigation showed

the reactions to proceed by a dissociative mechanism involving

breaking of the metal-halogen bond, and the rates increased

in the order, Cl<Br<I, (6 8) possibly indicating their

relative bond strengths. The exchange of halide substitu-

ents of XMn(CO)g and cis-Fe(C0)^X2 with the corresponding

radioactive halogens (a process which is governed by a

halide independent rate law) has been studied (80) but the

results seem to indicate little about the metal-halide bond

strengths. The widely divergent (and, in some instances,

highly negative) entropies of activation calculated from

the data of Hieber and Wollmann (8), as well, as lack of

trends as a function of X from metal to metal would lend a low

64

credence to their data. The reaction of XM(CO)~ (X=Cl,Br,I;

M=Cr,Mo,W) with CO in chloroform at room temperature was

seen to involve breaking of the M-X bond to give the hexa-

carbonyl, and the rate constants were seen to increase

Cl<Br<I with all three metal atoms (81) possibly indicating

the relative strengths of the M-X bonds. The order of

reactivity is consistent with the steric demands of the

halides. The steric properties are the only parameters

about which enough information is presently known to attempt

to explain the trends.

In conclusion, the reactions of the hexacarbonyls with

tetrabutylammonium halides were seen to proceed at rates

which were dependent upon the concentration of the halides.

The reaction of the chromium hexacarbonyl with the halides

also showed a term in the rate law independent of the halide

concentration. These results suggest mechanisms involving

an almost exclusive associative pathway; this pathway was

exclusive for the reactions of Mo(C0)r and W(C0)„ with the b 6

halides at the temperatures investigated. These results

can be contrasted to the results of the reactions of the

hexacarbonyls with Lewis base ligands (discussed in Chapter

I). The rate constants for the associative and dissociative

components were of comparable magnitude for the reactions

with these ligands, whereas the rates by the associative

pathway are much larger for reactions with halides than with

65

Lewis bases. The rate of reaction of W(CO)„ with tetra-D

(n—butyl)ammonium bromide was some three orders of magnitude

faster than its reaction with tri(n-butyl)phosphine (26).

These large differences in rates for the associative path-

ways may indicate the reactions of the hexacarbonyls with

these different nucleophiles to proceed by different

mechanisms. The reactions with the Lewis bases may proceed

by the Id mechanism, while reactions with the halides occur

through the "associative" process.

The site of ligand attack was inferred to be at the

metal atom in reactions of the halide ions with Mo(CO)_ and 6

W(CO)g. This position of attack was also observed for

reactions of the hexacarbonyls with Lewis bases (Chapter I).

Steric demands probably precluded attack of the halide ions

at the metal atom of Cr(C0)g; attack instead occurred at the

carbonyl carbon. Another system where initial attack at the

metal atom is indicated is the oxidative elimination reaction

Fe(CO)5 + HgCl2 2s» Fe(C0)4(HgCl)2 + C0C12 . (15)

This reaction is also important from other considerations

and will be discussed in detail in Chapter III.

CHAPTER III

STUDIES OF THE KINETICS AND MECHANISM OF THE

REACTION OF IRON PENTACARBONYL WITH

MERCURIC CHLORIDE IN ACETONE

Introduction

Although the first two chapters have dealt with the

reactions of metal carbonyl complexes with nucleophilic

ligands there is a considerable amount of information in the

literature about the reactions of metal carbonyls with elec-

trophilic reagents. The predominant reaction of metal car-

bonyls with nucleophiles is the nucleophilic substitution

reaction, whereas the main reaction with electrophiles is

the oxidative elimination reaction. Oxidative elimination

reactions of metal carbonyls with electrophilic reagents are

characterized by expulsion of one carbonyl ligand and its

replacement by two univalent ligands obtained from the elec-

trophilic species, as illustrated by the general process,

(CO)nM + TX X C O ^ M C j S + CO. (16)

The product could draw both univalent ligands from one mole

of the electrophile, such as (bipy)(CO)3XMoMX3 (M=Sn,Ge;

X=Cl,Br,I; Bipy=2,2-bipyridyl) (82), or from two moles of

ligand for each mole of substrate such as (bipy)W(CO)3(HgCl)2

66

67

(83) or X2M(CO)i+ (M=Fe,Ru,Os; X=C1, Br, I ,HgCl ,HgBr ,HgI) (84).

Oxidative elimination reactions have proven very useful in

the production of novel seven-coordinated metal carbonyl

derivatives (10).

An oxidative elimination reaction was observed as early

as 1928 for the reaction of iron pentacarbonyl with halogen

molecules (85). Since that time oxidative elimination

reactions have been performed with iron carbonyl derivatives

(83,86,87), Group VI-B carbonyl compounds (82,88), the

square planar iridium (I) complex, Ir (CO) (P<j>3) 2C1 (89), and

ruthenium and osmium complexes (84). The electrophilic

species may be a halogen molecule (85), a pseudo-halogen

such as NCX (90), or compounds which lead to the formation

of metal-metal bonds when reacted with metal carbonyl com-

plexes. Electrophilic species which have led to metal—metal

bonds with metal carbonyls are mercuric halides (83) and

Group IV-A halides and their organic derivatives (82).

The kinetic investigation of the reaction of

(5-X-phen)Mo(CO)^ (X=H,CH3,C1 and N02 substituted in the

5 position) with mercuric chloride (91) led to the proposal

of a mechanism (Equation 17) involving the formation of one-

to-one and one-to-two "adducts" of substrate and ligand as

intermediates.

68

kl (5-X-phen)Mo(CO)^ + HgCl2 (5-X-phen)Mo(CO)4 • HgClg

(5-X-phen)Mo(CO)^ * HgCl2 + HgCl2 — ^ — >

(5-X-phen)Mo(CO)4 • 2HgCl2 f a s t — > (17)

(5-X-phen)Mo(CO)^(CI)HgCl + CO + HgCl2

The kinetic studies of the reaction of trigonal bipyramidal

iron pentacarbonyl with iodine (6) to give cis-Fe(CO),I„

indicated a mechanism which involved the successive forma-

tion of 1:1 and 1:2 adducts, with each adduct ultimately

yielding the observed product through loss of CO. The

kinetics of the reactions of (bipy)W(CO)^ with tin tetra-

chloride and its organic derivatives to give capped octa-

hedral derivatives, and (phen)Mo(C0)4 (phen=l,10-phenanthro-

line) with Gel^ (92) to give a capped trigonal prismatic

product also indicated formation of these 1:1 and 1:2 inter-

mediates. These adducts were also indicated as intermediates

in the kinetic studies of the reactions of M(CO)4L2 (M=Mo, W;

L2=bipy, phen) with HgX2 (X=C1, Br) (93). The general

mechanism for oxidative elimination reactions may therefore

be similar to the proposed mechanism in Equation 17.

In one of these studies, the reaction of M(C0)4L2

(M-Mo, W; L2=bipy, phen) with HgX2 (X=C1, Br), an inter-

mediate, believed to be M(CO)^L2 • 2HgX2, was observed (93).

The formation of 1:1 and 1:2 adducts as intermediates also

seems likely since the adducts have been observed as products

with the proper choice of substrate. For example, the

6 9

reaction of Fe(C0)g with HgCl2 leads to the product

Fe(C0)1+(HgCl)2 (84), whereas the reaction of HgCl2 with

Fe(CO)3(PPhg)2 leads to Fe(CO)3(PPhg)2 * HgCl2 (94). Where

triphenyl phosphite is the coordinating ligand instead of

triphenylphosphine, the 1:2 adduct, Fe(CO)3[(PhO)3P]2 •

2HgCl2, is observed (95). Also, the reaction of HgC^ with

(diphos)W(CO)^ (diphos=bis(1,2-diphenylphosphino)ethane)

gave (diphos)W(CO)I+ " HgCl2 and with (diphos^W^O^ gave

(diphos)2W(CO)2 * 2HgCl2 (88).

The question as to the nature of the products and inter-

mediates of oxidative elimination reactions has been investi-

gated since it is possible that they may be either electrolytes,

neutral with both ligands bonded to the metal atom, or have

a ligand coordinated to a carbonyl group. The product

Fe(CO)^(HgX)2 has been determined by infrared analysis (84,

86, 96) to possess an octahedral arrangement of ligands with

two HgCl ligands cis to one another. The X-ray diffraction

work of Baird and Dahl (97) for Fe(CO)^(HgBr)^ supported

this conclusion. The structures of W(C0)3(bipy)(HgCl)2 (83)

and W(CO)3(bipy)I2 (94) were determined to be seven-

coordinated by infrared analysis and non-ionic from con-

ductance measurements. The (bidenate)(CO)gXMNXg(M=Mo,W;N=

Ge,Sn) and (CO)^XFeMX3 (M=Ge,Sn) complexes (87) were found

to be non-electrolytes (82) and were believed to involve one

metal-Group IV-A metal bond and a metal-halide bond. The

reactions of (bipy)M(CO)4 (M=Mo,W) with HgX2 (X=Cl,Br) gave

70

the products, (bipy)M(CO)g(HgX2), when reacted under dif-

ferent conditions (88) than was used to obtain the

(kipy)W(C0)3(HgCl)2 product (83). The (bipy)W(CO)3(HgX2)

products were believed to be seven-coordinated complexes

just as the (bipy)W(CO)3(HgCl)2 Pr0(3uct was (88), suggesting

the presence of a metal-halogen bond as well as a metal-

metal bond in these complexes. The investigation of the

closely related complexes, (5-X-phen)Mo(CO)3(HgCl2), in-

dicated these products may involve a bond between the HgCl

and CI ligands with the ligands arranged in a capped octa-

hedral configuration around the metal (91). This was

proposed by comparison with the structure of '

(bipy)Mo(CO)3(CI)SnCHgClg (98). This compound was reported

to be a seven-coordinated molybdenum . complex containing a

bridging bond between the SnCH3Cl2 and CI ligands. However,

the structure of (bipy)Mo(CO)3(HgCl)(CI) was shown by an

X-ray investigation (17) to approximate a capped trigonal

prismatic orientation of ligands about the metal with the

CI ligand in the axial position and the HgCl ligand across

the equatorial plane from it.

The structures of compounds which might be considered

as intermediate in the general reactions have also been

investigated. The phosphine substituted iron complexes lead

to the 1:1 and 1:2 adducts as products (99) in their reac-

tions with mercuric halides. Infrared analysis and

71

conductance measurements indicated the 1:1 adduct,

Fe(CO)g(PPhg)2 *HgClg, to be a six-coordinated complex with

HgX2 (X=C1,Br) functioning as a monodentate ligand bonded

to the iron atom with a metal-metal bond. The 1:2 adduct,

Fe(CO)g[P(OPh)g]2 2HgX2, was believed to be a 1:1 adduct

with a molecule of mercuric halide in the lattice. The

1:1 and 1:2 adducts of Fe(CO)5 with HgCl2 were also ob-

served and isolated during the same study (99). The sub-

stituted Group VI-B complexes with mercuric halides gave a

variety of products such as LM(CO)3*nHgX2(L=

IT-C gHgMe g 51, 3 j 5—Cy Hg j M-Mo,Wj n=l or 2j X=Cl,Br). The

arene substituted metal carbonyl complexes, where n=l, were

thought to be neutral and seven-coordinated complexes; the

structure of the 1:2 adduct was unknown (88). However, the

recent X-ray diffractometer investigation (10 0) of

^6^3^e3 *-"0) 3M0 (HgCl2 ) 2 indicated a dimeric structure with

the two tricarbonyl-mesitylenemolybdenum groups bonded to a

mercury atom of a central four-membered E ^ C ^ ring. All

mercury atoms were four coordinated, and the mercury atoms

not bonded to molybdenum are halogen-bridged to the four

membered ring. This bridgework structure is shown in Figure

19.

Problem

Most of the oxidative elimination reactions discussed

may be described as "external" processes, in that the

72

attacking electrophile is the reduced species. An "intsrndl"

process, where a group that is originally coordinated to the

metal is the species reduced, was reported as early as 1928

for the reactions of iron pentacarbonyl with mercuric halides

(101);

Fe(CO)5 + 2HgX2 » cis-Fe(CO)^(HgX)2 + C0X2 . (18)

X=Cl,Br,I

An internal reduction" reaction was also observed involving

the nitrosyl group, NO, coordinated to a cobalt complex (102);

Co(diars )2(NO) + NCS~ * Co(diars)2(NO) (NCS)+ . (19)

(diars=o-phenylenebis(dimethylarsine))

X-ray crystallographic studies have shown that the reactant

and product contain coordinated nitrosyls (10 3) which may be

considered as N0+ and NO , respectively, on the basis of the

N—Is photoelectron spectrum (104) and electronic spectrum

(102).

Internal reduction processes are of increased im-

portance in light of a relatively new development in biologi-

cal chemistry. Recently Chatt (105,106) proposed a chemical

model for the reduction of dinitrogen to diazene (N2H2):

trans-(dppe)2M(N2)2 + 2HX (dppe) 2M(X) 2 (N2H2 ) + N2 . (20)

(dppe=l,2-bis(diphenylphosphino)ethane; M=Mo,X=Br;M=W,X=C1,Br)

73

This system is a chemical model for a proposed intermediate

step in the reduction of dinitrogen to ammonia by the

nitrogenase enzyme. Reactions 18, 19 and 20 are analogous

in that the coordinated ligands which are reduced are iso-

electronic and all reactions are thought to involve two

electron processes. The stochiometry of reactions 18 and

20 also suggests that the mechanism proposed for external

reduction reactions (formation of 1:1 and 1:2 adducts as in

Equation 17) may be possible for these reactions also.

Equation 2 0 is a reasonable chemical model for the

reduction of dinitrogen to ammonia since the only definite

reaction of molecular nitrogen known to occur at ordinary

pressures and temperatures is the formation of dinitrogen

complexes from nitrogen gas (107). Also, two molybdenum

atoms in the Mo-Fe protein of nitrogenase are believed to

provide the sites where dinitrogen is bound prior to its

reduction to ammonia (107). However, it is believed that

these two atoms react independently since it is just as

likely for dinitrogen to be reduced as a mono-metal as a

bi-metal site (106). The two electron reduction of N2 to

diazene is thermodynamically endothermie (48.7 kcal/mole)

(108), but since Chatt's reduction occurs at ordinary

temperatures this value for free diazene formation is

markedly reduced by coordination of these intermediates to

metals. An excellent review article on the relatively

74

new field of dinitrogen reactions has been written by

Chatt (109).

Kinetic studies of internal reduction reactions have

not been undertaken thus far. The kinetic study of iron

pentacarbonyl with mercuric halides (Equation 18) was under-

taken in order to determine if the mechanism for internal

oxidative elimination reactions are similar to external

oxidative elimination reactions. The results can be ex-

tended to the analogous reaction involving the reduction of

dinitrogen to diazene (Equation 20). The results should

also give more information into the factors which determine

the site of ligand attack at a metal carbonyl complex.

Materials

The rate of the reaction of iron pentacarbonyl is very

dependent upon its purity (8). Thus, Fe(C0)g (Apache

Chemical Company, Rockford, Illinois) was rigorously puri-

fied by trap-to-trap distillation under high vacuum (five

times) and was stored under nitrogen, in the cold, in a

foil-wrapped container. Mercuric chloride was obtained

from Fisher Scientific Company and was twice recrystallized

from absolute ethanol, dried, and stored in a brown jar in

a desiccator. HgCl2 absorbs moisture and occasionally had

to be redried under high vacuum. Reagent grade acetone

(Fisher Scientific company) was purified by distilling from

anhydrous potassium carbonate (110), redistilling from

75

anhydrous potassium permanganate (111), drying over Linde

4A molecular sieves and redistilling three additional times

onto the sieves (112). The acetone was then saturated with

dry sodium iodide and was decanted and cooled to -15°C.

The resulting crystals of Nal'SC^HgO were recovered by

filtration and were warmed to about 30°C. The resulting

acetone was then fractionally distilled onto molecular

sieves with rejection of the first and last portions (113).

Reaction rates employing this highly purified solvent were

found not to differ significantly from the rates employing

spectrophotometric grade acetone (Fisher Scientific Company)

The latter was therefore employed in the majority of kinetic

runs.

Determination of Reaction Rates

The reaction rates were determined under pseudo—first-

order conditions (at least a 30-fold excess of mercuric

chloride) through monitoring the decrease in intensity of

the A2 carbonyl stretching band of iron pentacarbonyl at

1995 cm (114) on a Perkin-Elmer 6 21 grating infrared

spectrophotometer.

Solutions of HgC^ Fe(CO)g in acetone were found

to react with the sodium chloride windows and metal parts

of the observation cells, and thus it was necessary to

construct special cells which eliminated contact of the

solution with NaCl and all metals. These cells (Figure 11)

7 6

r*

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7 7

employed Irtran II5'5 windows (Barnes Engineering Co.), 1 mm

Teflon*5'4 spacers, and Kel-F*** hubs (Hamilton) on the front

plate. Due to the relative incompressibility of Teflon it

was necessary to place Parafilma sheets on each side of the

spacers to insure a leak-proof seal. The hubs were con-

structed so as to form a tight seal with the parafilm gasket.

To avoid the use of metal needles for sampling, a modified

reaction flask (Figure 12) was employed. Teflon tubing

(.022 inch i.d.; Hamilton), of sufficient length to reach

the bottom of a 2 5 ml volumetric flask, was inserted through

a rubber septum. The outside end of the tubing contained a

Kel-F Luer-lox hub to which a Delrin*3 valve (Ace Glass; 1

mm bore) was attached. An all-glass syringe (2 ml; American

Hospital Supply) was then employed for sample removal.

The following procedure was employed to prepare reaction

solutions. Mercuric chloride was weighed into a 25 ml

volumetric flask under a nitrogen atmosphere. About 2 0 ml

of acetone was added and the flask was then sealed as shown

in Figure 12. The reaction vessel was placed in a bath with

a Haake Model ED circulator. After it attained the reaction

^Registered trade mark Eastman Kodak Company.

sV&T.m. E.I. DuPont DeNemours and Company.

*'**T.M. Minnesota Mining and Manufacturing Company.

aRegistered trade mark Marathon Products.

^T.M. unknown.

Fig. 12--Reaction vessel which allows sample withdrawal with no metal contact.

79

temperature , the volume was brought to the mark by in-

jection of acetone through the septum employing a needle

and syringe. Iron pentacarbonyl (ca. 1.5x10 M) was then

added in the same manner, and the resulting solution was

well mixed. A volume of nitrogen equal to that of each

sample removed was added to the reaction vessel during

sampling.

Plots of ln(At-Aoo) vs. t (where A± and A^ are the

absorbances of the solution at time t and at infinite time

(essentially complete reaction of Fe(C0)5), respectively)

were non-linear indicating an unreliable t^ value. Initial

slopes in plots of In A_ vs. t were therefore used to deter-

mine the rate of disappearance of Fe(C0)g and exhibited

linearity to thirty per cent of reaction completion. The

unreliability in t^ values was found to be due to the dis-

appearance of the initial product, Fe(CO)^(HgCl)2, concurrent

with its formation.

Fe(CO)^(HgCl)2 was found to be reacting with HgCl2 to

give the final product Fe(C0)1+(Hg2Cl3)2 (vide infra). The

rate of appearance of this product was also monitored on the

IR by observing the increase in intensity of its A^2^ car-

bonyl stretching mode at 2094 cm"1. This assignment was

based on the previously assigned bands of Fe(CO)^(HgCl)2

(which apparently possesses the same symmetry (vide infra)).

The ligand was in at least a 2 0-fold excess of the substrate

80

- 3

(whose concentration was about 2x10 M). Plots of lnCA^-A^)

vs. t were linear to at least three half-lives, after an

initial induction period. The k , , and k values were calculated from a least-

obsd

squares computer program and the cited limits of error are

to one standard deviation. Values were rejected that varied

over two standard deviations.

Identification of Reaction Products

Scans of the carbonyl stretching region of the infrared

spectrum (2200-1900 cm-"*") during the course of the reaction

indicated the presence of the reaction product,

cis-Fe(CO)^(HgCDg (73). The complex was isolated as follows:

Fe(C0)g was reacted with excess HgC^ in acetone at 55°C

until precipitation occurred. The precipitate was filtered,

washed with water and n-hexane, and dried in vacuo.

Elemental analysis of the light yellow residue (Galbraith

Laboratories) indicated it to be the expected Fe(C0)^(HgCl)2

(Found: C, 7 . 4-6 ; Fe,8.79; CI,10.88. Calc. for C^Cl2FeHg20^:

C,7.51; Fe,8.72; CI,11.08%). The carbonyl stretching spectra

of this product in acetone, and as a nujol mull are given in

Table VIII. The spectra were calibrated against a water

vapor peak at 18 6 9.4 cm

In a typical kinetics run at 55°C, if the reaction is

quenched after Fe(C0)g is essentially depleted, the solvent

removed under vacuum, and the residue washed with water and

81

n-hexane, the nujol mull spectra of the residue indicated

the complex Fe(CO)^(HgCl)2 as the predominant species.

Bands arising from another substituent were also present.

If the reaction was not quenched after depletion of Fe(C0)g

but was allowed to react further (until no change in the

spectra at 2094 cm-1), or if Fe(CO)^(HgCl)2, prepared as

described above, was allowed to react further with excess

HgCl2 and a similar work-up was repeated, a second product

was obtained. Elemental analysis of this product indicated

the compound Fe(C0)1+(Hg2Cl3)2 (Found:C,4.31;.Fe,4.77; Hg,

67.81; Cl,16.70. Calc. for C^ClgFeHg^ : C, 4 . 06 ; Fe,4.72;

Hg,67.82; CI,17.98%). This compound, a brownish powder,

was stable in the air for long periods of time and was in-

soluble in most solvents. It was insoluble in water but

when agitated therein for one hour showed partial reversion

to the complex Fe(CO)4(HgCl)2. The infrared spectra (in

the carbonyl stretching region) for Fe(CO)1|(HgCl-HgCl2 )2 in

acetone and as a nujol mull are given in Table VIII, and

the spectrum of this complex and Fe(C0)1+(HgCl)2 (both as

nujol mulls) are reproduced in Figure 13.

Electrolytic properties of both products were studied

employing a Beckman Model RC 16B2 Conductivity Bridge with

a dip-type platinum electrode. For 10~ M solutions in

nitrobenzene, the conductivities (at 24°C) of Fe(CO)^(HgCl)2

and Fe(CO)^(HgCl*HgCl2)2 were found to be 6x10 ^mhos/cm

8 2

-1 P 2 p O , 2 0 p p | C n r r ' , 21.00 1900

Fig. 13--Carbonyl stretching spectrum of a) Fe(CO)^(HgCl) and b) Fe (CO) (H^Clg ) obtained from nujol mulls.

8 3

TABLE VIII

CARBONYL STRETCHING FREQUENCIES OF PRODUCTS OBTAINED FROM THE REACTION OF IRON PENTACARBONYL

WITH MERCURIC CHLORIDE

Compound Solvent v ( c m - 1 ) CO

Fe(CO)^(HgCl)2 Nujol 2095sh, 2087ms, 2032sh, 2008s, 1980sh

Acetone 2089ms, 2075s, 2046sh, 2015vs, 19 95sh

Fe(CO)4(Hg2Cl3)2 Nujol 2109ms, 2077ms, 2050s, 2040s, 2 015 sh

Acetone 2090m, 2076sh, 2020s, 2000sh

s strong, m medium, sh shoulder, v very

-4

and 2x10 mhos/cm, respectively. Thus both products are non-

electrolytes in nitrobenzene.

Although the presence of phosgene gas, C0C12 , as a

reaction product was not determined directly, the following

evidence indicates its formation as has previously been

reported (101). The effluent gas from the reaction of

Fe(C0)5 and HgCl2 in acetone was bubbled into distilled

water. The acidity of the solution was found to increase

over the course of the reaction, and the presence of Cl" in

the solution was confirmed through addition of silver nitrate.

The results are consistent with formation of HC1 through

hydrolysis of C0C12 (101).

(21) C0C12 + H20 C02 + 2HC1

Results

Rate data (k , , values) for the reaction of iron obsd

pentacarbonyl with mercuric chloride, obtained by observing

the disappearance of Fe(CO>5 (1995 cm-1), are given in

Table IX, and the data obtained by monitoring the appearance

of the product, Fe(C0)^(HgCl*HgCl2)2 (2094 cm ^) are

presented in Table X. Plots of In vs. ln[HgCl2]

show a slope equal to two indicating a squared dependence 2

upon the ligand concentration. Plots of k vs. [HgC^J

are linear with intercepts of zero within experimental error.

The k values are given in Table XI along with the activation

parameters for the investigations at the two different wave-2

lengths. The plots of vs• [HgClg] are shown in

Figures 14 and 15.

Discussion

The rate data and the identification of the reaction

products indicate the overall reaction to be,

Fe(CO)5 + 2HgCl2 > cis-Fe(CO)^(HgCl)2 + C0C12 (22)

cis-Fe(C0),[ )HgCl)2 + 2HgCl2 > cis-Fe (CO )1+ (HgCl • HgCl2 ) 2 . (23)

While cis-Fe(C0)l| (HgCl),, is a well-known compound whose

spectral and crystallographic properties have been determined

(84,97), cis-Fe(CO),(HgCl'HgCl2)2 has not previously been

observed.

TABLE IX

RATES OF REACTION OF IRON PENTACARBONYL WITH MERCURIC CHLORIDE IN ACETONE AT VARIOUS TEMPERATURES OBTAINED BY MONITORING THE DISAPPEARANCE

OF THE SUBSTRATE

T, °C [HgCl2], M lo4kobsd' S6C"

30.00(5)

35.00(5)

0.0662 0.428(21)

0.0727 0.559(23)

0.0897 0.812(18)

0.0979 0.983(9)

0.1131 1.10(1)

0.1202 1.40(4)

0.1299 1.59(5)

0.1308 1.64(5)

0.1421 1.80(4)

0.1497 2.11(5)

0.1606 2.38(5)

0.1657 2.67(7)

0.1698 2.78(2)

0.1778 2 . 84(6)

0.0775 0.655(13)

0.0894 1.10(6)

0.1000 1.18(2)

0.1099 1.48(3)

0.1191 1.85(5)

86

TABLE IX—Continued

T, °C [HgCl2], M 4 -1 10 k , ,, sec obsd'

0.1219 2.19(6)

0.1305 2.20(8)

0.134-8 2.12(6)

0.1416 2.39(5)

0.1451 2.51(3)

0.1498 3.00(2)

0.1552 2.82(1)

0.1583 3.20(12)

40.00(5) 0.054-3 0.281(5)

0.0678 0.941(68)

0.0876 1.08(5)

0.0938 1.25(11)

0.0976 1.47(5)

0.1016 1.90(7)

0.1151 2.09(4)

0.1182 2.38(12)

0.1222 2.22(5)

0.1294 3.01(20)

0.1333 3.09(8)

0.1358 2.76(5)

0.1449 3.68(15)

8 7

TABLE X

RATES OF REACTION OF IRON PENTACARBONYL WITH MERCURIC CHLORIDE IN ACETONE AT VARIOUS TEMPERATURES

OBTAINED BY MONITORING THE APPEARANCE OF THE PRODUCT

T, °C [HgCl2], M 10l+kobsd' S e C _ 1

35.00(5)

45.00(5)

0.0894 0.671(13)

0.0950 0.702(19)

0.1001 0.891(13)

0.1049 0.910(25)

0.1139 1.01(2)

0.1140 1.25(1)

0.1182 1.35(2)

0.1225 1.46(3)

0.1265 1.41(1)

0.1304 1.69(6)

0.1341 1.59(4)

0.1414 1.94(3)

0.1484 1.96(3)

0.1549 2.10(3)

0.1613 2.35(4)

0.1673 2.38(4)

0.1000 1.05(1)

0.1095 1.16(1)

0.1182 1.38(2)

88

TABLE X—Continued

T, °C [HgCl2], M 4 -1

10 k , ,, sec obsd'

5 5 . 0 0 ( 5 )

0 . 1 2 6 6 1 . 6 3 ( 2 )

0 . 1 3 4 1 1 . 7 3 ( 1 )

0 . 1 4 1 4 2 . 2 3 ( 2 )

0 . 1 4 8 3 2 . 3 0 ( 5 )

0 . 1 5 5 0 2 . 6 1 ( 5 )

0 . 1 6 1 2 2 . 3 7 ( 8 )

0 . 1 6 7 3 2 . 6 7 ( 8 )

0 . 1 7 3 3 3 . 1 9 ( 6 )

0 .1896 3 . 6 3 ( 9 )

0 .0549 0 . 3 8 7 ( 4 )

0 .0632 0 . 5 6 1 ( 9 )

0 . 0 7 0 7 0 . 5 8 4 ( 4 )

0 . 0 7 7 5 0 . 7 7 7 ( 7 )

0 . 0 8 3 6 0 . 7 6 1 ( 1 2 )

0 . 0 8 9 4 0 . 9 9 2 ( 1 3 )

0 . 0 9 4 8 0 . 9 6 6 ( 1 0 )

0 . 1 0 2 1 1 . 2 2 ( 1 )

0 . 1 0 9 5 1 . 5 2 ( 1 )

0 . 1 1 4 0 2 . 0 5 ( 2 )

0 . 1 3 2 8 2 . 0 0 ( 2 )

0 . 1 3 4 2 2 . 1 8 ( 1 )

0 . 1 4 4 8 2 . 3 9 ( 3 )

BSjUl(UBS

89

TABLE X--Continued

T, °C [HgCl,], M 1+

10 k , ,, sec o.b.sd.

-1

0 . 1 4 8 3 2 . 4 7 ( 3 )

0 . 1 5 1 6 2 . 8 4 ( 5 )

0 . 1 5 8 0 2 . 9 7 ( 1 1 )

0 . 1 6 4 3 3 . 2 2 ( 5 )

0 . 1 6 4 3 3 . 2 3 ( 5 )

0 . 1 7 0 3 3 . 4 6 ( 1 0 )

0 . 1 8 4 5 3 . 9 6 ( 6 )

90

TABLE XI

RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE REACTION OF IRON PENTACARBONYL WITH

MERCURIC CHLORIDE IN ACETONE

Disappearance of starting material

T* k- k /k x 102** aAs^*ft

(°C) (sec "'"M ) (kcal/mole) (cal/deg-mole)

30.00 0.91(2) 11.6(8) -29.7(14)

35.00 1.24(7)

40.00 1.75(11)

Appearance of product

1* k^kg/k x 102** bAH^** bAS^**

(°C) (sec "'"M ) (kcal/mole) (cal/deg'mole)

35.00 0.90(4) 2.01(2) -61.5(10)

45.00 1.01(6)

55.00 1.17(2)

*±.05°C.

**The limits of error in last digit(s) (in parentheses) are to one standard deviation.

a AH^AH^+AH^-AH^; AS* = AS.* + AS*-AS* .

bAH^ = AHj+AH^-AH^; AS^AsJ+Asjf-AS^.

91

/

CD O

0 /

' [ H g C I ^ I O 2 " M '

g; 1 4-- p i o^ s ? f kobsd vs. [.HgClo 3 for the reaction of ron pentacarbonyl with mercuric chloride in acetone obtained

by monitoring the rate of disappearance of the pentacarbonyl?

92

1 2 [HgClg (m2) x10 :

2 Fig. 15—Plots of vs. [HgCl21 for the reaction of

iron pentacarbonyl with mercuric chloride in acetone at 5 5°C obtained by observing the appearance of the product.

93

The similarity of the carbonyl stretching spectrum of

this complex and that of the well-characterized FeCCO)^(HgCl)2

(Table VIII) indicates Fe(CO)^(HgCl*HgCl2)2 to exhibit a cis

orientation of the HgCl'HgCl2 groups. The carbonyl stretch-

ing spectra of the two products in nujol mulls show an

average shift of ca. 35 cm of the carbonyl stretching

bands of Fe(CO)|+(HgCl*HgCl2)2 to higher energies. This is

consistent with electron withdrawal from Fe as HgCl2 is

complexed, presumably to the HgCl group. The complex could

have one of the two possible structures illustrated in

Figure 16. Figure 16a exhibits a possible structure contain-

ing the bridging lattice of HgCl2 groups as was found to be

1 .—CI

h 9 — CI

Fig. 16—Possible structures of Fe(CO)^(Hg2Cl^)

94

present in [fr-CgHgMe-g )Mo (CO) ^ (HgC^)^ by an X-ray crystal-

lographic study (100). However, the required distortions

of Cl-Hg-Cl bond angles from the expected tetrahedral con-

figuration about mercury would probably induce considerable

strain, and make this structure less probable than that

illustrated in Figure 16b.

Plots of ln(A^.-Aoo) vs. t for reaction of Fe(C0)g with

HgCl2 in acetone exhibit initial non-linearity, indicative

of the formation of at least two species, which are not in

a constant proportion. This is consistent with the isola-

tion of the two products, Fe(C0)^(HgCl)2 and

Fe(C0)1+(HgCl*HgCl2 • The rate of disappearance of Fe(C0)g

was therefore determined by the initial slopes method. By

this method no t^ is taken and the initial linearity (over

the first 30 per cent of the reaction) in plots of A^ vs. t

arises from the low contribution to the total absorbance at

1995 cm 1 by these two products. The subsequent linearity

of the plots measuresrate of conversion of Fe(C0)1+(HgCl)2

into Fe(CO)^(HgCl•HgCl2)2 after disappearance of Fe(CO)^.

The absorbances of these species at this wavelength were,

however, not large so the rate data was obtained at 20 94 cm"1

where absorbances from both species were large.

The rate of disappearance of Fe(C0)5 and rate of forma-

tion of Fe(C0)|+(HgCl*HgCl2 )2 were seen not to be equal

indicating buildup of Fe(C0)1+(HgCl)2. The Arrhenius plots,

9 5

shown in Figure 17, indicate that the rate of loss of sub-

strate increases with temperature faster than the rate of

formation of the final product. At approximately 28°C the

two rates should be equal and Fe(CO)^(HgCl)2 should not be

observed. This was observed as the only product from

earlier investigations of the reaction of Fe(C0)c and 0

HgCl2» probably because the reactions were in aqueous

acetone in which the reaction rate for formation of

Fe(CO)^(HgCl)2 is greatly enhanced (94). The concentrations

of Fe(CO)5 and HgCl2 were also large, and the Fe(CO)1+(HgCl)2

precipitated out and was filtered. The final product,

Fe(CO)4(Hg2Cl3)2, is much more soluble than Fe(CO)^(HgCl)2

in acetone and would not have been precipitated.

The rate data obtained from the initial slope method at

1995 cm is illustrated in Figure 14 and supports a rate law,

kobsd = W Hg c l2 ] 2 ' <21<>

so that,

d[Fe(CO)5] ^ = k[Fe(CO)5][HgCl2r . (25)

The dependence of the rate of disappearance of starting

material on the square of the ligand concentration and the

fact that the 1:1 (99,101) and 1:2 (99) adducts of Fe(C0)c

and HgCl2 have been observed indicate their presence in this

reaction. This mechanism may well be similar to the one

9 6

appearance of

prod uct

i 4.4"

disappearance of

s tar t ing mate r ia l

3.1 3.2 1/ T(°K~1)x 103

3.3

Fig. 17-—-Arrhenius plot of the rate constants of the reaction of iron pentacarbonyl with mercuric chloride in acetone at different temperatures (indicated temperatures are in °C).

97

proposed for the external reduction processes (91) for

which the same type of rate behavior has been observed.

This proposed mechanism is shown in Figure 18.

The rate law for disappearance of substrate (using the

representations shown in Figure 18 for the complexes) would

be,

- = klCS][L] - k_1[l1] . (26)

Intermediate 1^ is only observed at low temperatures (<0°C)

(99,101) and would be expected not to persist at higher

temperatures. Assuming the concentration of I does not

change with time (d[I1]/dt=0 (steady-state assumption))

simplifies Equation 2 6 to,

d[S] k-^CSHL] 2 ^ ( 2 y )

dt ~ k ^ + k^LL]

as is shown in Appendix I. In the limit of low ligand con-

centration, the relationship, k_1>>k2[L], must hold and the

rate expression would be,

d[S] k k [S][L]2

" — = k~[ ' (28'

consistent with the observed rate law (Equation 25). Higher

ligand concentrations should bring the value of k2[L] within

the range of k-1 and deviation from linearity in plots of

9 8

CM

CM

U U) I _

' i n 1 ^ "

O ~ y AT

L L

JI

R CM

0 3 D ) d .

1 +

i n

U SFL

¥ L L

0 O )

1

O J CM

i n H

o y 0

L L

H CM

CM

U CJ)

I +

CM

0 U)

1 i n

o Y

L L

CM

U

0

U +

CM

U U) 1

O y 0 )

L L

IL CO

j * :

CM

U D )

I

CM i n

o y ¥ LL

o bo K

rC "P •H

LO /""N o o

CL) •PH

4H O

O •H •P O rti CD Sh

0) rC •P

k O

Mh

s w

•H G rd rC o CD 5 n j 0) 0] O P-. 0 6 Ph 1 I

oo i—f

be •H Ph

99

r -i2

^obsd VS" would be expected. In the concentration

range employed in this investigation, this value was not

reached presumably because of the large value of k ^ as

compared to •

The intermediate I is probably a non-ionic, six

coordinated metal donor complex such as those which were obtained

from the reactions of substituted iron complexes with mercuric

halides (99,94). Its stability would be expected not to

allow its existence at temperatures this high, and indeed,

at room temperature the intermediate 1^ has been shown to

yield the intermediate, I2 (99). On the basis of conduc-

tivity and infrared studies of the analogous

Fe(CO)g[P(OPh)g^2'2HgX2 (X=Cl,Br) this intermediate was

believed to be a 1:1 adduct with a molecule of mercuric

halide in the lattice structure. This would be consistent

with the proposal of a structure similar to the known

structure of (CgH3Me3)(CO)3Mo(HgCl2)2 (100) involving

dimeric octahedrally coordinated metal atoms bridged

through a mercury-chlorine network of atoms as is illus-

trated in Figure 19.

Figure 19 was drawn to scale using the bridgework

angles and distances as in the molybdenum complex (100) and

the Fe-C and C-0 distances obtained from the gas phase

electron diffraction investigation of Fe(C0)5 by Beagley

e"t al • (115). The Fe-Hg bond distance was estimated as the

100

r i

\, / Hq*

X C

u

O v,.

0 —

/ // A'

\ / i

\ , - . / ci-.. y , '-J /

X I y \ \ \ I I-

/ / o / \

- p e-/1

c\

= o

\ c

o" o

Fig. 19—Possible structure of the 1:2 adduct (intermediate I2 in the text) obtained in the reaction of iron pentacarbonyl with mercuric chloride in acetone.

101

sum of the single-bond metallic radii of the two metals

(116). Calculations show that the closest distance between

the two CI atoms on the mercury atoms of the bridging

Hg-Cl system and a properly positioned carbonyl carbon are

around 3 and 4-.5A0. The C-Cl distance in phosgene is 1.7M-A0

as determined by X-ray diffraction (117), and it seems

likely that the angle deformations and bond length varia-

tions caused by vibrations within the molecule would bring

the atoms well within their bonding distance. The Fe-Hg

distance is around 5.5A° in the intermediate and when

bonded (in Fe(CO)^(HgBr^) is 2.59A° (97). However,

molecular vibrations should bring the former distance within

range of the latter distance.

The complex (bipy )Mo(C0)1+ with mercuric chloride gave

(bipy)Mo(CO)g(HgCl)(CI) instead of a product with two

mercury-metal bonds (17). This reaction may proceed through

an intermediate similar to the 1:2 adduct (10 0) obtained as

a product, (CgHgMe^)(CO)gMoCHgC^)^ • With such a structure

the CI atom is oriented below the basal plane of the approxi-

mate capped octahedral structure and would be in a favorable

position to bond. Expelling the CO group trans to the

bonded HgC^ group would give the capped trigonal prismatic

structure that is observed.

The intermediate would not be expected to exist in

a finite concentration since the formation of the product

from this intermediate would be a very favorable process.

102

The rate data obtained from the linear portion of the

plots of ln(At-Aoo) at 209^ cm-1 is illustrated in Figure 15

(data obtained at 5 5°C) and support a rate law,

d[Fe(C0) <Hg?Cl„)9] = k[Fe(CO)1+(HgCl)2][HgCl2] . (29)

This is consistent with a mechanism involving coordination

of two molecules of HgCl2 to Fe(C0)4(HgCl)2 as is illus-

trated in Figure 20. The rate law for such a mechanism

would be (using the notations in Figure 20),

d[Pf]

~~dt = k5^I3-"-L-' *

If you assume steady-state concentration of Ig (as shown in

Appendix II) Equation 31 would simplify Equation 30 to,

d[Pf] k1+ks[Pi][L]2

dt ~ k_4 + k5[L] '

In the limit of low ligand concentration the condition

w o u ld hold and Equation 31 would simplify to,

d[P ] k k [P ][L]2

- a r - * • <32)

consistent with the observed rate law (Equation 29).

The entropy of activation calculated from the data ob-

tained from monitoring loss of substrate (Table XI) is

negative and would be consistent with the proposed mechanism

103

U

U )

I

1 * )

u

O )

I

O

u

LL

0 1

O

U

© LL

0 2 -

CJ •H & 3 O 0) S p

•H £

o bC

O o S_/ 0) Ph Mh o a G •H P a rd a) 5H <D rC P

o Mh 6 CD •H £ rd rC a a) e na a; 0] o ft 0 fn Plh 1 I o OJ

bO CD •H nj PH H Pi O rH rC O

10 *+

since the entropy value is actually a combination of

individual values, AS^AS^+AS^-AS^. For the steps whose

rates are controlled by ^ and k^, the processes should be

associative and unfavorable from the entropy standpoint and

therefore should be negative. The entropy value for k ^

should be positive but should contribute negatively to the

total value. The entropy of activation calculated from the

data obtained by monitoring formation of final product

(Table XI) is also a combination of entropy values, ± i i i

AS=AS^+ASg-AS_^. The steps whose rates are controlled by

and kg should show negative entropies and the return step,

whose rate is controlled by k should yield a positive

AS* value but will contribute negatively to the overall

value.

The solubilities of mercuric bromide and mercuric iodide

and their products with Fe(C0)5 were much less than mercuric

chloride in acetone so quantitative rate data could not be

obtained. However, qualitative information, obtained by

monitoring the loss of substrate, indicated the mercuric

bromide to react faster and the mercuric iodide to react

slower than mercuric chloride. This is the same order of

reactivity that was observed by Lewis and Wild for these

reactions (86). Both of these ligands would be expected to

be slower to react because of their greater steric demands

relative to mercuric chloride. However, the polarizabili-

ties are expected to be greater which would cause the rates

105

to be enhanced relative to mercuric chloride. The polariza-

bility effect may predominate in mercuric bromide, and the

steric repulsion may predominate for mercuric iodide.

However, conclusions would be premature from the informa-

tion known about the reaction of these ligands with metal

complexes.

In conclusion, the reaction of iron pentacarbonyl with

mercuric chloride in acetone was seen to lead initially to

the formation of the products cis-FeCCO)^(HgCl) and COCl^.

The rate of this process was found to be dependent upon the

concentration of the iron pentacarbonyl and the square of the

mercuric chloride concentration. The mechanism for this

process is inferred to be similar to the ones proposed for

other oxidative elimination reactions, involving the formation

of one-to-one and one-to-two adducts of substrate and ligand as

intermediates. The initial product, FeCCO)^(HgCl)^> w a s found

to react further with mercuric chloride to yield the previously

unobserved product, cis-Fe(CO)^(HgCl•HgC^)Q• The rate of this

process was found to be dependent upon the concentration of

cis-Fe (CO )f| (HgCl)^ and the square of the mercuric chloride

concentration. The mechanism proposed for this process is the

successive abstraction of HgC^ by each of the HgCl substituents

of cis-Fe(CO(HgCl) .

The formation of the product, COCI2, indicates interaction

of the chloride substituents at the carbonyl carbon. This seems

106

likely since "the electrophilic addition of mercuric chloride

"to iron pentacarbonyl would be expected "to remove electron

density from the iron atom. A more positive metal, atom

would result in a more positive carbonyl carbon which would

be favorable for abstraction of CI. Examples of nucleophilic

attack at the carbonyl carbon have been presented in Chapters

I and II.

Higher substituted iron carbonyl complexes, L2Fe(CO)3,

gave stable 111 and 1;2 adducts in their reactions with

mercuric halides (99) instead of phosgene and an elimination

product. This behavior seems likely due to the higher electron

density expected at the metal atom for substituted iron

carbonyl complexes and their adducts with mercuric halides,

relative to the unsubstituted complexes. Fe(C0)g and

presumably Fe(C0)g•2HgCl2 show higher carbonyl stretching

frequencies (114) than the corresponding phosphine and phosphite

substituted derivatives (99).

Metal carbonyl complexes which contain a low VQQ would

contain a relatively negative carbonyl oxygen would be

susceptible to attack by electron-deficient species at that

atom. A review article has been written by Shriver (118)

which deals with such interactions. The analogous nitrogen

fixation reaction (eqn. 20) is believed to proceed by attack

of the hydrogen ion of strong acids, H+, at the terminal

nitrogen (119). This is also supported by relatively low \>NN

of the highly substituted trans-(diphos)?W(N2)9 species.

APPENDIX I

DERIVATION OF THE RATE LAW FOR THE MECHANISM PROPOSED

FOR THE REACTION OF IRON PENTACARBONYL

WITH MERCURIC CHLORIDE

From the proposed mechanism in Figure 18, it can be

seen that the rate of disappearance of iron pentacarbonyl

(S) can be expressed as,

d[S] k, [S][L] - k ,[!-,] . (1)

dt

To get the rate law in terms of ligand (mercuric chloride)

and substrate concentration, the concentration of 1^ (the

1:1 adduct of substrate and ligand) must be determined.

The rate of formation of 1- would be,

d[I,] d t

X = k-^[S] [L] - k_1[l1] - k2[I1][L] . (2)

Using steady state assumption of 1^ (d[l- ]/dt = 0) simplifies

Equation 2 to Equation 3.

k,[S][L] l-Il-' = k_x + k2[L]

( 3 )

Substitution of Equation 3 into Equation 1 gives the expression,

d[S] k1k2[S][L]2

~ ~dt~ = k_]_ + k2[L] ' ( 4 )

10.7

APPENDIX II

DERIVATION OF THE RATE LAW FOR THE MECHANISM PROPOSED

FOR THE REACTION OF Fe(CO^(HgCl)2

WITH MERCURIC CHLORIDE .

From the proposed mechanism in Figure 20, the rate of

formation of P^ (Fe(CO)1+(Hg2Cl3)2) can be expressed as,

d[Pf]

- a t — = k5 [ I3 ] [ L ] • ( 1 )

The rate of formation of 1^ (the substrate with one molecule

of HgC^ abstracted) would be,

d[Iq] d t = kl|[Pi][L] - k_4[I3] - k5[l3][L] . (2)

Assuming steady state on I3 would allow the concentration

of I^ to be determined.

kJP.][L]

C I3 ] = + k.LLj C 3 )

Substitution of Equation 3 into Equation 1 gives,

d[P ] kuk,[P.][L]2

i_ = ** 5 1

dt k_4 + k5LL] '

10

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