Pyrolysis studies of main group metal-alkyl bond dissociation energies: VLPP of GeMe4, SbEt3, PbEt4, and PEt3

Pyrolysis Studies of Main Group Metal- Alkyl Bond Dissociation Energies:

VLPP of GeMe4, SbEts, PbEt4, and PEt3

GREGORY P. SMITH and ROGER PATRICK Department of Chemical Kinet ics , SRI International, Menlo Park, California 94025

Abstract

The bond dissociation energies of tetramethyl germane, triethyl stibine, tetraethyl lead, and triethylphosphine were determined using the technique of very-low-pressure pyrolysis. Arguments are presented for log A 2 17.0. The respective dissociation energies AH298 are 83, 57, 54, and 68 (*2) kcal/mol. A consistent set of methyl bond energies to main group metals is determined from these and previous results, and is examined for trends. Bond energies for various radicals to tin are also derived.

Introduction

The thermochemistry of many hydrocarbon compounds and interme- diates has been measured to the degree that it is now possible to accurately and quickly determine the thermodynamics and kinetics of many related species without experiment [l]. Some progress has also been made for silicon [2] and sulfur [3] compounds, and to a lesser degree for oxygen, nitrogen, and many halides [4]. There is, however, a vast group of metal- organic compounds about which few bond dissociation energies are known. Since such bonds and their variations from metal to metal are important in the growing fields of catalysis and inorganic chemistry, we have begun a systematic investigation of organometallic bond energies and trends.

The main group metal alkyls, from Al(CH3)3 to Bi(CH3)3, offer a large regular set of compounds on which to begin such an effort. In particular, there is a body of information built over the years by Price and co-workers [5-191 who studied the pyrolysis of many of these compounds by the toluene-carrier technique. Recently a very-low-pressure pyrolysis (VLPP) study of Sn(CH3)4 was made [20], and a shock-tube experiment [21] was performed on Ge(CH3)4. The efforts reported here concentrate on group IV and V compounds. Variations between groups were examined by in- vestigating triethylstibine, and by conducting the first phosphorus-carbon bond dissociation energy measurements. New bond dissociation energy

International Journal of Chemical Kinetics, Vol. 15,167-185 (1983) 0 1983 John U’iley & Sons, Inc. CCC 0538-8066/83/020167-19$02.90

168 SMITH AND PATRICK

values for ethyl compounds of some late main group metals were determined including the antiknock gasoline additive tetraethyl lead.

Experimental Method

The VLPP technique and apparatus have been described in detail previously [22]. Gas flows at very low pressures through a heated Knudsen cell reactor, where it is pyrolyzed. Since the rate of escape into the analysis mass spectrometer is known, the decomposition rate can be measured relative to it. The 134-cm3 reactor has two interchangeable exit apertures (small and large) with escape rate constants of 0 . 2 0 8 8 m s-l and 2 . 5 5 7 1 a s-l (T in K and M in amu). The collision frequency with the walls, which heat the gas, is 4 . 9 8 2 d m s-l for this reactor, but gas-gas collisions are mostly avoided at the low millitorr reactor pressures. Under these conditions the decomposition reaction is in the falloff region, that is, the measured rate is partly determined by the energy transfer process. Fortunately the wall collisions are efficient, and the rate is known exactly, so unimolecular rate theory may be applied to derive the Arrhenius parameters, and bond energy, for the decomposition.

The compound concentrations were monitored mass spectrometrically relative to peaks for SFs or benzene (which do not decompose), which were added to the flow gas mixture in small amounts. The peaks used for Ge(CH3)4, Sb(C2H5)3, Pb(C2H5)4, and P(C2H5)3 were a t masses M-15, M, M-29, and M, respectively. For those compounds without parent peaks, contributions to the monitored peak from the pyrolysis products are a possible complication. There are several arguments against this possibility. The radical products should be easily destroyed on the walls, with which they typically undergo many collisions. Furthermore, the second bond dissociation energy (the radical) should be less than the first [2], and thus further, fast decomposition is probable. The VLPP experiment on tetramethyl tin [20] demonstrated no product involvement in the M-15 peak. Finally, the current experiments give identical decomposition rates for large- and small-exit apertures (short and long reaction times) using these daughter peaks, and produce results consistent with previous investigations [16,18,21].

Results

The VLPP decomposition rate constants for Ge(CH3)4, Sb(C2H&, Pb(CzH5)4, and P(C2H& as a function of temperature for both size apertures are shown in Figures 1-4. Flow rates of -2 X l O I 4 molecls were used, and no dependence of the decomposition rates.on the flow (that is, reactor pressure) was observed. Also, rates do not vary with aperture size (that

169 PYROLYSIS OF METAL-ALKYL BOND ENERGIES

loo 3

-

10 - - - - -

7

I 111 - Y -

- -

1.0 - - - - - - - - - - - - - - - -

/ 0.1 I I I I 1000 1050 1100 1150 1200 1250 1300

T(K) JA-2056-5

Figure 1. VLPP decomposition rate constants versus temperature for tetramethyl-germanium. 0-small aperture; 0-large aperture; -Gorin transition state fit of Table 11; - - - deviation of a fixed vibrational transition state fit.

is, escape rate or reaction time). These results indicate an absence of bi- molecular or surface reactions.

A VLPP study of tetraethyl tin was attempted. Decomposition rates from monitoring the M-29 peak differ by factors of 3-5 for the two aperature sizes. Cleaning the reactor walls or coating them with tin or carbon pro- duced no improvement. Only slight flow rate dependence was observed. The large-aperture data could be fit only by using a much lower A factor and activation energy than the known values [20] for the related compound Sn(CH3)4. These observations indicate possible surface reaction difficulties, or perhaps interferences at the M-29 mass peak by some eventual products. The ratios of the various other daughter ion peaks are observed to vary with temperature. Attempts to diagnose and cure these problems have thus far failed. The tetraethyl tin difficulties are unexpected in view of the straightforward success of VLPP studies of tetraethyl lead and tetramethyl tin [20].

The VLPP of Al(CH& was also examined briefly, since there are no reported values for aluminum-alkyl bond dissociation energies. Very


Figure 2.

T(K) JA-2056-7

VLPP results for triethylantimony. Symbols as in Fig. 1.

rapid wall-catalyzed decomposition was observed above 750 K, and abated when the reactor temperature was lowered. Attempts to oxidize the wall coating, or to “season” the reactor by flowing Al(CH3)s through at higher temperatures, failed to alleviate these difficulties. Previous high-temperature kinetics studies [23] of larger aluminum alkyls, which tend to undergo P-elimination reactions, also indicate the surface sensitivity of these compounds.

Data Analysis

As previously mentioned, the low VLPP wall collision frequency means that the pyrolysis reaction is in the falloff and unimolecular reaction theory must be used to fit the data. Typically [20] RRKM theory is used, but this program is expensive and time consuming to run properly for the methyl compounds and quite impractical for the larger ethyl systems. As an alternative, we use Kassel integrals [24] (RRK theory) to correct for the falloff and fit the data. The suitability of using RRK theory to fit VLPP and other data near the high-pressure limit has been investigated and demonstrated

PYROLYSIS OF METAL-ALKYL BOND ENERGIES 171

600 650 700 750 800 850 900 T(K)

JA-20564

Figure 3. VLPP results for tetraethyllead. Symbols as in Fig. 1.

in the past [25]. To make a direct comparison more relevant to the compounds of this work, we fit the VLPP data on tetramethyl tin [ZO] using Kassel integrals and the transition state of the RRKM fit of [20]. Equally good fits with very similar parameters were obtained. At 850 K, log k(RRK) = 18.1 - 69/0 and log k(RRKM) = 18.0 - 69.3/0; a t 1100 K, log k(RRK) = 17.0 - 6718 and log k (RRKM) = 16.9 - 67.4/0. The input parameters for the RRK calculation are the A factor and the activation energy at temperature T , the known gas-wall collision frequency, and the effective number of oscillators S = C,ib/R = C,/R - 4 [24]. The molecular frequencies determine Cvib and the values of S at various T. The A factor and the model (frequencies and rotations) of the transition state are also similarly related. The transition state frequencies and rotations, and any temperature variation in them (that is, A ) , determine how E, varies with T . Specifically, the bond dissociation energy is

(1) AH898 = E T , ~ - ( AC:) 6 8 ( T - 298) - ( AC;,,) TT where E T , ~ is the activation energy, ACE is the heat capacity difference for decomposition, and ACZ,, is the difference in heat capacity between the

172

100

10

- c

VI 1

Y

1.0

0.1

- I 1 I I - - - - - -

-

- - - - - - -

-

-

- - - - -

- -

-

1 I I I I 800 850 900 950 1000 1050 1100

transition state and the radical fragment (that is, for recombination) [4]. There are two widely used models for the transition state, a vibrational

one and a hindered rotational (Gorin) one. The choice of model has a direct bearing not on the VLPP experimental results, but on the extrapolation of E, values to AH298. This is due to the difference between vibrational and rotational heat capacities. Consistent with our previous practice, we have chosen a rotational model, although others may prefer vibrational models, provided attention is paid to following uniform procedures.

For the typical vibrational model fixed frequencies are used, giving rise to a constant A factor and to a nearly constant activation energy at reactive temperatures. With no A variation, the interpretation of data over a limited temperature range is straightforward from an Arrhenius plot. In general, the transition state frequencies, which differ from the molecular ones, are four fragment bending vibrations, the bond stretch which becomes the reaction coordinate, and the torsion about the breaking bond. The four


frequencies are reduced to match the chosen A factor. For the metal alkyls, the torsion is assumed to be a free rotation for both the molecule and the transition state. This model is best suited to low A-factor reactions whose relatively tight transition states are physically best represented by vibrational modes. Since the frequencies of the modes that change all decrease (become looser) in the transition state, and lower frequency modes approach classical behavior a t lower temperatures (Cvib approaches 2), the heat capacity difference is positive and E, > Eo, the barrier energy. For example, at 800 K, with the ratio of moments of inertia I+/I = 1.5 and a fourfold reaction degeneracy, an A factor of 1016 corresponds to five molecular vibrations of 500 cm-l becoming the reaction coordinate and four transition state vibrations of 215 cm-l. (One free rotation remains constant.) Then the activation energy at 800 K is 2.5 kcal above Eo and 1.2 kcal above the bond energy. This changes little at higher temperatures. Relationship (1) must be examined in each individual case, particularly if higher frequencies are involved, and depends on the details of the chosen vibrational transition state. Generally E, is approximately equal to AH298.

For A factors of 1OI6 and higher, transition state frequencies of below 200 cm-l are usually required, and these modes increasingly resemble re- stricted rotors rather than harmonic oscillators. The hindered rotation, or modified Gorin model for the transition state [1,26], describe these modes as rotations of the nascent fragments whose entropy (that is, state density) is reduced by a hindrance parameter N . This reduction is proportional to the amount of phase space (that is, solid angle) excluded for a rotation by the presence of the other fragment, and corresponds to a hard-sphere square-well potential.

One key feature of this Gorin model is an easy accommodation of a temperature-dependent A factor. The transition state is not fixed. As the temperature is raised, the average rotational energy and the centrifugal potential increase, moving the effective potential maximum inward. Since the location of the transition state is closer in, the hindrance is greater and A decreases with temperature.

There are several arguments in favor of the rotational model. Bond scission reactions generally have no energy barriers save the centrifugal one, so the transition state separation of the fragments is sizable. A re- stricted rotation is the physically better description of the modes at such distances. Only nonvalence forces, that is, square-well-like repulsions rather than strong chemical attractions, are involved. Second, the low vibrational frequencies required to fit reasonable A factors correspond to parabolic potentials over large distances with low force constants. This seems unlikely without large and hard to estimate anharmonicities. Many pyrolysis studies [l] indicate high A factors that is, loose transition states best characterized by rotations: C(CH3)4,17.3 [20], Si(CHd4,17.6 [27]; Ge(CH3)4,17.0 [22]; and (CH&, 16.7 [22]. There is no reason to expect


significantly tighter vibrational transition states (low A ) for similar organometallic compounds such as Pb( C2H5)4.

A direct comparison of fixed vibrational and fixed rotational models was made previously for CF3 + CF3 recombination [as], using room temperature rates and VLPP data at -1000 K. The vibrational model which fit the data predicted unreasonably large high-pressure rate constants a t 1000 K and an unexpected rise in the recombination rate with temperature. The vibrational model also predicted a significant activation energy for recombination a t high temperatures, contrary to experimental experience.

The Gorin model offers the flexibility of a temperature-variant A factor, despite some added uncertainty in the fitted Arrhenius parameters at- tributable to how much temperature dependence is used. At high temperatures the heat capacity of the Gorin transition state is lower than that for classical vibrations (R/2 versus R) . The activation energy thus falls as temperature rises, partly offsetting the declining A factor from the point of view of the fixed transition state model. At lower temperatures where the molecular frequencies are still well within the quantum limit (low Cu), the activation energy rises with temperature. To continue our previous 800K example with relevant molecular vibrations of 500 cm-l, the Gorin model activation energy at 800 K is 0.1 kcal below Eo and 1.4 kcal below the bond energy. A t 1200 K, however, the activation energy is 1.5 kcal below Eo and 2.8 kcal below the bond energy AH2g8. Thus the extrapolation from the measured E, to AH298 depends on the temperature and the molecule, but not on the transition state hindrance. This feature of the Gorin model is easy to use, but lacks the vibrational model feature that AH298 - E,. Considerable differences exist a t higher temperatures, and thus attention to model consistency is required when applying kinetically derived AH298 values back to higher temperatures. To the extent that the true transition state mode potentials lie between the vibrational and the rotational extremes, the true bond energy lies between these two extrapo- lations of E, to AH2g8.

Unfortunately VLPP experiments cannot independently determine both A and E,, but rather determine a range of E, (and hence AH2g8) for a reasonable set of A values. Although evidence cited [20,21,27] argues for high values of A > 1017, and hence the Gorin model, the toluene carrier data [5-191 to which comparisons will be made have lower A factors. Much of these data appear to be in the pressure falloff, and are most easily and successfully reanalyzed using a fixed transition state over a short temperature range. The new VLPP data reported here will be analyzed using what we believe to be the more appropriate and accurate Gorin model with high A factors. To derive a useful, consistent set of AH298 values, we will use the rotational model throughout, although the alternative option will also be considered.

Table I gives the parameters used for the RRK calculators for tri- and

PYROLYSIS OF METAL-ALKYL BOND ENERGIES

TABLE I. RRK fit parameters.

175

Mode

STR

DIST

*M--C

**M--C

a l k y l STR

a l k y l DEF

* * a l k y l ROCK

a l k y l TOR

a l k y l C-C

S (600 K )

S (800 K )

S (1000 K)

S (1200 K)

M(CH3) 4

500 (3)

150 (3)

3000 (9)

1400 (9)

800 (6)

RTN (3)

15.5

19

21.5

23

M(CH3)k

500 (4)

150 (5)

3000 (12)

1400 (12)

800 (8)

RTN ( 4 )

M(C,Hq) 9 M(C>H,)4

500 (3) 500 (4)

150 (3) 150 (5)

3000 (15) 3000 (20)

1400 (21) 1400 ( 2 8 )

800 (6) 800 (8)

RTN ( 3 ) RTN ( 4 )

1000 (9) 1000 (12)

22 26.5 36

26 34 46

30 39 53

32 4 3 57

1000 1200 ~ _ _ 600 ~ 800 T (K) __.__

Gorin ACZg8-Ea ( k c a l ) .80 1.37 2.00 2.73

Vbn AH298-Ea ( k c a l ) .02 .03 .10 . 2 4

A = 10l6 a t BOO; u+ = 300,75 cm-I.

tetramethyl and ethyl compounds. These values determine S , and are approximated from the known frequencies of Sn(CH3)4, Hg( CH3)2, ethyl and methyl 1291. The signified frequencies (*) are those that change in the transition state. For the Gorin model they determine how E, varies with temperature. For the fixed vibrational model the transition state frequencies must be determined from the A factor used before E, can be calculated. Values of AH298 - E, for the A = 1016 vibrational model and for the Gorin model are given for several temperatures in the table. These frequencies were also used to calculate AC, in equation (1).

The correct range of A factors for tetramethylgermane, and the other compounds considered here, can be estimated from the equilibrium constant and the recombination rate constant, since k d = K k R . The equation

A e - E J R T = k R e A S / R e - A H / R T

reduces to

A = ekReAs/R

We estimate k R a t 700 K using hard-sphere collision diameters of 5.2 and


3.8 A (from propane and methane) [30], and assuming that 45” cones above the below the “planes” of both radicals define the effective solid angle for recombination. This gives h~ = 2.7 X cm3/molec.s. The entropy change of 34.6 eu can be calculated from the methyl value [l] and the frequencies of Table I, assuming a planar M(CH&. This procedure gives log A - 16.9. An A factor below 1016 implies a recombination rate constant below the reported 700 K value for two isopropyl radicals 1311 and a solid angle (steric factor) below 2%.

VLPP Bond Energy Values

The measured VLPP decomposition rate constants for Ge(CH3)4, Sb(C2H5)4, Pb(C2H& and P(CzH& were given in Figures 1 4 as functions of temperature for both small- and large-escape apertures, along with lines representing the best Kassel integral fits. The corresponding high-pressure Arrhenius parameters are given in Table I1 for both the low-A fixed vibrational transition state and the preferred high-A Gorin model. The previous VLPP results for Sn(CH3)4 [20], fit here by Kassel integral rather

TABLE 11. Fit Arrhenius parameters.

Ea

Other r e s u l t s ‘

Ge(CHg)r

1050

(b )

17.5-81.010

1175

16.0-7810

16.5-80.510

1300

16.0-7810

16.2-80.010

(78)

83.2

78-80.5

17.0-7710

Sn(CH3) +a 850

16.0-6310

18.0-68.010

1000

16.0-6310

17.2-67.510

1100

16.0-6310

17.0-67.210

Sb(CpH5) 3

725

16.2-52/0

17.9-56.310

775

16.2-5210

17.8-86.210

850

16.2-5210

17.6-55.910

Pb(CzHs) 4

650

16.3-4910

18.0-53.6/0

725

16.3-4910

18.0-53.3/0

800

16.3-4910

18.0-53.110

P(CzH 5) 3

850

16.0-6310

17.5-67.010

950

16.0-6310

17.1-66.710

1050

16.0-63/0

16.7-66.410

63 52 49 63

69.5 57.5 54.5 68.5

63-67.5 52-56 49-53 63-67

15.7-64.510

a From [20], RRK analysis. log k = 16 - 78/8 only gives 0.4k at 1050 K. From (21,141.


than the previously used full RRKM treatment, are also included for comparison.

The tabulated results reveal several interesting features. The Gorin model A factors used drop from 10IR near 800 K to 1017 near 1100 K, and even lower for the high-temperature tetramethyl germane pyrolysis. The sharp drop for Ge, P, and Sn, however, does contrast with the nearly constant A factors of the low-temperature Sb and Pb studies. The Kassel fit of Sn(CH3)4 matches the earlier RRKM fit [20] within 1 kcal, although the latter did not use the correct molecular frequencies [as]. Also note that the fixed vibrational transition state fails to fit the Ge(CH3)4 data at low temperatures. The bond energies derived from the two models differ by approximately 7 kcal/mol. The differing A factors and the different transition state heat capacities (vibration versus rotation) contribute equally to this discrepancy. The results for Ge(CH3)4 and Sn(CH3)4 are consistent with previous measurements within typical 3 kcal/mol error limits.

Obviously the choice of transition state and A factor produces some ambiguity in the correct bond dissociation energy AH298. In accord with the arguments presented earlier, we present AH298 values in terms of the Gorin transition state, and believe these higher values to be most accurate. The possibility of values 3-5 kcal lower should be kept in mind, however, and care exercised when comparing AH298 values of different compounds derived from E, values via differing transition state models. For the purpose of comparing metal-alkyl bond dissociation energies simply and consistently in this paper, values of the activation energy in the midrange of the temperatures are used to derive AH298 via a rotational model.

Other Bond Energy Values

A variety of bond dissociation energies for main group alkyls can be derived from the literature, and are listed in Table 111. The butyl-lithium value is directly derived from calorimetric data and heats of vaporization. This assumes that the vapor is not associated, as ethyl-lithium is known to be, and is a lower limit. The lithium-methyl value is estimated to be -5 kcal higher.

For boron thermodynamic values can be derived from gas-phase heats of formation for phenyldichloro- [32] and methyldifluoro- [33] boron, and the values for BC12 [34] and BF2 [35] radicals. The higher phenyl bond energy is as expected. By analogy with carbon compounds, the substitution of halogen atoms for alkyl groups should have little effect on the B-C bond strength.

Neopentane decomposition activation energies (and A factors) vary from 78 to 85 kcal, but fortunately hydrocarbon thermochemistry is fairly well known. Following the discussion of [20], an 82.4-kcal bond energy was


TABLE 111. Alkyl bond dissociation energies.

Ref T - _ Ga(CH3)3 10 800

Ga(CzH5)j 11 600

In(CH3)3 12 600

TI(CH3)3 13 500

Pb(CH3), 16 700 Sn(C2H5)4 15 800

As(CH;)j 17 800 Sb(CH3); 18 700

Bi(CH3)3 19 650 Zn(CH3)i 5 850

Zn(CZH5)Z 6 750

Cd(CH3)Z 7 750

Hg(Ctl3)Z 8 700

Hg(C2Hg)2 9 650

T ~

298

2 98 298 1100

1000 1150

1000

298

log k

15.5-59.51:) 15.7-47.2lC

15.7-47.210

15.1-36.410

14.7-4 9.41c 16.0-59.3/0

15.8-62.810

15.3-55.910 14.0-44.OlO 13.3-54.010

14.3-49.010

13.4-48.nio 15.7-57.510

15.4-45.7 I0

80.4

84.8 77.0 67.5

- l o g k l

16.1 -61.0/0 16.2 -47.510

16.1 -48.010

16.0 -38.0ii 16.05-54.010 16.0 -59.310

16.15-64.0/O 15.9 -58.010

15.9 -49.0/0 16.0 -65.0/0

16.0 -54.010

16.0 -57.010 16.2 -58.010

16.0 -47.010

61.5 121 112 82.4

86.8 79.6 69.5

76

Ea' ~

61 47.5

48

38

54 59.3

64 58

49 65

54

57 58 47

AH298(G)

62.4

48.3 48.8

38.5 55.1 60.7

65.4 59.1 50.0

66.5 5 5 . 2

5 8 . 2

59.1 48 .0

____

chosen, with log A = 17.3. A more recent review of thermodynamic data suggests an 84-kcal value, and thus a slightly higher A factor. For tetramethyl silane, [27] gives reasonable parameters of log k = 17.6 - 84.8/8, although the discussion of [2] suggests a final AH298 value 2 kcal higher. Ge and Sn results are also included [21,20] (log A = 17.0, 17.6). Group IV A H 2 9 8 values are derived using Gorin model transition states. The tri- methylamine bond energy is thermodynamically derived from VLPP results on dimethylbenzylamine [36] using recent henzyl AH values [4].

The remaining main group organometallic bond energies are based on toluene-carrier studies by Price and co-workers [5-191. In general these results show very low A factors (see Table III), a fault previously noted for this technique [37]. There is no logical expectation that A should fall two orders of magnitude on changing from C to Pb, or from Hg to Zn. Fur- thermore, the 1014 value for Bi(CH& at 650 K requires the transition state to be as tight as the molecule. As mentioned in [20], RRK calculations show that these results are in the pressure falloff. (This is evident in some [5,7,8,12,17] of the data.) Clearly, Kassel integral fits of these data must

PYROLYSIS OF METAL-ALKYL BOND ENERGIES

TABLE IV. Toluene-carrier data fits.

179

a 730

830

610

649

578

600

610

610

502

67 1

743

7 64

858

836

836

705

767

655

680

823

875

700

750

745

745

800

800

700

750 755 626

po- 13 8.0

13 8.0

12 8.0

12 8.0

13 8.0

13 8.0

8 8.0

30 8.6

22 8.0

14 8.0

12 8.0

23 8.2

25 8.2

6.3 7.5

36

164

171

16

16

26

145

54

169

15

15

20

100

23

80

320

110 325

5

8.3

9.0

9.0

8.0

8.0

8 .3

9.0

8.5

9.0

8.0

8.0

8.3

9.0

8.3

8.8

9.2

8.7 9.2 7.5

S

18

20

16

17

15

16.5

16

16

14

23

25

18

20

20

20

17

18

16

16

12

12

12

12

2 0

21

12

12

12

12

11

- k/ km .764

.705

.673

.522

.547

.491

.40

.56

.367

.888

.84 1

.764

.719

.526

.741

.883

.866

.503

.442

.334

.527

.327

.454

.759

. 7 1 1

.305

.491

.244

.365

,469

-

11.5 .309 11.5 .410

18 .484

kfit%3 .0069

,976

.131

1.12

,0074

1.00

,052

.073

.134

.033

1.56

.0070

.645

.177

.249

.0096

.260

.17

.79

.024

.037

.24

.34

.134

1.65

.074

.12

. 8 2

1.23

.007 5

. I 0 2

.135

.24

(5-1)

.0069

.941

.148

1.21

.0076

1.05

.061

,073

.145

.038

1.39

.007 1

.663

.190

.270

.0115

.255

.21

.81

.027

.041

.21

.31

.126

1.30

.07

.12

.57

1.15

.0066

.092

.148

.27

be made to get accurate activation energies, using higher A factors. For efficiency, convenience, and consistency we have utilized a fixed transition state with log A > 16 to reanalyze these data. The net effect is not severe, raising the activation energies by 1-4 kcal. The use of a looser transition state would raise E, and m 2 9 8 , several additional kilocalories. A H 2 9 8

is again derived via a Gorin model. Table I11 lists the Arrhenius parameters for the reanalysis of these data,

log h' and Eh, at the pyrolysis temperature 7'. Examples of the fits to the actual data are given in Table IV. The remainingunspecified parameter needed for the Kassel integral is the collison frequency w, approximated here by -108 s-l at 15 torr and 750 K. This corresponds to a rate constant


of 5 X cm3/s for energy-thermalization collisions between the organometallic and toluene. This is derived assuming unit collision efficiency using hard-sphere collision diameters [30] of 5.8 A (from pentane) and 5.3 A (from benzene). Values of S are from Table I, and the dimethyl compound frequencies used are from [29].

Several points are evident from an examination of Table IV. First, the revised, higher A factor, fixed transition state parameters adequately fit the data. All results are indicated to be in the pressure falloff with the exception of Sn(C2H5)4 [15], although Pb(CH3)4 and Sb(CH3)3 are only slightly so. Furthermore, most of the observed pressure dependences of the decomposition rate constants, where observed, are adequately fit as well. Also, note the limited temperature range of usable values, and hence greater uncertainty in E,, for Ga(C2H5)3, Tl(CH3)3, Bi(CH3)3, and

Table I11 summarizes the resulting activation energies with a probable accuracy of 2-3 kcal. A consistent set of bond dissociation energies A H 2 9 8

is also given, using a Gorin transition state for the extrapolation. If log A - 17 rather than 16, these E, and AH298 values should be increased by -3 kcal. Attempts to fit the toluene-carrier data using the larger A factor predict an even greater degree of falloff and fail to fit the limited available pressure dependence data. If one chooses to use a fixed vibrational transition state model, with log A - 16, AH298 is -1 kcal lower than are the corresponding Gorin values, and roughly equal to the reported E, values. In both cases Eo is 1.0 kcal below AH298.

Hg(C2H5)2.

Discussion

Table V summarizes the results of these studies, reanalyses, and other investigations on the activation energies for methyl-main group bond dissociations. Data sources and analyses have been described previously. For the toluene-carrier results the larger of the bracketing values reflects the effect of assuming a 1017 A factor. It is more consistent with the rest of the data in the table, but less consistent with the actual experiments. For tin and germanium, the quoted values are averages of VLPP and other nearly identical values [14,21]. Errors of 3 kcal are possible, given the uncertainties of measurement and data treatment.

The tetravalent group IV elements form the strongest methyl bonds, excepting the unusually strong methyl-boron bond. The group I11 metals form the weakest. In each group, among the later period metallic elements, the methyl bond energies decrease, going down the group. This trend is weakest for group IIB (that is, mercury seems anomalous) and is broken only by the strong silicon-methyl bond energy. The germanium-methyl bond strength resembles more that of carbon or silicon than the metallic tin, according to the results of this study and of [21]. Measurements are


TABLE V. i w 2 9 8 for methyl (ethyl) bond dissociation (kcal/mol).

B C

112 83(a)

(79)

A1 si a7

Ga Ge

62-65 a 1

(48)

In Sn

49-52 69 I f l l - h b )

T 1 Pb

39-41 55-58 (54)

rl Bu (61)

N 0 F 76 83(2) 109(a)

(82) (106)

P s c1 - 7 2 77(b) 84(a)

(68) (82)

As Se Br

65-68 71 (a)

(68)

Sb Te I 59-62 57(a) ( 57) (54)

Bi

50-53

Zn

66-69

(55)

Cd 58-61

Hg

0 59-62

a From [1,39,4]. From [38].

still needed for lithium, boron, aluminum, magnesium, selenium, and tel- lurium, while thallium and bismuth are the least precise.

The bond dissociation energies for ethyl compounds are shown in the table in parentheses. The tin result of [15] agrees with the thermodynamically derived value for Sn(CH&-C2HS (see Table VII). This value and the current results for antimony and lead indicate that the effect of ethyl substitution is a 5-kcal weakening of the group IV or V metal-carbon bond. The corresponding value for hydrocarbons is a comparable 4 kcal. The effect appears much greater (11 kcal) for group IIB, and possibly group I11 based on the limited, solitary gallium data.

With the newly measured ethyl-phosphorus bond dissociation energy of 68 kcal/mol we can estimate a P-CH3 value of 72 kcal. This produces a smooth trend in group V, as the alkyl bond energies drop for larger more metallic elements. Both nitrogen and phosphorus are atypical in that the alkyl bond dissociation energies are close to the average values for all three bonds.

The somewhat different behavior of group IIB, noted in the ethyl effect and the relatively strong mercury bond, is particularly apparent in the second bond dissociation energies D (M-CH3). These values, derived from gas-phase heats of formation [33] and the first bond energies, are given in Table VI. While second bonds, those from a radical, are typically weaker, in this case the contrast is extreme. The effect is progressively larger going down the group to mercury. These observations may be explained in terms of an unstable high-energy CH3Hg radical relative to Hg. The Hg s-orbital lone pair is embedded in the large number of d and f electrons, and being


TABLE VI. Second bond dissociation energies [33] AH298

(kcal/mol).

less shielded from the nuclear charge is a t much lower energy than the more exposed s p hybrid orbitals involved in the CH3Hg unpaired and bonding electrons. Rehybridization compensates for the bond scission. This effect is greatest for the later transition series. For later groups, hybridized p orbitals, which extend further out from the d and f shells, are involved for the nonbonded valence electrons. Table VI graphically illustrates that average bond energies should never be used as bond dissociation energy estimates. These “bond enthalpy contributions” are useful for group additivity estimates of thermodynamic AHf values, but not for bond scission kinetics or energetics.

Gas-phase heats of formation have been measured [32] for many tetramethyl tin derivatives. Since the Sn(CH& bond energy determines AHf (Sn(CH3)3), these derivative bond energies can be calculated from thermodynamic cycles:

D(Sn(CH& - X) = AHf(Sn(CH&) + AHf(X) - AHf(Sn(CH&X)

Several such values are listed in Table VII, along with the corresponding carbon bond energies. Some mercury values from both kinetic [37] and thermodynamic [33] sources are also given. For comparison, the differences from methyl for various other groups are given for the three elements. The metal-halogen (and hydrogen) bonds are stronger than those for carbon, which is indicative of their greater ionic character. In germanium, however, the hydrogen bond strength [40] in trimethyl germane is 81 kcal, roughly equal to the methyl value. This resembles the case for silicon (AD = +3) and carbon (AD = +9) more than that for tin (AD = +26) and again illustrates the nonmetallic character of germanium.

The other organometallic values for tin follow the same trends as carbon, with roughly the same magnitudes. 7r-Electron systems, such as phenyl, benzyl, and vinyl, show no back bonding with the metal d orbitals. Mer- cury values as discussed before seem slightly larger. The bond energies in Sn(CH3)3-CzH5 and Sn(CzH5)4 are the same. Also, note that Table I11 indicates that the boron-phenyl bond is 9 kcal stronger than the methyl


TABLE VII. Derived bond dissociation energies (CH3),M-X.

X M=CA M=S$ & b D ( C ) h D ( S n ) w

CH 2 8 3 6 9 5 8 0 0 0

C2H 5 7 8 6 3 47 -5 -6 -11

iPr 7 4 5 8 4 1 -9 -11 -17

tBu 70 5 4 - 1 3 - 1 5

,+ 92 81 68 +9 +12 + I 0

WH, 6 8 5 3 - 1 5 - 1 6

CZH3 92 7 1 +9 +8

M(CH3)n 7 0 6 7 5 - 1 3 -2

B r 6 4 9 0 5 7 7 5 - 1 9 +2 1 + I 9

I 5 1 7 5 5 6& -32 +6 +8

H 9 2 9 5 5 +9 +26

E t j S n - S n E t 3 9 0 5 Sn(C2H5)4 6 4 E t 3Ge-GeEt 3 92 Ge(C2Hg)h % 7 6 Me3Si -S iMe3 8 If. S i ( C H j ) $ 87 Me3C -CMe3 7@ C(CH3)4 8 3

a From [1,4]. From [32]. From (331. From [37]; MX2. From (411; Et3SnH. From [2].

bond, similar to carbon. These derived results and this comparison indicate that the type of general group additivity rules used successfully to estimate hydrocarbon heats of formation may be applied to other group IV compounds easily and confidently, and most likely will also find applications to other organometallics as well.

Finally, Table VII also lists the derived metal-metal bond energies compared to the methyl values, using similar thermodynamic cycles. A value 5 kcal below that measured for methyl was assumed for the ethyl- germanium bond energy. The results show an increasingly strong bond compared with the metal-carbon bond as the metallic character and period of the element increases. The germanium-germanium bond is particularly strong.

To summarize our results, we have measured alkyl bond energies for phosphorus, germanium, lead, antimony (and previously tin [20]). These and other values were analyzed, presented, and discussed in terms of systematic, observable trends. Group IV-methyl bonds are the strongest, and bonds weaken for heavier members of each group, although certain anomalies apply for group IIB. Ethyl bonds are typically 5 kcal weaker than methyl bonds, and predictable differences appear to hold for other tin alkyls. Thus accurate estimates of a wide variety of main group or-


ganometallic bond dissociation energies can be based on known hydrocarbon trends, the compilation of methyl bond energies, tabulated enthalpy values, and thermochemical estimation techniques.

Acknowledgment

The authors appreciate useful discussions with Drs. Robin Walsh, David Golden, and Richard Laine. Special thanks go to Mr. Walter Ogier for his initial experimental assistance. This work was supported by the National Science Foundation under grant CHE-79 23569.

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Received May 13,1982 Accepted August 30,1982

Documents

Pyrolysis studies of main group metal-alkyl bond dissociation energies: VLPP of GeMe4, SbEt3, PbEt4, and PEt3