Reactions of organometallic complexes

Textbook H: Chapter 5.1 – 5.5

Textbook A: Chapter 5

Probing the mechanism of a reaction The rate law

From the kinetic order: the number and types of each molecule participating in the formation of the transition state of the rate-determining step. Method employed: pseudo-first order conditions (one of the reactants, A, is

kept in high excess of the other, B, while the concentration of B is varied. The process repeats for A.

From the rate constant: the rapidity of the reaction.

2

A + B C

d[C]

dt= k[A][B]2

does not necessarily mean a termolecular reaction; it means that, in the series of elementary steps leading to a transition state, 2 molecules of B react sequentially to 1 molecule of A.

Measurement of reaction rates Inert species, t1/2 > 1 min

Classical techniques (absorption spectroscopy, pH measurements, etc.)

Labile species, 1 ms < t1/2 < 1 min Stop flow measurements, rapid mixing, fast spectroscopy

Rapid reactions, t1/2 < 1 ms Relaxation techniques, fast spectrophotometry

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Thermodynamic parameters

Notes: One cannot conclude a mechanism from the kinetic parameters or the

rate law. One can only disprove a mechanism.

H# reflects differences in bond strengths S# reflects solvation effects T can only be varied over 20-30° otherwise the mechanism might

change and the information is not meaningful.

Electronic and steric effects difficult to separate easier to establish electronic effects (similar to Hammett plots in OC)

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k =kBT

hexp

G#

RTG# H S=

Ligand substitution reactions: overview Studied systematically for the reactions of

phosphines with metal carbonyls (Basolo) Classification

D Dissociative (comparable to the SN1 limiting case) A Associative (comparable to the SN2 limiting case) I Interchange / Intermediate

Ia (comparable to typical SN2 reactions) Id (comparable to typical SN1 reactions)

Notes: "Labile" and "Inert" are kinetic terms

"Stable" and "Unstable" are thermodynamic terms

Substitution mechanisms

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Associative vs dissociative

Associative Dissociative

Type of complex 16e and 17e 18e

Rate law 1st order in entering ligand 0 order in entering ligand

Activation parameters

Large negative S‡

Large negative V‡

Small positive S‡

Small positiveV‡

Electronic effects Ligand: favored for more basic entering ligandMetal: Favored for more electrophilic centers

Not determining

Effect of departing ligand

Not affected Affected strongly by the BDE to the departing ligand

Steric effects Favored by sterically accessible metal centers

Favored for sterically hindered metal centers

Other factors Reduction weakens M-L and accelerates dissociation of L

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Thermodynamic considerations BDE of M-Y must be greater than BDE of M-X

Bonds between M and neutral, 2e donors are less than half as strong as typical bonds in organic chemistry (M-CO: 25-46 kcal/mol; M-PR3: 30-60 kcal/mol; H3C-CH3: 88 kcal/mol; H-CH3: 104 kcal/mol).

Polyhapto/polydentate ligands bind stronger than monodentate ligands; multiple equiv of a monodentate ligand, however, can displace a polydentate ligand. Also, be aware of the chelate effect.

General binding trends for low-valent M: Cp > MexC6H6-x > C6H6 > CO ~ PMe3 ~ ethylene > PPh3 > py > CH3CN > N2 > H2 > THF, acetone, EtOH

M-L BDEs increase down in a group.

Steric properties are important: more hindered ligands bind more weakly than less hindered ligands (even though the former are stronger Lewis bases than the latter).

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LnM X LnM Y+ Y + X

Dissociative displacements

Observed for 18e carbonyls Rates: TBP d8 > Td d10 > Oh d6

Y-intermediate is favored by L being a good -donor; T-intermediate is favored by a high trans-effect L.

Rates for TM: 3rd row < 2nd row > 1st row

Dissociation is accelerated for large ligands.

Weakly bound solvent molecules are often useful ligands synthetically.

LnM CO- CO, k1

+ CO, k-1

LnM + L'k2

LnM L'

rate = k1[LnM-CO]

rate =k1k2[L'][LnM-CO]

k-1[CO] + k2[L]

if [L] or k-1 are very small

ML

L L

CO

L

L

ML

L L

L

L

square pyramid (T-shape)

retention

LML

L

L

Lretention

inversion

inversion

trigonal bipyramid (Y-shape)

OR

Associative substitutions

Often adopted by 16e complexes:

Found for 18e complexes that have a ligand which can rearrange (slip):

MLn+ L', k1

LnM L'- L

f astLn-1M L'

rate = k1[complex][L']

slow

M

L

L

Lt Ld Li

slowM

L

L

LtLd

- Ld

f ast

Li

M

L

L

Lt Li

MLn

Li

slow

MLiLn

- L

slow

MLiLn-1

5-indenyl5-indenyl 3-indenyl

The shift mechanism

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12

3 and 1 intermediates are sufficiently stable to be trapped at high conc of PMe3.

Rearrangements of coordinatively unsaturated species When a ligand dissociates, one of the remaining ligands rearranges to fill the

vacant site created: the reverse of the slippage process. Analogous to neighboring group participation in organic chemistry

M

O

OM

O

O

MoMe2PhP

Me2PhP PMe2Ph

PMe2Ph

N2

N2

Mo

Me2PhP

Me2PhPPMe2Ph

heatPMe2

WOC

OC CO

PCy3

PCy3

H

H

WOC

OC CO

Cy2P

PCy3 H

PMe3

FeMe3P PMe3

PMe3Fe

Me3P

Me3P CH2

H

Me2P

PMe3

Oxidative addition / Reductive elimination

Textbook H: Chapter 6 - 8

Textbook A: Chapter 3.3.1 – 3.3.6

LnMxA

BLnM

A

BLnMx+2

A

B

OA product-complexRE product

Outline

Oxidative addition of C-H bonds

Nucleophilic displacement (SN2) mechanism

Radical mechanism

Reductive elimination

OA/RE in catalytic cycles

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Electron transfer (ET) reactions Types of mechanisms:

Inner-sphere: the two reagents bridge during electron transfer; substitution processes are important.

Outer-sphere

Barriers to ET between metal sites distance the electron has to travel

solvent reorganization coordination sphere reorganization

different metal-ligand distances for different oxidation states atom movements are slower than electron movements: intermediate with

intermediate distances

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