Reaction chemistry of complexesThree general forms:1. Reactions involving the gain and loss of ligands

a. Ligand Dissoc. and Assoc. (Bala)b. Oxidative Additionc. Reductive Eliminationd. Nucleophillic displacement

2. Reactions involving modifications of the liganda. Insertionb. Carbonyl insertion (alkyl migration)c. Hydride elimination (equilibrium)

3. Catalytic processes by the complexesWilkinson, MonsantoCarbon-carbon bond formation (Heck etc.)

a) Ligand dissociation/association (Bala)

• Electron count changes by -/+ 2

• No change in oxidation state

• Dissociation easiest if ligand stable on its own(CO, olefin, phosphine, Cl-, ...)

• Steric factors important

MBr

+ Br-M

b) Oxidative Addition

Basic reaction:

• Electron count changes by +/- 2(assuming the reactant was not yet coordinated)

• Oxidation state changes by +/- 2• Mechanism may be complicated The new M-X and M-Y bonds

are formed using:• the electron pair of the X-Y bond• one metal-centered lone pair

LnM +X

YLnM

X

Y

One reaction multiple mechanisms

Concerted addition, mostly with non-polar X-Y bondsH2, silanes, alkanes, O2, ...

Arene C-H bonds more reactive than alkane C-H bonds (!)

Intermediate A is a σ-complex

Reaction may stop here if metal-centered lone pairsare not readily available

Final product expected to have cis X,Y groups

X

YLnM

X

YLnM + LnM

X

YA

Stepwise addition, with polar X-Y bonds– HX, R3SnX, acyl and allyl halides, ...

– low-valent, electron-rich metal fragment (IrI, Pd(0), ...)

Metal initially acts as nucleophile

– Coordinative unsaturation less important

Ionic intermediate (B)

Final geometry (cis or trans) not easy to predict

Radical mechanism is also possible

X YLnM

B

LnM X Y LnMX

Y

OC Ir ClPEt3

Et3P

OC Ir H

PEt3

Et3P

H

Cl

OC Ir I

PEt3

Et3P

H

Cl

OC Ir Cl

PEt3

Et3P

CH3

Br

Ir(I)

Ir(III)

Ir(III)

Ir(III)

H2

cis

cis

trans

HI

CH3Br

Cis or trans products depends on the mechanism

c) Reductive elimination

This is the reverse of oxidative addition - Expect cis elimination

Rate depends strongly on types of groups to be eliminated.

Usually easy for:• H + alkyl / aryl / acyl

– H 1s orbital shape, c.f. insertion

• alkyl + acyl

– participation of acyl p-system• SiR3 + alkyl etc

Often slow for:• alkoxide + alkyl• halide + alkyl

– thermodynamic reasons?

We will do a number of examples of this reaction

Complex Rate Constant (s-1) T(oC)

PdCH3Ph3P

Ph3P CH3

PdCH3MePh2P

MePh2P CH3

PdCH3P

P CH3

PhPh

PhPh

1.04 x 10-3 60

60

80

9.62 x 10-5

4.78 x 10-7

Relative rates of reductive elimination

Most crowded is the fastest reaction

PdCH3L

L CH3

+ solv

-L

PdCH3L

solv CH3

RELPd(solv) + CH3 CH3

Special case:Nucleophilic Attack on a Coordinated CO acyl anion

Fisher carbene

This is Fischer carbene It has a metal carbon double bond

Such species can be made for relatively electronegativemetal centers N.B. mid to late TMs

Fischer carbenes are susceptible to nucleophilic attack atthe carbon

Fischer carbenes act effectively as σ donors and π acceptors

The empty antibonding M=C π orbital is primarily on the carbon making it susceptible to attack by nucleophiles

Other type is called a Shrock carbene (alkylidene)

Characteristic Fischer-type Schrock-typeTypical metal (Ox. State)

Middle to late T.M.Fe(0), Mo(0) Cr(0)

Early T.M.Ti(IV), Ta(V)

Substituents attached to carbene carbon

At least one highly electronegative heteroatom

H or alkyl

Typical other ligands

Good p acceptors Good s and p donors

Electron count 18 10-18

Nucleophilic displacement

Ligand displacement can be described as nucleophilic substitutions

O.M. complexes with negative charges can behave as nucleophilesin displacement reactions Iron tetracarbonyl (anion) is very useful

RX R[Fe(CO)4]2- [ Fe(CO)4]-

CO

H+

OX

R

[ Fe(CO)4]-RO

H+ OH

R

R H

O

XR

X2

O2

R'X O

OHR

O

R'R

Modifications of the ligand

a) Insertion reactions

Migratory insertion!

The ligands involved must be cis - Electron count changes by -/+ 2

No change in oxidation state

If at a metal centre you have a σ-bound group (hydride, alkyl, aryl)

a ligand containing a π-system (olefin, alkyne, CO) the σ-bound

group can migrate to the π-system

1. CO, RNC (isonitriles): 1,1-insertion

2. Olefins: 1,2-insertion, β-elimination

M

R

MR

MR

COM

O

R

1,1 1,2

1,1 Insertion

The σ-bound group migrates to the π-system

if you only see the result, it looks like the π-system has inserted into the M-X bond, hence the name insertion

To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion (Both possible tutorial)

The reverse of insertion is called elimination

Insertion reduces the electron count, elimination increases it

Neither insertion nor elimination causes a change in oxidation state

α- elimination can release the “new” substrate or compound

In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate.

The metal-bound substrate atom increases its valence

CO, isonitriles (RNC) and SO2 often undergo 1,1-insertion

1,2 insertion (olefins)

Insertion of an olefin in a metal-alkyl bond produces a new alkyl

Thus, the reaction leads to oligomers or polymers of the olefin

• polyethene (polythene)• polypropene

MMe

SO2

MS Me

O OM

Me

CO

MMe

O

MR

MR

M

R

MR

Standard Cossee mechanism

Why do olefins polymerise?

Driving force: conversion of a π-bond into a σ-bondOne C=C bond: 150 kcal/molTwo C-C bonds: 2´85 = 170 kcal/molEnergy release: about 20 kcal per mole of monomer(independent of mechanism)

Many polymerization mechanismsRadical (ethene, dienes, styrene, acrylates)Cationic (styrene, isobutene)Anionic (styrene, dienes, acrylates)Transition-metal catalyzed (a-olefins, dienes, styrene)

Two examples

β Hydride elimination (usually by β hydrogens)

Many transition metal alkyls are unstable (the reverse of insertion)the metal carbon bond is weak compared to a metal hydrogenBond Alkyl groups with β hydrogen tend to undergo β elimination

M -CH2-CH3 M - H + CH2=CH2

To prevent beta-elimination from taking place, one can use alkyls that:

Do not contain beta-hydrogensAre oriented so that the beta position can not access the metal centerWould give an unstable alkene as the product

A four-center transition state in which the hydride is transferred to the metal An important prerequisite for beta-hydride elimination is the presence of an open coordination site on the metal complex - no open site is available - displace a ligand metal complex will usually have less than 18 electrons, otherwise a 20 electron olefin-hydride would be the immediate product.

The Monsanto acetic acid process

Methanol - reacted with carbon monoxide in the presence of a catalyst to afford acetic acid

Insertion of carbon monoxide into the C-O bond of methanol

The catalyst system - iodide and rhodium

Iodide promotes the conversion of methanol to methyl iodide,

Methyl iodide - the catalytic cycle begins:

1. Oxidative addition of methyl iodide to [Rh(CO)2I2]-

2. Coordination and insertion of CO - intermediate 18-electron acylcomplex

3. Can then undergo reductive elimination to yield acetyl iodide and regenerate our catalyst

Catalysis (homogeneous)Reduction of alkenes etc.

Alternative starting material

The size of the substrate has an effect on the rate of reaction

Same reaction different catalyst

Homogeneous cross coupling reactions: Heck reaction

CH2=CH2 > CH2=CH-OAc > CH2=CH-Me > CH2=CH-Ph > CH2=C(Me)Ph

krel: 14,000 970 220 42 1

Pd(0)

Pd(II)

Y = H, R, Ph, CO2R, CN, OMe, OAc NHAc

R-Pd(II)-XR-Pd(II)-X

R-Pd(II)-X

R-X

Y

Y

R

Y

HHR

YH

Pd(II)-X

HX

Wacker process (identify the steps)

Identify the steps