04/22/23 Ligand Substitution and Activation 1

Ligand substitution reactions:dissociative

Example:

Factors influencing ease of dissociation:• 1e row < 2e row > 3e row

• d8-ML5 > d10-ML4 > d6-ML6

• stable ligands (CO, olefins, Cl-) dissociate easily (as opposed to e.g. CH3, Cp).

LnM CO LnM LnM L'+ COL'

18 e 16 e 18 e

04/22/23 Ligand Substitution and Activation 2

Dissociative substitution in ML6

16-e ML5 complexes are usually fluxional;the reaction proceeds with partial inversion, partial retention of stereochemistry.

18-e

oct

16-e

SP distortedTBP

or

04/22/23 Ligand Substitution and Activation 3

Example:

Sometimes the solvent is involved:

Ligand substitution reactions:associative

16 e 18 e

L'Ln-1M L'LnM LnM L'

- L

16 e

L2MCl2 L2M(Cl)(Br)

L2M(Cl)(H2O)+

Br-

- Cl-

H2O - Cl- Br-- H2O (cis-platin !)

04/22/23 Ligand Substitution and Activation 4

Ligand rearrangement

Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactions:

M N

O

M N O

3-e 1-e

M M

3-e5-eMCO

RM

O

R

(1+2)-e 1-e

04/22/23 Ligand Substitution and Activation 5

Ligand substitution reactions:redox-induced

For 17-e ⇋ 19-e usually fast equilibrium:

• Reduction promotes dissociative substitution.

• Oxidation promotes associative substitution.

• In favourable cases, the product oxidizes/reduces the starting material redox catalysis.

LnM L'LnM + L'

04/22/23 Ligand Substitution and Activation 6

Initiation by added reductant.

Sometimes, radical abstraction produces a 17-e species(see C103).

CO

L

Fe(CO)4L

Fe(CO)4L

Fe(CO)4

Fe(CO)5

Fe(CO)5

04/22/23 Ligand Substitution and Activation 7

Ligand substitution reactions:photochemical

Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (LF transition). This results in fast ligand dissociation.

Requirement: the complex must absorb,so it must have a colour.

d

d

h

M(CO)6

04/22/23 Ligand Substitution and Activation 8

Some ligands have a low-lying * orbital and undergo MLCT excitation. This leads to easy associative substitution.

The excited state is formally (n-1)-e !

M-M bonds dissociate easily on irradiation (n-1)-e associative substitution

d

d

h

M(CO)4(phen)

*

04/22/23 Ligand Substitution and Activation 9

Ligand activation:electrophilic and nucleophilic attack

• Electron-rich metal fragment:ligands activated for electrophilic attack.

N4: strong -donor

NN

N

Rh SN

++

SH+N

N

N

RhN

+

04/22/23 Ligand Substitution and Activation 10

Ligand activation:electrophilic and nucleophilic attack

• Electron-poor metal fragment:ligands activated for nucleophilic attack.

-

CrOCOC

CO

Bu

H

Li+

CrOCOC

CO

BuLi

CO: strong -acceptor

04/22/23 Ligand Substitution and Activation 11

Electrophilic attack on ligand

• Hapticity may increase or decrease.

• Formal oxidation state of metal may increase.

H+MI

+

MI

H+M(0) MII

+

04/22/23 Ligand Substitution and Activation 12

Electrophilic addition

Electrophilic abstraction

Alkyl exchange also starts with electrophilic attack.

O

Fe(CO)3

OEt+

Fe(CO)3

Et3O+

Cp2ZrMe

Me

B(C6F5)3Cp2Zr

Me

+

MeB(C6F5)3-

16 e 14 e

+

04/22/23 Ligand Substitution and Activation 13

Competition: attack of electrophile on metal(may be followed by shift to ligand)

Fe

+

Fe H

Ni

H+

H+

Ni

+

04/22/23 Ligand Substitution and Activation 14

Electrophilic attack on metal

Can be the start of oxidative addition

Key reaction in the Monsanto acetic acid process:

I2(CO)2Rh Me I I2(CO)2RhMe + I- I3(CO)2RhMe

MeOH + CO MeCOOHHI

"Rh"

04/22/23 Ligand Substitution and Activation 15

MeOH

MeI

HI

H2O

MeCOOH

MeCOI

Rh(CO)2I2

Rh(CO)2I3Me Rh(CO)2I3(COMe)

CO

oxidativeaddition

reductiveelimination

insertion

04/22/23 Ligand Substitution and Activation 16

Nucleophilic attack on ligand

Nucleophile "substitutes" metal hapticity usually decreases

Oxidation state mostly unchanged

Competition: nucleophilic attack on metalusually leads to substitution

04/22/23 Ligand Substitution and Activation 17

Nucleophilic abstraction

NaH

CrOCOC

CO

Na+

-

CrOCOC

CO

Cp2WH2BuLi

Cp2WH Li

04/22/23 Ligand Substitution and Activation 18

Nucleophilic addition

Key reaction of the Wacker process:

(H2O)Cl2Pd + OH- (H2O)Cl2Pd

OH-

C2H4 + ½ O2PdCl2, H2O

CH3CHOCuCl2

04/22/23 Ligand Substitution and Activation 19

H2O PdCl

Cl

H2O PdH

ClOH

H2O PdH

Cl O

Cl Pd ClCl

Cl 2--

Cl PdCl

Cl

H2O PdCl

OH

H2O PdCl

OH-H elim

- 2 e (CuCl2 CuCl)

C2H4 H2O

H2O PdCl

ClOH

-

ins

Cl-CH3CHO

H2O Pd ClH

Cl-

Pd(0) + H+ +H2O + 2 Cl-

OH-

- Cl-

-H elim

04/22/23 Ligand Substitution and Activation 20

How can you distinguish between internal and external attack of OH- ?

Use trans-CHD=CHD and trap the intermediatePd-C-C-OH with CO:

H2O PdCl

Cl

H2O PdOH

Cl-

OH-H2O Pd

Cl

ClOH

-

-

H2O PdO

Cl

H

ins

OH- - Cl-??

H2O PdCl

ClOH

-

PdCO

OH

O

O

CO

- Cl-

Pd

O

HO

ins

nuclattack

04/22/23 Ligand Substitution and Activation 21

OH-

O

OD

D

Pd

HO

D

D

Pd

HO

D

D

Pd

D

D

O

OD

D

PdD

HO

D

Pd

OHD

D

CO

CO

04/22/23 Ligand Substitution and Activation 22

Could acetaldehyde be formed directly as vinyl alcohol ?Perform reaction in D2O:

diss-H elim OPd

D

ins

CH3

OH

Pd

OD

D2O

CH2D

OHdiss

ODOD

Pd

H