Chem 1140; Introduction to Organometallic Chemistry

• General Mechanistic Principles• Reactions with Wilkinson’s Catalyst

A. Organometallic Mechanisms

Oxidation State: The oxidation state of a metal is defined as the charge left

on the metal after all ligands have been removed in their natural, closed-shell

configuration. This is a formalism and not a physical property!

d-Electron Configuration: position in the periodic table minus oxidation state.

18-Electron Rule: In mononuclear, diamagnetic complexes, the total number

of electrons never exceeds 18 (noble gas configuration). The total number of

electrons is equal to the sum of d-electrons plus those contributed by the

ligands.

18 electrons = coordinatively saturated

< 18 electrons = coordinatively unsaturated.

P d

C l

P d

C l

for each Pd: Ox. state

, C l

P d ( I I )

d: 10 (4d

10

5s

0

) - 2 = 8

electron count:

bridging by lone

pairs on Cl;

each Cl acts as a

2-electron, mono

negative ligands to

one of the Pd's,

and a 2-electron

neutral donor ligand

like PPh3

to the other

: 4e

-

C l: 2e

-

C l: 2e

-

8e

-

+ d

8

= 16e

-

unsaturated

Z r

C l

H

Ox. State: 2 Cp

-

, 1 H

-

, 1 Cl

-

→ ( )Zr IV

: 4 (4d d

2

5s

2

) - 4 = 0

: 2 electron count Cp

-

: 12

H

-

: 2

Cl

-

: 2

: 0Zr

16 e

-

, unsaturated

Bonding considerations

donation:

M

vacant

dsp hybrid

or

π

σ

:backdonation

M

filled d orbital π∗

for M-CO:

M OC

dsp

n

acceptor

σ− donor

Structure

• saturated (18 e-) complexes:

- tetracoordinate: Ni(CO)4, Pd(PPh3)4 are tetrahedral

- pentacoordinate: Fe(CO)5 is trigonal bipyramidal

- hexacoordinate: Cr(CO)6 is octahedral

• unsaturated complexes have high dx2-y2;

16e- prefers square planar

x

y

z

Basic reaction mechanisms

- ligand substitution: M-L + L’ M-L’ + L

can be associative, dissociative, or radical chain.

trans-effect: kinetic effect of a ligand on the role of substitution at

the position trans to itself in a square or octahedral complex (ground-state

weakening of bond).

L M, repels negative charge to trans position.

M XL

- - -+ + +

Lt

P t

Lc

X

Lc

+ Nu

-

Lt

P t

Lc

X

Lc

N u

Lt

P t

X

N u

Lc

Lc

Lt

P t

Lc

N u

Lc

X

Lt

P t

Lc

N u

Lc

+ X

-

- oxidative addition:

[Ph3P]4Pd [Ph3P]3Pd [Ph3P]2Pd-L -L Ph

Br

strong σ-donor

16 e- 14 e-

Ph

H H

BrL2Pd

(2agostic e-/3-center) bond interactions

Ph

L2PdBr(+II)

16 e-16 e-

- reductive elimination: the major way in which transition metals are used

to make C,C- and C,H-bonds!

P h3

P

P d

M e P P h3

P h

P h3

P

P d

P h3

P M e

P h

P d

P h3

P M e

P h

-L

cis!

14e

-

?

P h3

P - P d

12e

-

P h

+L

- migratory insertion:

R

Z r

C p

C p

C l

16e

-

C O

R

Z r

C p

C p

C l

18e

-

C

O

Z r

C p

C p

C l

16e

-

R

O

- -elimination/hydrometalation:

R

Z r

C p

C p

C l

R

Z r

C p

C p

C l

R

Z r

C p

C p

C l

H H

H

-elimination

hydrometalation

- olefin metathesis:

( O C )5

C r

O M e

P h

18e

-

Fischer carbene complex

O E t

Δ , 80 °C

P h

O M e

+( O C )

5C r

O E t

( O C )5

C r

O M e

P h

E t O

- transmetalation:

R-M + M’-X R-M’ + M-X

Summary of Mechanisms:

- ligand substitution

- oxidative addition/reductive elimination- migratory insertion/-elimination (carbo-, hydrometalation)

- alkene metathesis

- transmetalation

Reactions with Wilkinson’s Catalyst

Alkene Hydrogenation

CO2Me

H2

cat. RhCl(PPh3)3

H2

cat. PtO2

CO2Me CO2Me

96:4

49:26

PPh3

Rh H

PPh3Cl

H

PPh3

Rh H

PPh3

Cl

H

R

RH

H

coordination

R

migratoryinsertion

reductiveelimination

oxidativeaddition

-PPh3

+PPh3

[RhCl(PPh3)2] RhCl(PPh3)3

H H

PPh3

Rh H

PPh3

HCl

R

Mechanism

Oi-PrOH, KOH

cat. RhCl(PPh3)3

OHi-PrOH, KOH

cat. RhCl(PPh3)3

OH

Reductions

Where does the hydrogen come from????

Et or EtHSiMe2Ph

cat. RhCl(PPh3)3

SiMe2PhEt

Hydrosilylation

THPO

OTHP

CHO

0.4 equivRhCl(PPh3)3

CH2Cl240 oC, 20 h

OTHPO

H

H

OTHP

Hydrocarbonylation

PhO

Et 1.0 equiv RhCl(PPh3)3

Ph H

Et

93% retention

Decarbonylation

NH

O H2, CO

cat. RhCl(PPh3)3

NH

O

CHO

other minor products

Hydroformylation

Ph

1. catecholborane cat. RhCl(PPh3)3 2. H2O2, OH-

OH

Ph

primary alcoholif the catalyst ispartially oxidized

Markownikow Hydroboration

TBDMSO

cat. RhCl(PPh3)3

TBDMSO

H

H

Cycloisomerization