– Isolobal Analogy Inclusion of the ligand η-C5H5

- which, as a donor of 3 π-electron pairs formally occupies 3 coordination sites, yields the analogies:

The following molecules are related:

1

– Isolobal Analogy

Another analogy is that between cyclopropane and µ-alkylidene complexes:

2

– Isolobal Analogy

3

xxxx

– Isolobal Analogy

4

– Isolobal Analogy The realm of isolobal connections is considerably extended if the mutual replacement of σ-donor ligands and metal electron pairs is introduced:

5

– Isolobal Analogy

6

– Isolobal Analogy

7

– Isolobal Analogy

8

– Isolobal Analogy

9

– Isolobal Analogy

Examples of isolobal fragments:

10

Ligand Substitution Reactivity of Coordinated Ligands

H2O PdCl

Cl

H2O PdH

ClOH

H2O PdH

Cl O

Cl Pd ClCl

Cl 2--

Cl PdCl

Cl

H2O PdCl

OH

H2O PdCl

OH"β-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-

β-H elim

Ligand Substitution 12

Why care about substitution ?

Basic premise about metal-catalyzed reactions: •  Reactions happen in the coordination sphere of the metal •  Reactants (substrates) come in, react, and leave again •  Binding or dissociation of a ligand is often

the slow, rate-determining step

This premise is not always correct, but it applies in the vast majority of cases.

Notable exceptions: •  Electron-transfer reactions •  Activation of a single substrate for external attack

–  peroxy-acids for olefin epoxidation –  CO and olefins for nucleophilic attack

Ligand Substitution 13

Dissociative ligand substitution

Example: Factors influencing ease of dissociation: •  1st row < 2nd row > 3rd 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'+ CO L'

18 e 16 e 18 e

Ligand Substitution 14

Dissociative substitution in ML6

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

The 5-coordinate intermediates are normally too reactive to be

observed unless one uses matrix isolation techniques.

18-e

oct

16-e

SP distortedTBP

or

Ligand Substitution 15

Associative ligand substitution

Example: Sometimes the solvent is involved.

Reactivity of cis-platin:

16 e 18 e

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

- L

16 e

(NH3)2PtCl2 (NH3)2Pt(Cl)(Br)

(NH3)2Pt(Cl)(H2O)+

Br-

- Cl-

H2O - Cl- Br-- H2O

NucleoBase - H2O

(NH3)2Pt(Cl)(NB)+

slow

fast

fast- Cl- slow

Ligand Substitution 16

Ligand rearrangement

Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactions of an apparently saturated complex:

M NO

M N O

3-e 1-e

M M3-e5-eM

CO

RM

O

R

(1+2)-e 1-e

Ligand Substitution 17

Redox-induced ligand substitution

Unlike 18-e complexes, 17-e and 19-e complexes are labile. Oxidation and reduction can induce rapid ligand substitution.

•  Reduction promotes dissociative substitution. •  Oxidation promotes associative substitution. •  In favourable cases, the product oxidizes/reduces

the starting material ⇒ redox catalysis.

L'

LnM

LnM+ LnM L' +

LnM- Ln-1M- + L

- e-

+ e-18-e

17-e 19-e

19-e 17-e

Ligand Substitution 18

Redox-induced ligand substitution

CO

L

Fe(CO)4L

Fe(CO)4L

Fe(CO)4

Fe(CO)5

Fe(CO)5

Initiation by added reductant.

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

Ligand Substitution 19

Photochemical ligand substitution

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

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

or use UV if the complex absorbs there

dπ

dσhν

M(CO)6

Ligand Substitution 20

Photochemical ligand substitution

Some ligands have a low-lying π* orbital and undergo Metal-to-Ligand Charge Transfer (MLCT) excitation. This leads to easy associative substitution.

–  The excited state is formally (n-1)-e ! –  Similar to oxidation-induced substitution

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

dπ

dσhν

M(CO)4(bipy)

π*

HOMO LUMO

Ligand Substitution 21

Electrophilic and nucleophilic attack on activated ligands

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

H2O is acidic enough to protonate this coordinated ethene. Without the metal, protonating ethene requires H2SO4 or similar.

NN

NRh S

N

++

SH+

NN

NRh

N

+

Ligand Substitution 22

Electrophilic and nucleophilic attack on activated ligands

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

-

CrOCOC

CO

Bu

HLi+

CrOCOC

CO

BuLi

BuLi does not add to free benzene, it would at best metallate it (and even that is hard to do).

Ligand Substitution 23

Electrophilic attack on ligand

Hapticity may increase or decrease. Formal oxidation state of metal may increase.

H+MI

+

MI

H+M(0) MII

+

Ligand Substitution 24

Electrophilic addition

•  Is formally oxidation of Fe(0) to FeII (the ligand becomes anionic). •  Ligand hapticity increases to compensate for loss of electron.

O

Fe(CO)3

OEt+

Fe(CO)3

Et3O+

Ligand Substitution 25

Electrophilic abstraction

Electrophilic abstraction also by Ph3C+, H+

Alkyl exchange also starts with electrophilic attack:

Cp2ZrMe

Me

B(C6F5)3 Cp2ZrMe

+

MeB(C6F5)3-

16 e 14 e

+

Zn

Me

MeZnMe

Me

MeZn

Me

MeZn

Me

Ligand Substitution 26

Electrophilic attack at the metal

If the metal has lone pairs, it may compete with the ligand for electrophilic attack

Transfer of the electrophile to the ligand may then still occur in a separate subsequent step

Fe

+

Fe H

Ni

H+

H+Ni

+?via?

Ligand Substitution 27

Electrophilic attack at the metal

Can be the start of oxidative addition (although this could also happen

via concerted 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"

H2O

HI CH3OH

CH3I

CH3COOH

CH3COI

Rh(CO)2I2-

MeRh(CO)2I3-

MeCORh(CO)I3-

MeCORh(CO)2I3-

CO