lecture 5: organometallic reactions i ligand substitution

LECTURE 5: ORGANOMETALLIC REACTIONS ILIGAND SUBSTITUTION

LIGAND SUBSTITUTION

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

Ligand Substitution 4

LIGAND 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

LIGAND SUBSTITUTION

This premise is not always correct, but it appliesin 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

2 MAIN MECHANISTIC PATHWAYS

ASSOCIATIVE (A):

LnML’ + L’’ slow LnML’L’’ fast LnML’’ + L’

DISSOCIATIVE (D): L’’LnML’ slow LnM fast LnML’’

+ L’

Ligand Substitution 7

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 8

DISSOCIATIVE SUBSTITUTION at 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 9

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

trans influence and trans effect

• In square planar complexes, some ligands direct substitution to a position trans to themselves.

• When reaction is controlled by factors influencing the ground state energy of the complex – trans influence

• Reaction is controlled by factors affecting the transition state energy.

trans influence

Ligands that form strong bonds or tend to weaken the metal ligand bond trans to the metal.

In the ground state this property is called the trans influence. H- > PR3 > SCN- > I-, CH3

-, CO, CN- > Br- > Cl- > NH3 > OH-

trans kinetic effect

• Tendency of certain ligands to direct incoming groups to trans position with reactions under kinetic control.

C2H4, CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3> OH-

overall trans effect

CO, CN-, NO C2H4 > PR3, H- > CH3-, S=C(NH2)2 > Ph-

NO2- SCN-, I-, > Br- > Cl- > Py, NH3, OH- H2O

Exercise 7.1

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

rate

Rate = ks[ML4] = k1 [ML4][Y]

Mechanism: Square pyramidal – trigonal bipyramid – with retention of configuration.

Associative substitution with 18 e systems

• Can occur if the metal can delocalize a pair of electrons onto one of its ligands

Ligand Substitution 18

LIGAND REARRANGEMENT

Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactionsof 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 19

DISSOCIATIVE LIGAND SUBSTITUTION

Example:

Rate = k [ML6]

LnM CO LnM LnM L'+ CO L'

18 e 16 e 18 e

Rate of substitution of Ligands

Rate of substitutions of a particular ligand is a function of ligand type.

Ligands that are nuetral in their free state dissociate rather easily.

Ligand Substitution 22

Redox-induced ligand substitutionUnlike 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 23

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 abstractionproduces a 17-e species

Ligand Substitution 24

Photochemical ligand substitutionVisible 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

d

dh

M(CO)6