Reactions: Oxidative-Addition (OA) and Reductive-Elimination (RE)

Oxidative-Addition LnM + X-Y → LnM(X)(Y)

ƒ increases by 2 units ƒ common for electron counts of 16 e- or less

Vaska’s complex (shown below) is a classic example of 16 e-d8 compound that undergoes oxidative addition with a wide range of substrates and does so by a variety of mechanisms. Some of these mechanisms are dealt with in the following sections. a) Three-centre concerted additions

H-H, C-H and C-C additions are usually of this type

H2 addition

H P Cl P H Ir + H2 Ir OC P OC P

Cl thermodynamics

o ∆H is roughly 2 D(Ir-H) – D(H-H) = 2(-60) – (104) = -16 kcal/mol (-67 kJ/mol) ∆So is negative and large: -30 eu (-125 J mol-1 K-1) therefore ∆Go = -16 – (298)(30)/1000 = -7 kcal/mol (-29 kJ/mol)

kinetics rate = kobs[Complex][H2]

consistent with a concerted process

from: Organometallic by Spessard and Miessler M.O. picture for this reaction is straightforward:

(i) σ-donation from filled H2 σ orbital

(ii) π-back donation from metal orbital to H2 σ*

It is interaction (ii) that causes the H-H bond to break.

• metals with positive charges, higher oxidation states or competing π-acceptors cannot back donate well, so stable η2-

H2 complexes are obtained:

Compare:

H H HH R P PR R P CO 3 Mo 3 3 Mo R P PR OC PR 3 3 3 PR OC 3 7-coordinate 6-coordinate 18 electron 18 electron dihydride dihydrogen

HH + H H H + Ir IrH5(PCy3)2 H + Cy3P H

PCy3 6-coordinate 18 electron dihydrogen C-H addition

‘C-H activation’ is one of the most sought after reactions in : potential economic impact is huge

• intermolecular C-H activation is still rare but intramolecular cases (cyclometallation) are common (entropy effect)

hν R-H Ir - H Ir (solvent) Ir H 2 H Me3P Me3P Me3P H R Ir(III) 18 e- Ir(I) 16 e- Ir(III) 18 e- R = Me, Ph, Cy

- PEt Et P 3 3 Pt Et P Pt 3 Et3P Pt Et3P H H

+ PEt3 - CMe4

Et P 3 Pt Et3P b) Nucleophilic oxidative addition of RX

filled dz2 R L L slow L L M + RX M X- L L r.d.s. L L +

rate = k [M][RX] fast obs

R L L M L L

X

‡ ∆S is large and negative suggesting an SN2-like transition state: L L H δ− δ+ δ− M C X L L H H

- - - - - • rate depends on leaving group: CF3SO3 > I > OTs , Br > Cl • inversion observed at C

Ph Ph L - 2 L XC + PdL4 C Pd X

H R R H L

• polar solvents increase rate (polarized transition state) • electron-releasing increase rate (metal more nucleophilic)

Reductive-elimination

X L Y L L M M + XY L L L L

L Mn+2 Mn

• very important as the C-C bond forming step in catalytic cycles (where X and Y are organic groups) • cis orientation of X and Y is required for concerted elimination • proceeds with retention of stereochemistry at C:

nucleophilic O.A. PR PR Ph 3 (inversion) 3 Br Br + Me Pd PR3 Me Pd PR3 H D Me Ph Me H D concerted R.E. (retention)

Ph PR3 Me + Me Pd PR3 D H Br

• favoured by:

ƒ bulky ligands (relief of crowding) ƒ high oxidation state ƒ ancillary ligands that can stabilize the lower

oxidation state (CO, , PR3 etc.)

• intermolecular processes can be ruled out by conducting the crossover experiment:

PR3 PR3 heat D C Pd PR H C Pd PR 3 3 + 3 3 D3CCD3 + H3C CH3

CD3 CH3 but NO H3C CD3