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. 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.

Cl for each Pd: Ox. state Pd Pd , Cl Cl bridging by lone Pd(II) pairs on Cl; d: 10 (4d10 5s0) - 2 = 8 each Cl acts as a 2-electron, mono electron count: negative ligands to one of the Pd's, - and a 2-electron : 4e neutral donor ligand like PPh to the other 3 - Cl : 2e - Cl : 2e 8e- + d8 = 16e- unsaturated Ox. State: 2 Cp-, 1 H-, 1 Cl- H → Zr(IV) Zr Cl d: 4 (4d25s2) - 4 = 0

electron count: 2 Cp-: 12 H- : 2 Cl- : 2 Zr : 0 16 e-, unsaturated

Bonding considerations donation:

M or vacant σ dsp hybrid π backdonation:

M

filled d orbital π∗ for M-CO:

M C O

σ−donor dspn acceptor

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 z

y • unsaturated complexes have high dx2-y2; 16e- prefers square planar

x

Basic reaction mechanisms

- ligand substitution: M-L + Lʼ → M-Lʼ + L

can be associative, dissociative, or radical chain. L M X + - + - + -

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.

Nu Lc

Lc X Nu Lc X L Pt Pt + Nu- Pt t Lt Lc X Lt Lc Lc

L Nu Lc Nu c - Pt Pt + X L L Lt Lc t c X

- oxidative addition:

Br -L -L Ph [Ph3P]4Pd [Ph3P]3Pd [Ph3P]2Pd 16 e- 14 e- strong σ-donor (+II) Br Br L2Pd L2Pd H H Ph Ph 16 e- 16 e- agostic (2e-/3-center bond) interactions - reductive elimination: the major way in which transition metals are used to form C,C- and C,H-bonds!

Ph P Me -L Ph P Me Me PPh3 3 3 Pd Pd Pd Ph P-Pd Ph P 3 Ph3P 3 - 14e- 12e Ph cis! Ph ? Ph +L Ph

- migratory insertion:

Cp O Cp Cp CO C Zr Cp Zr Cp R Zr Cp R R Cl Cl Cl O 16e- 18e- - 16e