The Role of Electron Transfer and Charge Transfer in Organometallic Chemistry

The principal objective of this lecture is to describe the efficacy of organometals as electron donors, and to show how this property dominates many aspects of their chemistry. First, the structural effects of alkyl ligands (B) on the highest occupied molecular orbital (HOMO) of the neutral alkylmetals, R4Sn, R4Pb and R2Hg, are probed by photoelectron spectroscopy. The chemical properties of the resultant paramagnetic cation is then discussed. Electron transfer from alkylmetals to iron(III) complexes is shown to proceed via an outer-sphere mechanism, whereas that to iridate(IV) and tetracyanoethylene are inner-sphere processes. The difference lies in their response to steric effects in the alkylmetals. Steric effects are quantitatively evaluated with the aid of charge transfer transition energies in the absorption spectra of alkylmetal-tetracyanoethylene complexes. After correction for the steric effect, the rates of electron transfer from alkylmetal to both hexachloroiridate(IV) and tetracyanoethylene follow a linear free energy relationship with a Brönsted slope = 1, predicted by Marcus theory for inner-sphere mechanisms. The generalized concept of charge transfer is applied to a variety of organonickel systems, including (1) the oxidative addition of aryl halides to triethylphosphinenickel(0) complexes, (2) biaryl synthesis from the induced decomposition of arylnickel(II) halides, (3) oxygen atom transfer from organic nitro compounds to coordinated phosphines in nickel(0) complexes and finally even to (4) n-ligand substitution of benzophenones into phosphinenickel(0) complexes.