Unified View of Marcus Electron Transfer and Mulliken Charge Transfer Theories in Organometallic Chemistry. Steric Effects in Alkylmetals as Quantitative Probes for Outer-Sphere and Inner-Sphere Mechanisms

Electron transfer rate constants for various homoleptic organometals, especially tetraalkyltin, tetraalkyllead, and dialkylmercury, with tris( 1,10-phenanthroline)iron(III), hexachloroiridate(IV), and tetracyanoethylene are compared under standard reaction conditions. Although electron transfer from alkylmetals to FeL₃³⁺ (where L = 1,10-phenanthroline) follows the Marcus correlation with the predicted slope α = 0.5 for an outer-sphere mechanism, both IrCl62- and TCNE show deviations which vary with the steric hindrance in the alkylmetal, RM. Inner-sphere mechanisms forthe latter are also indicated by electron-transfer rates which can be 107-9 times faster than those predicted by the Marcus equation using measured values of the reduction potentials and the rorganizational energies of IrCl₆^2- and TCNE.This conclusion is supported by differences in the cleavage selectivities S(Et/Me) for a series of methylethyltin compounds as well as the activation parameters for electron transfer. The Mulliken theory of charge transfer (CT) in TCNE complexes is used to evaluate steric effects in various alkylmetals. The difference AE in the charge transfer transition energy hv ct of the CT complex relative to that of a reference alkylmetal is assigned to steric effects on the interaction energy associated with ion-pair formation in the successor complex for the inner-sphere mechanism. The result is the linear free energy relationship, ΔG = ΔG° + ΔE, which can be applied to the kinetics of electron transfer from alkylmetals in the absence of steric effects. The unification of Marcus electron transfer and Mulliken charge transfer theories in this manner allows outer-sphere and inner-sphere mechanisms of electron transfer to evolve from a single viewpoint.