The Orbital-Overlap Factor in Electron Transfer: Sensitivity of Homogeneous Self-Exchange Kinetics for Some Metallocenes to Electronic Structure.

Abstract : Rate constants, k(h)(ex), for the electron self exchange of cobaltocenium-cobaltocene, Cp2Co(+/o), and for the decamethyl derivative (Cp-Me5)2Co(+/0) obtained by using the NMR line broadening technique in acetonitrile and dimethylsulfoxide are compared with corresponding data for the ferrocene couples Cp2Fe(+/0) and (Cp-Me5)(2)Fe(+/0) and bis(benzene)chromium (I)/(O). The rate constants in a given solvent display and marked sensitivity to the reactant structure, the kh(ex) values being about tenfold larger for Cp2Do(+/0) relative to Cp2Fe(+/0); decamethyl substitution yields tenfold increases in kh(ex) for both these couples. A relationship is established between these ca. 100 fold rate variations and the nature of the donor and acceptor orbitals. In particular, the markedly slower self-exchange kinetics observed for Cp2Fe(+/0) relative to Cp2Co(+/0) are consistent with the much greater ligand-delocalized character of the Ze(lg) orbital involved in the latter electron transfer are compared with the 4e(2) or 8a(lg) orbital for the former reaction. Keywords: Line broadening, Electron transfer, Metallocenes, Orbital overlap factor.