STRUCTURAL-CHANGES ASSOCIATED WITH ELECTRON-TRANSFER REACTIONS - ONE-STEP VS 2-STEP REACTIONS IN THE OXIDATION OF W(ETA(5)-C-5(CH3)(5))(CH3)(4)

In accord with the original interpretation of Liu et al. (J. Am. Chem. Soc. 1987, 109, 4282-4291), it has been demonstrated that electrochem ical oxidation of the title compound in methylene chloride proceeds by a two-step process by which the square-pyramidal neutral reactant (SP ) is oxidized to a cation of similar structure (SP+) followed by isome rization to the preferred trigonal-bipyramidal cation (TBP+). A combin ation of slow cyclic voltammetry near room temperature and fast scan e xperiments at low temperature has allowed determination of the thermod ynamic and kinetic parameters for the conversion of SP+ to TBP+ (Delta H degrees = -6.05 kcal/mol; Delta S degrees = -3.97 cal mol(-1) K-1; Delta G degrees(298) = 9.5 kcal/mol). The reversible formal potential for the SP/SP+ couple is +1.20 V vs cobaltocenium/cobaltocene in the same solvent and the reversible formal potential for the direct one-st ep oxidation of SP to TBP+ is 0.99 V, i.e., the direct oxidation requi res 5 kcal/mol less energy than the two-step reaction. The fact that t he reaction actually proceeds via the two-step pathway is explained by a large barrier to the direct oxidation, calculated to be greater tha n about 12 kcal/mol. It is argued that this large barrier is reasonabl e in view of the expected contribution from the outer and inner reorga nization energies.