Coupled Two-Electron/Halide Transfer Reactions of Ruthenoceniums and Osmoceniums

Rate constants for the coupled two-electron/halide transfer self-exchange reactions of [Cp2MX]+ with Cp2M (M = Ru, Os; X = Cl, Br) have been measured in CD3CN at 298 K by 2D NMR spectroscopic techniques. Including data for the iodo complexes, the rates vary over 6 orders of magnitude depending on the metal and the halide, with the relative orderings I > Br > Cl and Ru > Os. The relative reactant state free energies for the ruthenium complexes have been determined from halide substitution equilibria and indicate that the reactant state energy ordering is Cl < Br < I. From these data and the free energies of activation for the self-exchange reactions, the relative transition state energies have been determined. A reaction coordinate and orbital correlation is proposed which rationalizes the observed data.