Theoretical and Empirical Relationships between the Redox Potentials and 1(δ → δ*) transition energies of quadruply bonded complexes

An analysis of the available electrochemical and electronic-spectroscopic data for quadruply metal-metal bonded complexes reveals that the (1)(delta --> delta*) transition energy is an approximately linear function of the difference between the electrode potentials at which they are reduced and oxidized to the bond-order 3.5 complexes. A theoretical framework for interpreting this correlation is provided by extending to the redox potentials a simple zero-differential-overlap model previously applied to (1)(delta --> delta*) transition energies. It is suggested on the basis of this model that the simplicity of the empirical relationship between (1)(delta --> delta*) energies and redox potentials is a consequence of the particular one- and two-electron energy terms that characterize these complexes; a linear relationship is not required by theory.