Half-Wave Potential Splittings ΔE1/2as a Measure of Electronic Coupling in Mixed-Valent Systems: Triumphs and Defeats

This review is a cautionary note against the often purported direct relation between the half-wave potential splitting ΔE1/2 (or ΔE°) for stepwise, consecutive electron transfer from systems featuring two or more identical redox sites and the true electronic coupling HAB and charge or spin distribution in the ground state of intermittently formed mixed-valent (MV) systems. Several examples where these different quantities go in parallel are contrasted with other examples where this is not the case. Different kinds of such “non-conformist” behavior are outlined with the aid of representative examples. These include cases of fairly strong electronic couplings and large degrees of ground-state delocalization despite small values of ΔE1/2—sometimes just above the statistical limit or even below that—as well as examples for just the opposite behavior of no detectable electronic coupling despite appreciable electrochemical half-wave potential splitting. The crucial roles of the nominal bridges that interconnect the individual redox sites and of the environment (solvent, supporting electrolyte) in determining ΔE1/2 and HAB are emphasized. We also seek to provide some guidelines for the practitioner as to how to discriminate between these various types of behaviors and how to determine the strength of the electronic coupling between the redox sites.