Magnetic perturbation of the redox potentials of localized and delocalized mixed-valence complexes

The comproportionation constant ( K c) is larger for M = Fe than for M = Ru, but the metal–metal coupling in the mixed-valence states is larger for the Ru–Ru compound than for the Fe–Fe one. The potential differences (Δ E) between the two one-electron events observed for symmetrical mixed-valence (MV) complexes is generally used as a measurement of the thermodynamic stability of the MV state and often extended to the evaluation of the strength of the coupling between the redox centers through the bridge. In this review article, selected examples illustrate how the Δ E values to assess the degree of electronic communication between metals must be approached very judiciously. The role of the magnetic exchange which can take place between the unpaired spins carried by the redox sites in the doubly oxidized complexes is emphasized.