Mixedvalence

Mixed-valence describes compounds in which two or more metal centers within the same molecule or solid have different oxidation states but are sufficiently electronically linked to allow some degree of charge redistribution between centers. The centers are typically connected by bridging ligands, direct metal–metal bonds, or through delocalized ligand frameworks, enabling interaction between the valence sites.

Key features of mixed-valence systems include electronic communication between sites, possible localization or delocalization of valence, and distinctive spectroscopic or electrochemical signatures. A hallmark is the intervalence charge transfer, or IVCT, band observed in visible or near-IR regions when electrons can move between centers through the bridging pathway. Redox properties often reveal a balance between localized and delocalized valence states, giving rise to characteristic multiple oxidation states and conductivity behavior in solids.

The Robin–Day classification system, widely used for mixed-valence chemistry, divides compounds into three classes based on the strength of electronic interaction between centers. Class I shows no significant interaction, with fully localized valence states and no IVCT band. Class II exhibits intermediate interaction: one valence center is partially localized in the ground state and a noticeable IVCT band is present. Class III features strong interaction with full delocalization of the valence electron over the centers, effectively producing a single, averaged valence state and often a weak or absent IVCT band.

Classic examples include Prussian blue, Fe4[Fe(CN)6]3·xH2O, which contains Fe(II)–Fe(III) pairs linked by cyanide. Other well-studied systems are dinuclear ruthenium and iron complexes and various oxide- or cyanide-bridged clusters.

Mixed-valence concepts inform understanding of electron transfer, conductivity, and redox behavior in chemistry and materials science.