Valence tautomerism in metal complexes: Stimulated and reversible intramolecular electron transfer between metal centers and organic ligands

• Valence tautomeric (VT) transitions in metal complexes involve a stimulated intramolecular electron transfer between ligand and metal.
• Cobalt-dioxolene complexes and complexes with other redox-active metal–ligand combinations display VT transitions.
• Valence tautomerism has been reported for mononuclear, polynuclear and polymeric complexes.
• The combination of VT behavior with other physical and chemical properties can generate bifunctional molecule-based materials.

The defining characteristic of a valence tautomeric (VT) transition in a metal complex with a redox-active organic ligand is a stimulated intramolecular electron transfer between metal and ligand. Reversible VT behavior is well established for octahedral cobalt complexes with o-dioxolene ligands, with other combinations of metal centers and organic ligands also capable of exhibiting the phenomenon. Although the thermodynamic basis of VT transitions in cobalt-dioxolene systems has been understood for some time, it has not necessarily been elucidated for the less common systems. In general a number of factors additional to the redox-active metal–ligand unit influence the manifestation of the VT transition. These include the ancillary ligands, counterions, solvent molecules and intermolecular interactions, with cooperativity between VT molecules important for hysteresis and bistability. Recent advances in the field include the development of new metal–ligand combinations that can display VT transitions and the recognition that stimuli in addition to heat, light and pressure can induce the transitions. Considerable effort is also currently being directed toward assembling multiple VT moieties into polynuclear and polymeric complexes and combining VT behavior with other physical and chemical properties to generate bifunctional molecule-based materials.

Valence tautomeric (VT) transitions involve stimulated intramolecular electron transfer between a redox-active metal center and a redox-active organic ligand; reversible VT transitions induced by temperature, pressure, light-irradiation and other stimuli have been observed for systems involving d- and f-block metals with a variety of redox-active ligands. This review focuses on VT transitions involving new metal–ligand combinations, transitions in polynuclear and polymeric complexes and bifunctional materials that combine VT transitions with other chemical and physical properties.Figure optionsDownload high-quality image (123 K)Download as PowerPoint slide