Valence tautomerism: More actors than just electroactive ligands and metal ions

Valence tautomeric complexes combining electroactive ligands and transition metal ions are known to exhibit two nearly degenerated electronic states (tautomers) with localized electronic structures. The intramolecular electron transfer (IET) led between both redox-active units, is activated by an external stimuli such as photons, temperature and/or pressure, brings to a reversible interconversion between the degenerated electronic states, indicating an important sensibility of the charge distribution onto the environment. Moreover, the IET confers different chemical and physical properties to both tautomers converting these molecules as promise for applications as storage memories and/or devices, for example. Since this phenomenon was first discovered in the late 1980s, most of the valence tautomeric complexes so far reported are based on quinone or quinone-type ligands with a series of transition metal ions such as Cu, Rh, Ir, Ni, Mn and Co, which makes them good candidates for studying the role of various parameters governing the IET rates. But finally recent studies focus the attention on the external centers where the IET is taking place, such as counterligand, the partial oxidation or reduction of the complex and the nature of the matrix in which the molecule is surrounded. These factors are important for tuning and controlling the temperature of the electron transfer. In this review we are going to give an overall view of all the five actors that influence the IET scenario, as the key for constructing new molecules, enhancing or combining multifunctional properties.We acknowledge financial support of the Ministerio de Educación y Ciencia under grant (MAT2006-13765-C02) and the network of Excellence Magmanet. E.E. thanks the Ministerio de Educación y Ciencia for a predoctoral grant.Peer Reviewe