Mixed‐Valent Compounds and their Properties – Recent Developments

Recent evolutions, essentially in the last 15 years, are reviewed in the field of molecular mixed‐valent compounds and their properties. The considered systems are of the type M n –BL–M n where M n is either a monometallic terminal site ( n = 1) or a polymetallic cluster ( n = 2, 3), and BL is a bridging ligand. A large variety of bridging ligands is considered: non‐innocent, long conjugated, ambidentate, cross‐conjugated, H‐bonded, switching. The theoretical descriptions are briefly reviewed and sorted according to their degree of complexity/accuracy: simple orbital models, modified Marcus–Hush diagrams, 3D diagrams with two nuclear degrees of freedom, and finally quantitative models (mainly from DFT) with geometrical optimization. With improved DFT functionals, the prediction of the class II or III nature of a symmetrical complex can now be addressed. Finally, applications in NIR electrochromism and molecular electronics are briefly reviewed.