Non-innocent behaviour in mononuclear and polynuclear complexes: consequences for redox and electronic spectroscopic properties

This article reviews recent work in the area of non-innocent behaviour in polynuclear metal complexes. Non-innocence, which occurs when metal-based and ligand-based redox orbitals are similar in energy, has been known since the first dithiolene complexes of the Ni triad. Our recent work in this field is with complexes of two distinct types: polynuclear complexes of Ru(II) with dioxolene-type bridging ligands; and dinuclear complexes based on tris(pyrazolyl)borato-Mo(V) or -Mo(I) units linked by bis-phenolate or bis-pyridyl bridging ligands. Detailed redox and UV/Vis/NIR spectroelectrochemical studies on these complexes have been carried out. An important point which emerges is that non-innocent behaviour in dinuclear complexes is an essential prerequisite for strong metal–metal electronic coupling across extended bridging ligands. Many of the complexes studied show intense charge-transfer transitions in the near-IR region of the spectrum, and the use of these in prototypical optical devices is briefly discussed.