Polynuclear carbon-rich organometallic complexes: clarification of the role of the bridging ligand in the redox properties.

In this Perspective, we highlight the non-innocent behaviour of the bridging ligand in organometallic polynuclear metallic complexes displaying metal-carbon σ bonds between the metallic units and a strongly coupled conjugated carbon-rich bridging ligand. With the help of representative experimental and theoretical studies on polymetallic systems, but also on monometallic complexes, we point out that the level of implication of the carbon rich ligand in the redox processes is very sensitive to the nature of (i) the metal(s), (ii) the ancillary ligands and (iii) the carbon-rich ligand itself, and that this participation is frequently found to be major. Consequently, the general denomination M((n + 1)) that is usually used for oxidized species gives the picture that only the metal density is affected, which is misleading. Moreover, for polymetallic species, these elements make the mixed valence denomination and the use of standard methodologies to rationalize intramolecular electron transfer, such as the Hush model inaccurate. Indeed, these theoretical treatments of mixed-valent complexes have at their core the assumption of metal-based redox state changes. Quantum mechanical calculations, coupled with spectroscopic methods, such as EPR spectroscopy, turn out to be a valuable suite of tools to both identify and better describe those systems with appreciable ligand redox non-innocent character. Finally, some examples and perspectives of applications for this carbon-rich type of complexes that take advantage of their peculiar electronic structure are presented.