Electronic Communication through Unsaturated Hydrocarbon Bridges in Homobimetallic Organometallic Complexes

Metal-metal interactions have been studied in a large number of different carbon bridged homobinuclear complexes. The bridging ligands considered are sp2- and/or sphydrocarbon chains, bridging aromatic rings linked directly as fulvalene and fulvalene-like; and fused aromatic spacers as indenyl, permethylpentalene-diide, and permethylfluorene between others. The efficiency of the intramolecular electronic communication between the metal centers depends on the nature of the metals, their oxidation state, the strength of the coupling between the acceptor and the donor sites, the ancillary ligands and the structure of the bridging ligand. Generally, the electronic communication decreases with increasing the distance between the donor and the acceptor sites. Also, electronic coupling and other donor-acceptor interactions are stronger through alkene than alkyne bridges. Bridging aromatic rings and fused aromatic linkers are also suitable spacers to promote metal-metal interactions. However, if their planarity is broken due to steric effects, the loss of aromaticity decreases the electronic communication between the metal centers. Metal-metal interactions have been mainly evaluated on the basis of IR, EPR, optical, and electrochemical measure-ments of the mixed-valence species. According to these data, the numerous complexes presented on this review have been categorized following the Robin and Day classification.