Group 9 Metal-Metal Bonds

In chapter 9, metal-metal bonded complexes of cobalt, rhodium, and iridium that emerged since the comprehensive monographs in 2005 are highlighted. The studies of newly synthesized dirhodium complexes, predominantly supported by carboxylate ligands, however, outnumber by far the others, primarily due to their long-standing importance, and high efficiency/activity in catalytic reactions, e.g., cyclopropanation, cyclopropenation, functionalization of C-H bonds, C-N bond formation, and others, which are critical for the synthesis of natural products and pharmaceuticals. Other notable advances highlighted herein pertain to the development of novel dirhodium photocatalysts as mediators for O2 reduction to H2O, HCl-splitting photocatalysts for H2 production and solar energy conversion, peptide-modifying agents and metallopeptide catalysts, promising candidates as sensitizers in solar cells, flexible molecular frameworks as hosts for gas-adsorption, e.g., H2, O2, CO2, CH4, N2, NO, chromogenic sensors or inhibitors of small molecules such as CO or NO, heterometallic chains, and others. New developments in catalysis, photocatalysis, and materials research in conjunction with the ongoing studies of dirhodium compounds as metallo-pharmaceuticals and photodynamic therapy agents, which are not detailed herein, will direct further exploration of these bimetallic species.