Metal-ligand bond-energies in organometallic compounds

The energy contributions of metal-carbon and metal-ligand bonds in a variety of organometallic compounds are examined to seek trends as the metal or the ligand changes. In general, the bond energies D(M-X) for metals from the same group increase as the energy of atomization of the metal increases. Metal—carbon bond dissociation energies fall along the sequence D0(MC) in diatomic carbides > D(M-Cp) in cyclopentadienyls > D(M-arene) > D(M-alkyl) > D(M-CO) in metal carbonyls. For transition metals, the ligand bonding power of phosphine donors (PEt3 > PPh3) is larger than for nitrogen donors (pyridine > MeCN > NH3), and olefine donors have similar bonding power to CO and pyridine. The irregular changes in the dissociation energies, D(M2), of transition dimetals on moving across the Periodic Table are considered in relation to the 'valence-state' adopted by the metal in forming metal-metal bonds.