Theoretical aspects of the bonding in organometallic clusters containing exposed dicarbon (C-2) entities .2. High-nuclearity systems

The electronic and geometrical structures of a variety of LmMn-C-2 organometallic systems (n greater than or equal to 5) in which the dicarbon unit is solely bonded to metal atoms are analyzed and compared by use of molecular orbital calculations. It is shown that the arrangement of the M-5 and M-6 cores can be derived from a square-pyramidal and a trigonal-prismatic geometry, respectively. The bonding of the C-2 ligand with its metallic host follows the Dewar-Chatt-Duncanson model, resulting from an important forward electron donation from occupied C-2 orbitals toward acceptor metallic orbitals, accompanied by a back-donation from occupied metallic orbitals into vacant C-2 pi* orbitals.