METAL CLUSTER TOPOLOGY .14. FUSION OF OCTAHEDRA IN METAL-CARBONYL CLUSTERS

Two metal carbonyl cluster octahedra can be fused by sharing a vertex to give an M11 cluster, by sharing an edge to give an M10 cluster, or by sharing a face to give an M9 cluster. In a globally delocalized met al carbonyl cluster consisting of fused octahedra, vertices unique to a single octahedron use three internal orbitals, vertices of a face sh ared by two octahedra use four internal orbitals, vertices of an edge shared by two octahedra use five internal orbitals, and vertices of a face shared by two octahedra use six internal orbitals. Using this pri nciple the iridium carbonyl clusters Ir9(CO)203- and Ir12(CO)262-, con structed by face-sharing fusion of two and three octahedra, respective ly, are seen to be electron- and orbital-precise and thus may be regar ded as three-dimensional analogues of naphthalene and anthracene, resp ectively. These iridium carbonyl clusters are the second and third mem bers of a homologous series Ir3+3n(CO)(21+11n)/2; the end member of th is series is the polymer [Ir3(CO)11]n consisting of an infinite chain of octahedra sharing opposite faces. Other electron- and orbital-preci se clusters constructed by the face-sharing fusion of two octahedra ar e the mixed metal derivative Ir3Ni6(CO)173- and the Ni9 core in nickel clusters Ni12(CO)21H4-nn-; in the latter clusters the three symmetry- related edges of the central Ni9 core are bridged by Ni(CO)2 groups. T he rhodium carbonyl clusters Rh9(CO)193- and H2Rh12(CO)25 both have tw o skeletal electrons less than their iridium carbonyl counterparts Ir9 (CO)203- and Ir12(CO)262- , respectively, and can both be regarded as missing a pair of core bonding electrons; this arises by interaction b etween the symmetric S(sigma) core molecular orbitals in adjacent face -fused octahedra in the rhodium clusters which raises the energy of on e these orbitals to antibonding levels. The gold-nickel 'spiro' mixed cluster Au6Ni12(CO)242- consists of four Ni3Au3 octahedra which are fu sed by sharing each of the six gold vertices between two octahedra lea ding to overall T(d) symmetry; this cluster as well as Ru10C2(CO)242- consisting of two edge-sharing octahedra are electron- and orbital-pre cise.