Metal cluster topology. 1. Osmium carbonyl clusters

Abstract

Important theoretical approaches to metal cluster bonding including the Wade-Mingos skeletal electron pair method, the Teo topological electron count, the King-Rouvray graph theory derived method, and Lauher's extended Hückel calculations are shown to agree in their apparent skeletal electron counts for the most prevalent metal cluster polyhedra including the tetrahedron, the trigonal bipyramid (both ordinary and elongated), square pyramid, octahedron, bicapped tetrahedron, pentagonal bipyramid, and capped octahedron. The graph theory derived method is used to treat osmium carbonyl clusters containing from five to eleven osmium atoms. In this connection most osmium carbonyl clusters can be classified into the following types: (1) Clusters exhibiting edge- localized bonding containing multiple tetrahedral chambers (e.g., Os 5(CO) 16, Os 6(CO) 18, H 2Os 7(CO) 20 and HOs 8(CO) 22 −); (2) Capped octahedral clusters derived from osmium carbonyl fragments of the type Os 6+ p (CO) 19+2 p ( p = 0, 1, 2, and 4) (e.g., Os 6- (CO) 18 2−, Os 7(CO) 21, Os 8(CO) 22 2−, and H 4Os 10- (CO) 24 2−). Other more unusual osmium carbonyl clusters such as the planar Os 6(CO) 17 [P(OCH 3) 3] 4, the Os 9 cluster [Os 9(CO) 21C 3H 2R] −, and the Os 11 cluster Os 11C(CO) 27 2− can also be treated satisfactorily by these methods. The importance of the number of ligands around isoelectronic Os n systems in determining the cluster polyhedron is illustrated by the different cluster polyhedra found for each member of the following isoelectronic pairs: HOs 6- (CO) 18 −/H 2Os 6(CO) 18. Os 7(CO) 21/H 2Os 7(CO) 20, Os 8(CO) 22 2−/HOs 8(CO) 22 −. The tendency for osmium carbonyl clusters frequently to form polyhedra exhibiting edge-localized rather than globally delocalized bonding relates to the facility for osmium carbonyl vertices to contribute more than three internal orbitals to the cluster bonding. In this way Wade's well-known analogy between boron hydride clusters and metal clusters, which assumes exactly three internal orbitals for each vertex atom, is frequently no longer followed in the case of osmium carbonyl clusters.