Structural systematics in penta- and hexanuclear metal clusters containing carbocyclic ligands

This article describes the structural chemistry of penta- and hexanuclear ruthenium and osmium carbonyl clusters which contain one or more arene (η⁶-Ar) or cyclopentadienyl (η⁵-Cp) groups, and compares and contrasts their structures with those of related binary carbonyl and carbonyl hydride clusters. The pentanuclear structures described are based on the trigonal bipyramidal metal framework, or on frameworks derived from this by the cleavage of one or more metal-metal edges. The steric and electronic factors which determine the position of the M(η⁶-Ar) or M(η⁵-Cp) units in the framework are discussed. For the hexanuclear clusters two basic metal core geometries are observed, the bicapped tetrahedron, comprising three fused tetrahedra, and the octahedron. The two series of compounds exhibit quite different structural chemistries. The bicapped tetrahedral structures exhibit quite extensive isomerism for the carbocyclic ligand substituted complexes, the positions of the M(η⁶-Ar) or M(η⁵-Cp) being dependent on both steric and electronic factors. For the octahedral carbocyclic ligand substituted complexes isomerism is less prevalent, but an analysis of the ligand polyhedra in these systems shows that low energy transformations, consistent with the fluxional properties of these complexes, are possible. The metal core geometries and ligand polyhedral geometries of related series of hexanuclear clusters are analysed statistically using half-normal probability plots and scatter plots.