Metal cluster topology. 2. Gold clusters

Abstract

Previously developed methods for the treatment of polyhedral boranes, carboranes, and metal clusters are extended to the treatment of gold clusters, which present a variety of new problems. In most cases gold atoms in such cluster compounds do not employ the usual 9-orbital sp 3d 5 spherical bonding orbital manifold. Instead almost all non-tetrahedral gold clusters consist of a center gold atom surrounded by a puckered polygonal belt of peripheral gold atoms generally with one or more additional peripheral gold atoms in distal positions above and/or below the belt. The peripheral gold atoms in such clusters use a 7-orbital spd 5 cylindrical bonding orbital manifold, but their residual two orthogonal anti- bonding p orbitals can receive electron density from the filled d orbitals of adjacent peripheral gold atoms through dσ → pσ * and/or dπ → pπ * backbonding leading to bonding distances between adjacent peripheral gold atoms. Centered gold clusters can be classified into either spherical or toroidal clusters depending upon whether the center gold atom uses a 9-orbital sp 3d 5 spherical bonding orbital manifold or an 8-orbital sp 2d 5 toroidal bonding orbital manifold, respectively. The topology of the core bonding in gold clusters is generally not that of the K n complete graph found in other clusters but instead mimics the topology of the polyhedron formed by the surface atoms. This apparently is a consequence of the poor lateral overlap of the cylindrical spd 5 manifolds of the peripheral gold atoms. Examples of non-centered gold clusters treated in this paper include the squashed pentagonal bipyramidal Au 7(PPh 3) 7 + and the edge-fused bitetrahedral (Ph 3P) 4Au 6[Co(CO) 4] 2 which may be regarded as a ‘perauraethylene’ in which the six cluster gold atoms correspond to the six atoms of ethylene including a double bond between the two gold atoms corresponding to the two ethylene carbon atoms.