Chemical Bonding Topology of Metal-Centered Polygonal Wheels: Two-Dimensional Analogues of Metallaboranes Related to Benzene and Cyclopentadienide.

The anion [Au@Ru(CO)(μ-CO)] has a pentagonal wheel structure that can be derived from a hypothetical pentagonal ruthenium carbonyl cluster Ru(CO) by insertion of a gold atom in the center, thereby splitting the original Ru pentagon in Ru(CO) into five AuRu triangles. The six electrons used to form 3c-2e bonds in three of the five AuRu triangles suggest a relationship to the aromatic sextet of the likewise pentagonal cyclopentadienide anion. Furthermore, the pentagonal wheel framework of [Au@Ru(CO)(μ-CO)] can be derived from a pentagonal bipyramid, such as that found in the deltahedral borane anion BH, by bringing the two C axial vertices together at the center of the equatorial pentagon. Similarly, the hexagonal wheel complexes Ni@PR and Pd@Pd(μ-N═CtBu) with six triangular faces can be derived from a hexagonal bipyramid, such as that found in the dirhenaborane (η-MeC)ReBHCl, by bringing the two C axial vertices together at the center of the equatorial hexagon. A reasonable chemical bonding model for the hexagonal wheel complexes has three-fold symmetry with 3c-2e bonds in three of these six triangular faces analogous to the C═C double bonds in a Kekulé structure of benzene.