Static and Dynamic Stereochemistry of the Organo-Metallic Cluster Complexes [(CpCo)₃(μ₃-Arene)]

Abstract

Nearly twenty years ago, and based on even earlier propositions [1], Earl L. Muetterties stated that “discrete, molecular metal clusters may be reasonable models of metal surfaces in the processes of chemisorption and catalysis” [2]. This “cluster surface analogy” appeared immediately obvious when the first molecular cluster complexes with face capping benzene ligands, [Os₃(CO)₉(μ₃-C₆H₆)] 1 and [Ru₆-(CO)₁₁(η⁶-C₆H₆)(μ₃-C₆H₆)] 2, were reported in 1985 [3]. In 1 and 2, a benzene ligand is coordinated in the μ₃-η²:η²:η² -bonding mode to a triangular array of metal atoms, very much like the so-called hollow [C _3V (ϭ_d)] adsorption site of benzene on a close packed metal surface [4]. In one of the best studied systems, Rh(lll)/C₆H₆/CO, benzene is adsorbed in such a hollow site and kept in place by the coadsorbed CO molecules. For this system, a strong Kekulé-type distortion of the C₆ ring was derived from an electron diffraction study, with alternating short (1.33(15) Â, “on top” of a metal atom) and long (1.81(15) Å, “between” two metal atoms) carbon carbon bonds [5]. In contrast, photoelectron spectroscopy pointed to an essentially sixfold symmetry for the adsorbed benzene, and a large distortion was ruled out [6]. This discrepancy still remains unsolved, although an interesting suggestion involving frustrated translations of the benzene parallel to the surface was recently made [7].