Comparison of the Bonding of Benzene and C₆₀ to a Metal Cluster: Ru₃(CO)₉(μ³-η²,η ²,η²-C₆H₆) and Ru₃(CO)₉(μ³-η²,η ²,η²-C₆₀)

The electron distributions and bonding in Ru₃(CO)₉(μ³-η²,η ²,η²-C₆H₆) and Ru₃(CO)₉(μ³-η²,η ²,η²-C₆₀) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru₃(CO)₉ inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C₆₀ are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru-C_fullerene bond lengths are shorter than the corresponding Ru-C_benzene distances and the Ru-Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru₃(CO)₉(μ³-η²,η ²,η²-C₆₀) than for Ru₃(CO)₉(μ³-η²,η ²,η²-C₆H₆). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru-C_fullerene than Ru-C_benzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal-metal bonds to empty orbitals of C₆₀ and C₆H₆. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C₆₀ than to C₆H₆, thus accounting for the longer metal-metal bonds in the fullerene-bound cluster. The principal metal-arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C₆H₆ permits a greater degree of electron flow and stronger bonding between the Ru₃(CO)₉ and C₆₀ fragments. Of significance to the reduction chemistry of M₃(CO)₉(μ³-η²,η ²,η²-C₆₀) molecules, the HOMO is largely localized on the metal-carbonyl fragment and the LUMO is largely localized on the C₆₀ portion of the molecule. The localized C₆₀ character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C₆₀. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C₆₀.