Comparison of the bonding of benzene and C-60 to a metal cluster: Ru-3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C6H6) and Ru-3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C-60)

The electron distributions and bonding in Ru-3(CO)(9)(mu(3)-eta(2),eta(2),e ta(2)-C6H6) and RU3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C-60) are examined vi a electronic structure calculations in order to compare the nature of ligat ion of benzene and buckminsterfullerene to the common Ru-3(CO)(9) inorganic cluster. A fragment orbital approach, which is aided by the relatively hig h symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures sho w that both benzene and C-60 are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the en ergies of these distortions are similar. The experimental Ru-C-fullerene bo nd lengths are shorter than the corresponding RU-C-benzene distances and th e Ru-Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are sli ghtly higher for Ru-3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C-60) than for RU3( CO)(9)(mu(3)-eta(2),eta(2),eta(2)- C6H6). As a whole, these observations su ggest 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 a re 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 met al-metal bonds in the fullerene-bound cluster. The principal metal-arene bo nding 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 g reater degree of electron flow and stronger bonding between the Ru-3(CO)(9) and C-60 fragments. Of significance to the reduction chemistry of M-3(CO)( 9)(mu(3)-eta(2),eta(2),eta(2)-C-60) molecules, the HOMO is largely localize d on the metal-carbonyl fragment and the LUMO is largely localized on the C -60 portion of the molecule. The localized C-60 character of the LUMO is co nsistent with the similarity of the first two reductions of this class of m olecules to the first two reductions of free C-60 The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the met al fragment, thus accounting for the relative ease of the third reduction o f this class of molecules compared to the third reduction of free C-60.