Electronic structure of exohedral interactions between C 60 and transition metals

Abstract

The electron distribution and orbital interactions of C 60 with metals coordinated at different sites on the outside of the fullerene are evaluated. These sites include the position of a metal atom directly above a carbon atom (η 1 site), the metal atom centered above two carbons of a pentagon or above two carbons between two pentagons (both η 2 sites), the metal atom centered above a pentagon (η 5 site), and the metal atom centered above a hexagon (η 6 site). The frontier orbitals of C 60 are illustrated first with three-dimensional orbital contour plots. A palladium atom is then used to probe the bonding at the different sites on the C 60 surface. The results with Pd 0 are compared to our earlier study with the harder Ag + ion in order to examine the effects of metal electron richness and size. In addition, these results are compared with the bonding to more traditional ligands that represent the hapticity of these sites, such as methyl (η 1), ethylene (η 2), cyclopentadienyl (η 5), and benzene (η 6). The strength of the metal-C 60 interaction and the amount of charge delocalized from the metal to C 60 is sensitive to the site of coordination, the electron richness of the metal, and distortions in the geometry of C 60. As discussed in our previous work, the frontier orbitals of C 60 are well-suited for synergistic bonding of a metal atom to a carbon-carbon pair in an alkene-like fashion, in which the HOMO of C 60 donates carbon-carbon π bonding electron density to the metal, and the LUMO of C 60 accepts electron density from the metal into a carbon-carbon π* antibonding orbital. Although the HOMO and LUMO of C 60 describe the basic interaction, many frontier orbitals are involved. The site above the CC bond between two pentagons is favored over the site above the CC bond within a pentagon, and the interaction above the other sites is indicated to be net repulsive by these calculations. The differentiation between these sites increases with the electron richness of the metal center. The bonding of the metal to C 60 is generally weaker than to the small ligands, except for very electron rich metal centers where the bonding to the η 2 site between pentagons apparently becomes stronger than the bonding to ethylene.