Electronic structure of exohedral interactions between C₆₀ and transition metals

The electron distribution and orbital interactions of C₆₀ 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 (η¹ site), the metal atom centered above two carbons of a pentagon or above two carbons between two pentagons (both η² sites), the metal atom centered above a pentagon (η⁵ site), and the metal atom centered above a hexagon (η⁶ site). The frontier orbitals of C₆₀ 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 C₆₀ surface. The results with Pd⁰ 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 (η¹), ethylene (η²), cyclopentadienyl (η⁵), and benzene (η⁶). The strength of the metal-C₆₀ interaction and the amount of charge delocalized from the metal to C₆₀ is sensitive to the site of coordination, the electron richness of the metal, and distortions in the geometry of C₆₀. As discussed in our previous work, the frontier orbitals of C₆₀ 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 C₆₀ donates carbon-carbon π bonding electron density to the metal, and the LUMO of C₆₀ accepts electron density from the metal into a carbon-carbon π* antibonding orbital. Although the HOMO and LUMO of C₆₀ describe the basic interaction, many frontier orbitals are involved. The site above the CC bond between two pentagons is favored over the site above the CC 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 C₆₀ is generally weaker than to the small ligands, except for very electron rich metal centers where the bonding to the η² site between pentagons apparently becomes stronger than the bonding to ethylene.