Electronic structure and bonding of C60 to metals

The electron distribution and orbital interactions of C60 with metals coordinated at different sites on the outside of the fullerene are evaluated with the Fenske-Hall molecular orbital method. The characters and nodal properties of the frontier orbitals of C60 are first evaluated in terms of basis transformations to the C2 units that join the pentagons and to the C5 units of the pentagons in the C60 molecule. The highest occupied molecular orbital (HOMO, hu symmetry) of C60 is largely π bonding between the carbon atom pairs that join adjacent pentagons. The lowest unoccupied molecular orbital (LUMO, t1u symmetry) is predominantly π antibonding between these carbon atom pairs. These orbital characters and energies are well situated for synergistic bonding of a metal atom to the carbon-carbon pair between the pentagons, in which the HOMO of C60 donates σ electron density to the metal, and the LUMO of C60 accepts π electron density from the metal. The electron donation and acceptance between the individual molecular orbitals of the C60 molecule and the orbitals of a metal at different possible bonding sites of C60 are probed with a Ag+ ion. It is found that the bonding is favored at the site between the pentagons and that many different orbitals of C60 are involved in the interaction. The net bonding of Ag+ to C60 is weaker than to ethylene. Calculations are also carried out on the organometallic complexes C60Pt(PH3)2 and (C2H4)Pt(PH3)2. The net bonding of ethylene and C60 to platinum is found to be very similar in these cases. A significant difference in this case is that the net negative charge on C60 is more delocalized in the carbon cluster in contrast to the localized charge on ethylene.