Dl. Lichtenberger et al., ELECTRONIC-STRUCTURE OF EXOHEDRAL INTERACTIONS BETWEEN C-60 AND TRANSITION-METALS, Journal of organometallic chemistry, 478(1-2), 1994, pp. 213-221
The electron distribution and orbital interactions of C60 with metals
coordinated at different sites on the outside of the fullerene are eva
luated. These sites include the position of a metal atom directly abov
e a carbon atom (eta1 site), the metal atom centered above two carbons
of a pentagon or above two carbons between two pentagons (both eta2 s
ites), the metal atom centered above a pentagon (eta5 site), and the m
etal atom centered above a hexagon (eta6 site). The frontier orbitals
of C60 are illustrated first with three-dimensional orbital contour pl
ots. A palladium atom is then used to probe the bonding at the differe
nt 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 co
mpared with the bonding to more traditional ligands that represent the
hapticity of these sites, such as methyl (eta1), ethylene (eta2), cyc
lopentadienyl (eta5), and benzene (eta6). The strength of the metal-C6
0 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 C60. As discussed in our
previous work, the frontier orbitals of C60 are well-suited for syner
gistic bonding of a metal atom to a carbon-carbon pair in an alkene-li
ke fashion, in which the HOMO of C60 donates carbon-carbon pi bonding
electron density to the metal, and the LUMO of C60 accepts electron de
nsity from the metal into a carbon-carbon pi antibonding orbital. Alt
hough the HOMO and LUMO of C60 describe the basic interaction, many fr
ontier 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 pentago
n, and the interaction above the other sites is indicated to be net re
pulsive 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, ex
cept for very electron rich metal centers where the bonding to the eta
2 site between pentagons apparently becomes stronger than the bonding
to ethylene.