METALLOAROMATIC STABILIZATION OF METALLOCARBOHEDRENES

1. Guo, B.C.; Kerns, K.P.; Castleman, AW. JR.; Science 1992, 255. 1411.Author Institution: Department of Chemistry, The Ohio State UniversitySince their detection by mass $spectrometry^{1}$, metallocarbohedrenes (‘met cars’) of the general formula $MgC_{12}$ (M = Ti, V, Hf, Zr, etc.) have been the subject of numerous investigations. The most widely accepted idealised structure of met-cars is a $T_{h}$ structure analogous to the unknown icosahedral fullerene $C_{2}O$. We have used local density functional calculations to investigate the stability of $T_{h} M_{8}C_{12}$ vis-aA-vis the likely instability of $C_{20}$-Results indicate that the twenty-membered (fullerene is subject to anti-aromatic destabilization resulting from the partial population of a $\pi$-anti bonding HOMO. By contrast, the presence of energetically accessible metallic d orbitals in met-cars is seen to alleviate this effect by converting this non-bonding/antibonding HOMO to a purely bonding orbital, an interaction analogous to metalloaromaticity. It is further seen that striking parallels exist between the electronic structure of metalloaromaticity and that of $C_{60}$, and that these similarities constitute an alternative description of the met-cars that sheds light on their stability