Structural proposals for endohedral metal—fullerene complexes

A systematic search of isolated-pentagon fullerence cages reveals at least one closed-shell C ^2-_n anion for all even n in the range 70 to 90 except n = 72. It is argued that the more stable of these anions provide likely candidates for the structures of higher endohedral metal—fullerene complexes of the type MC _n, MBa, La, etc., in which the central metal atom donates two valence electrons from a high-lying large-radius s orbital to the delocalised π system of the fullerene cage. The recently observed LaC ₈₂ complex is predicted to be polar, with a C _3v symmetry fullerene cage.