Ionicity of the MC60 bond in M@C60 endohedral complexes

The character of the bond between encapsulated atoms and the enclosing C60 shell in M@C60 endohedral complexes is revisited using the density functional formalism. Mulliken population analysis indicates that this bond is strongly ionic in the case of K@C60, predominantly covalent in the case of O@C60, and has a mixed ionic and covalent character for Li@C60. In all these cases, van der Waals forces play only an insignificant role in the MC60 interaction. The significant polarizability of the C60 shell lies at the origin of the energy gain upon an off-center displacement of the encapsulated alkali ions.