Exohedral 1:2 Complexes of Lanthanide Atoms with the C60 Fullerene: What Hapticity Is Preferred, η10, η11, or η12?

The structure of crystalline lanthanide-C60 fullerides allows for the Ln atom coordination to two rings of adjacent fullerene molecules simultaneously, where the resulting Ln hapticity can be η10, η11, or η12. We analyze the geometries, bonding strength, charge, spin, and frontier orbital characteristics for the 1:2 exohedral complexes of selected lanthanides (Ln = La, Ce, Pr, Eu, Gd, Tb, Yb, and Lu) with an unsubstituted C60 cage, for the three possible coordination patterns. We demonstrate that the increase in Ln hapticity from η10 to η12 favors stronger lanthanide-fullerene bonding and a lower degree of charge transfer from the metal to the carbon nanocluster. The interaction of the fullerene cage with an unpaired electron-rich Ln atom converts C60 to an open-shell structure, due to which the molecular spin of the complex can be about 2 e higher than the spin of the isolated Ln atom in its ground state. The study provides key atomistic details for application of lanthanide-C60 complexes in modern molecular and materials science and technology.