Divalent metals can reside on bonds in fullerenes

DFT calculations consistently suggest that a lanthanide will sit on either the 6/6 bond inside C₆₀ having a divalent state or the hexagonal center having a trivalent state. Some lanthanides can stay only above the 6/6 bond inside C₆₀ to form stabilized structures, despite the greatly reduced metal-cage coordination numbers. The preference for C-C bonds by a divalent metal has been confirmed by revisiting the structures of Yb@C_2v(3)-C₈₀, Yb@C_s(6)-C₈₂ and Yb@C_2v(9)-C₈₂, for which the calculations suggest that the Yb atoms are indeed situated above the C-C bonds, close to the reported structures obtained by single crystal XRD experiments. The result will guide the characterization of structures and electronic structures of endohedral metallofullerenes (EMFs), especially C₆₀ mono-EMFs, in the future.