DFT calculations consistently suggest that a lanthanide will sit on either the 6/6 bond inside C-60 having a divalent state or the hexagonal center having a trivalent state. Some lanthanides can stay only above the 6/6 bond inside C-60 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)-C-80, Yb@C-s(6)-C-82 and Yb@C-2v(9)-C-82, 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 C-60 mono-EMFs, in the future.
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