Crystallographic X-ray analyses of Yb@C(2v)(3)-C80 reveal a feasible rule that governs the location of a rare earth metal inside a medium-sized fullerene.

Single crystal X-ray diffraction studies of Yb@C(2v)(3)-C(80)·Ni(II)(OEP)·CS(2)·1.5C(6)H(6) (OEP = octaethylporphinate) reveal that a relatively flat region of the fullerene interacts with the Ni(II)(OEP) molecule, featuring shape-matching interactions. Surprisingly, it is found that the internal metal is located under a hexagonal carbon ring apart from the 2-fold axis of the C(2v)(3)-C(80) cage, presenting the first example of metallofullerenes with an asymmetrically positioned metal. Such an anomalous location of Yb(2+) is associated with its strong ability to pursue a large coordination number and the lack of hexagon along the C(2) axis of C(2v)(3)-C(80). It is accordingly assumed that a suitable cage hexagon is most likely to be preferred by the single rare earth metal to stay behind inside a medium-sized fullerene, such as C(80) and C(82).