Some dimetal fullerenes M₂@C₆₀ (M = Cr, Mo, W) have been studied with computational quantum chemistry methods. The transition metal diatomic molecules Cr₂, Mo₂, W₂ form exohedral complexes with C₆₀, while U₂ forms a highly symmetric endohedral compound and it is placed in the center of the C₆₀ cavity. This highly symmetric structure is an artifact due to the small size of the C₆₀ cavity, which constrains U₂ at the center. If a larger cavity is used, like C₇₀ or C₈₄, U₂ preferentially binds the internal walls of the cavity and the U−U bond no longer exists.