Some dimetal fullerenes M-2@C-60 (M = Cr, Mo, W) have been studied with computational quantum chemistry methods. The transition metal diatomic molecules Cr-2, Mo-2, W-2 form exohedral complexes with C-60, while U-2 forms a highly symmetric endohedral compound and it is placed in the center of the C-60 cavity. This highly symmetric structure is an artifact due to the small size of the C-60 cavity, which constrains U-2 at the center. If a larger cavity is used, like C-70 or C-84, U-2 preferentially binds the internal walls of the cavity and the U-U bond no longer exists.