For the donor bis(dimethylthieno)tetratellurafulvalene (BDMT-TTeF), al
ready known to form red triclinic crystals (space group Fl), a new bla
ck monoclinic polymorph (space group P(2)1/n) was found, with a differ
ent molecular geometry. A stable 1:1 complex of BDMT-TTeF is formed wi
th buckminsterfullerene, C-60 and CS2 (space group C-2/c). which is ne
utral and electrically insulating, and has a magnetic transition at 26
0 K, probably associated with incipient rotation of C-60.