Insertion of tetrafluoroethylene into the iron-iron bond of (.mu.(SCH3)Fe(CO)3)2, its thermal rearrangement to a bridging carbene ligand, and the transformation of the carbene to a perfluoromethylcarbyne ligand. Structures of .mu.(SCH3)2.mu.(C2F4)Fe2(CO)6 and .mu.(SCH3)2.mu.(FCCF3)Fe2(CO)6 at - 162.degree.C