In the area of metal cluster chemistry, a butterfly cluster compound usually describes tetrametallic clusters containing five M-M bonds. A prototype of this motif is [Re4(CO)12]2−. Most butterfly clusters have additional bridging ligands. One example is the pentaphosphide [[Rh4(CO)5(PPh2)5]]− where all Rh---Rh edges are bridged by PPh2. A carbide-containing butterfly cluster is [Fe4C(CO)12]2− where the carbide is bonded to all four Fe centers.