Butterfly cluster compound

Most butterfly clusters have additional bridging ligands.

A carbide-containing butterfly cluster is [Fe4C(CO)12]2− where the carbide is bonded to all four Fe centers.

[2] Bonding in such clusters is often discussed in the context of polyhedral skeletal electron pair theory.

This theory predicts that tetrametallic clusters with 60 valence electrons will adopt tetrahedral geometry with six M-M bonds.

By addition of 2e, the 60e cluster opens one edge, as manifested in the butterfly motif.