Structure and chemistry of a metal cluster with a four-coordinate carbide carbon atom

Molecular metal clusters with carbide carbon atoms of low coordination number have been prepared; they are the anionic HFe₄C(CO)₁₂^-] and Fe₄C(CO)₁₂^2-] clusters. An x-ray crystallographic analysis of a tetraaminozinc salt of the latter has established a butterfly array of iron atoms with the carbide carbon atom centered above the wings of the Fe₄ core. Each iron atom was bonded to three peripheral carbonyl ligands. The distances from the carbide carbon to iron were relatively short, particularly those to the apical iron atoms (1.80 Å average). Protonation of the anionic carbide clusters reversibly yielded HFe₄(CH)(CO)₁₂, and methylation of the dianion gave {Fe₄CC(O)CH₃](CO)₁₂^-}. Oxidation of Fe₄C(CO)₁₂^2-] yielded the coordinately unsaturated Fe₄C(CO)₁₂ cluster, which was extremely reactive. Hydrogen addition to this iron cluster was rapid below 0°C, and a C—H bond was formed in this transformation.