A carbonyl group bridging four metal atoms in a homoleptic carbonylmetal cluster: The remarkable case of Co₄(CO)₁₁

The unsaturated Co₄(CO)₁₁ is predicted by density functional theory (B3LYP and BP86 functionals) to have a structure with one μ₄-CO group bridging all four cobalt atoms in a Co₄ butterfly, with five Co-Co bonds in the range 2.44-2.56 Å. This μ₄-CO group is predicted to have a relatively long C-O bond of 1.226 Å (BP86) with a correspondingly low ν(CO) frequency of 1636 cm^-1. Co₄(CO)₁₁ structures of higher energy have Co₄ tetrahedra (6 Co-Co bonds) or Co₄ butterflies (5 Co-Co bonds) with a collection of terminal, edge-bridging, and face-bridging CO groups forming a total of 15 or 16 Co-C bonds. The predicted molecular structures show no signs of cobalt-cobalt multiple bonding.