A Carbonyl Group Bridging Four Metal Atoms in a Homoleptic Carbonylmetal Cluster: The Remarkable Case of Co 4 (CO) 11

The unsaturated Co 4 (CO) 11 is predicted by density functional theory (B3LYP and BP86 functionals) to have a structure with one μ 4 ‐CO group bridging all four cobalt atoms in a Co 4 butterfly, with five Co–Co bonds in the range 2.44–2.56 Å. This μ 4 ‐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 . Co 4 (CO) 11 structures of higher energy have Co 4 tetrahedra (6 Co–Co bonds) or Co 4 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)