Dimetallocene carbonyls: The limits of the 18-electron rule and metal-metal multiple bonding in highly unsaturated molecules of the early transition metals

The structures and energetics of Cp₂M₂(CO) (M = Mn, Cr, and Ti) have been investigated by density functional theory (DFT) for comparison with previously reported studies on Cp₂V₂(CO). For all four metals energetically competitive structures are found with singlet, triplet, quintet, and septet spin multiplicities. However, significant spin contamination was found for the triplet Cp₂Mn₂(CO) and Cp₂Cr₂(CO) structures. The lowest energy structures for Mn and Cr are both septet states, namely Cp₂Mn₂(η²-μ-CO) with a four-electron donor bridging carbonyl group and an Mn{single bond}Mn distance of 2.523 Å and Cp₂Cr₂(μ-CO) with a two-electron donor bridging carbonyl group and a Cr{triple bond, long}Cr distance of 2.436 Å. However, for the Ti analogue the lowest energy structure is singlet Cp₂Ti₂(η²-μ-CO) with a four-electron donor bridging carbonyl group and a Ti{triple bond, long}Ti distance of 2.364 Å. The higher energy singlet structures of Cp₂M₂(CO) have the very short metal-metal distances of 1.879 Å (M = Mn) and 1.729 Å (M = Cr) suggesting very high formal bond orders of five and six, respectively.