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 MnMn distance of 2.523 Å and Cp2Cr2(μ-CO) with a two-electron donor bridging carbonyl group and a CrCr 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 TiTi 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.