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

Abstract

The structures and energetics of Cp 2M 2(CO) (M = Mn, Cr, and Ti) have been investigated by density functional theory (DFT) for comparison with previously reported studies on Cp 2V 2(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 Cp 2Mn 2(CO) and Cp 2Cr 2(CO) structures. The lowest energy structures for Mn and Cr are both septet states, namely Cp 2Mn 2(η 2-μ-CO) with a four-electron donor bridging carbonyl group and an Mn Mn distance of 2.523 Å and Cp 2Cr 2(μ-CO) with a two-electron donor bridging carbonyl group and a Cr Cr distance of 2.436 Å. However, for the Ti analogue the lowest energy structure is singlet Cp 2Ti 2(η 2-μ-CO) with a four-electron donor bridging carbonyl group and a Ti Ti distance of 2.364 Å. The higher energy singlet structures of Cp 2M 2(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.