Beyond the metal-metal triple bond in binuclear cyclopentadienylchromium carbonyl chemistry

The cyclopentadienylchromium carbonyls Cp₂Cr₂(CO) _n and Cp*₂Cr₂(CO)_n (Cp = η⁵-C₅H₅ and Cp* = η⁵-Me₅C₅; n = 3, 2) have been studied by density functional theory using the B3LYP and BP86 functionals. Triplet and singlet structures are found for Cp₂Cr₂(CO)₃, with the triplet isomer having an apparent Cr≡Cr triple bond (2.295 Å by BP86) and predicted to have a lower energy than the singlet isomer having an apparent Cr≡Cr quadruple bond (2.191 Å by BP86). Quintet, septet, and singlet structures, as well as a highly spin contaminated triplet structure, were found for the dicarbonyl Cp₂Cr₂(CO) ₂. In all of the Cp₂Cr₂(CO)_n (n = 3, 2) structures the carbonyls are asymmetric semi-bridging groups, typically with differences of 0.3-0.5 Å between the shortest and longest M-C distances. Very little difference was found between the structures and energetics of the corresponding Cp and Cp* derivatives. These DFT studies suggest that the reported unstable photolytic decarbonylation product of Cp* ₂Cr₂(CO)₄, characterized only by its infrared ν(CO) frequencies, is the singlet isomer of the tricarbonyl Cp*₂Cr₂(CO)₃.