The chromium methylidene cation: CrCH₂ ⁺

We have examined the electronic structure and bonding characteristics of the experimentally observed cation CrCH₂ ⁺. We find a ⁴B₁ ground state with a covalent double bond between ⁶S Cr⁺ and ³B₁ CH₂. These results are in contrast to previous theoretical studies which found a lowest state with ⁶B₁ symmetry and a single Cr-C bond. We calculate a direct bond energy of 44 kcal/mol and estimate the fully correlated limit to be 49 kcal/mol, which may be compared with the experimental value of 65 ± 7 kcal/mol and the previous theoretical results of 18.3 and 22.3 kcal/mol. The differences in results between the two theoretical studies on CrCH₂ ⁺ are discussed.