Bonding in Transition-Metal Methylene Complexes. 3. Comparison of Cr and Ru Carbenes; Prediction of Stable L_nM(CXY) Systems

The electronic structure of the lowest carbene state of a representative early-transition-metal complex, CrCH₂⁺ (⁶A₁ symmetry), has been examined by using ab initio techniques. Its properties reveal a complex with a single σ-donor bond from singlet CH₂ to high-spin (d⁵) Cr⁺ and no π-back-bond, resulting in a low bond energy (38.7 kcal/mol) and a large carbene–alkylidene state splitting (18.8 kcal/mol). These results are contrasted with Ru carbene (possessing both σ- and π-donor bonds) properties [D_e(Ru=C) = 65.8 kcal/mol and ΔE(carbene-alkylidene) = 12.9 kcal/mol]. This comparison enables, for the first time, a separation of σ-donor bond strengths from π-donor bond strengths. Finally, using only valence electron properties, we are able to predict stabilities of L_nM(CXY) complexes (e.g., how substituents at carbon affect the preference for bridging vs. terminal CXY), discussing trends for the entire transition series.