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 [De(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 LnM(CXY) complexes (e.g., how substituents at carbon affect the preference for bridging vs. terminal CXY), discussing trends for the entire transition series.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry