The relationship between spin state and metal-ligand bonding interactions in CpM(NO)-X2 species was investigated using density functional computational techniques. The geometries of CpM(NO)Cl2 (M = Cr, Mo), CpCr(NO)(NH2)X, and CpCr(NO)(CH3)X (X = Cl, CH3) were optimized at the DFT-B3LYP level for both the diamagnetic (S = 0) and paramagnetic (S = 1) electronic configurations. While the geometric parameters of the singlet compounds matched well with structures determined experimentally, the Cr-NO bond lengths in the triplet species exceeded the experimentally observed range by a significant margin, thereby indicating a propensity for nitrosyl-ligand dissociation from the high-spin complexes. The order of relative singlet vs triplet spin-state stability (expressed as ΔEs-t (kcal/mol)) was determined to be CpCr(NO)Cl2 (8.20) > CpCr(NO)(CH3)Cl (1.52) ≈ CpCr-(NO)(NH2)Cl (0.95) > CpCr(NO)(CH3)2 (-2.37) > CpCr(NO)(NH2)CH3 (-9.55) > CpMo(NO)-Cl2 (-17.62). The amide π-donation increases the HOMO-LUMO energy splitting, thus favoring the diamagnetic configuration. The alkyl ligand reduces the electron-electron repulsion through orbital expansion, thereby lowering the relative energy of the singlet state. Extended Hückel molecular-orbital calculations were performed on the DFT-optimized structures to help rationalize the metal-ligand bonding interactions, and interelectron repulsions were quantified by evaluation of the Coulomb (J) and exchange (K) integrals based on the B3LYP-optimized triplet spin-state geometries.
Publication Source (Journal or Book title)
Legzdins, P., McNeil, W., Smith, K., & Poli, R. (1998). Density functional study of spin state in CpM(NO)X2 (M = Mo, Cr; X = Cl, NH2, CH3): Spectrochemical and nephelauxetic effects in organometallic compounds. Organometallics, 17 (4), 615-622. https://doi.org/10.1021/om970973g