Density functional study of spin state in CpM(NO)X-2 (M=Mo, Cr; X=Cl, NH2, CH3): Spectrochemical and nephelauxetic effects in organometallic compounds

The relationship between spin state and metal-ligand bonding interactions in CpM(NO)Xa species was investigated using density functional computational techniques. The geometries of CpM(NO)Cl-2 (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 Delta Es-t (kcal/mol)) was determined to be CpCr(NO)Cl-2 (8.20) > CpCr(NO)(CH3)Cl (1.52) approximate to CpCr(NO)(NH2)Cl (0.95), > CpCr(NO)(CH3)(2) (-2.37) > CpCr(NO)(NH2)CH3 (-9.55) > CpMQ(NO)Cl-2 (-17.62). The amide pi-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 Huckel 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 (S) and exchange (K) integrals based on the B3LYP-optimized triplet spin-state geometries.