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 interacti ons in CpM(NO)Xa species was investigated using density functional com putational techniques. The geometries of CpM(NO)Cl-2 (M = Cr, Mo), CpC r(NO)(NH2)X, and CpCr(NO)(CH3)X (X = Cl, CH3) were optimized at the DF T-B3LYP level for both the diamagnetic (S = 0) and paramagnetic (S = 1 ) electronic configurations. While the geometric parameters of the sin glet compounds matched well with structures determined experimentally, the Cr-NO bond lengths in the triplet Species exceeded the experiment ally observed range by a significant margin, thereby indicating a prop ensity for nitrosyl-ligand dissociation from the high-spin complexes. The order of relative singlet vs triplet spin-state stability (express ed 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, thu s favoring the diamagnetic configuration. The alkyl ligand reduces the electron-electron repulsion through orbital expansion, thereby loweri ng the relative energy of the singlet state. Extended Huckel molecular -orbital calculations were performed on the DFT-optimized structures t o help rationalize the metal-ligand bonding interactions, and interele ctron repulsions were quantified by evaluation of the Coulomb (S) and exchange (K) integrals based on the B3LYP-optimized triplet spin-state geometries.