Electronic structures of osmium dinitrides and dinitrogen molecule activation by the osmium atom

Density functional theory and ab initio electron-correlation methods have been used to study structures, stabilities, and bonding properties of osmium dinitrides, and possible mechanisms for the direct insertion of osmium into the strong dinitrogen triple bond. At B3LYP, CCSD//B3LYP. and QCISD levels, the end-on linear OsNN species in the ³Σ^? state with a δ² electron configuration is found to be the most stable isomer, whereas CCSD(T)//B3LYP, QCISD(T)//QCISD, and CASSCF(12,11)//QCISD calculations result in an inserted NOsN species in the singlet ¹A₁ state as the lowest energy state. The direct insertion reactions of osmium into the dinitrogen triple bond, involved in specific low-lying electronic states of the osmium dinitrides, are exothermic, and the activation energies are in the range 3.9–21.8 kcal mol^?1 Natural population analyses show that the unoccupied 6p orbital in the osmium ground state configuration may play an important role in the dinitrogen molecule activation by osmium.