An ab initio study of the reaction mechanism of Co++NH3

Abstract

To investigate the mechanism for N–H bond activation by a transition metal, the reactions of Co+(3F,5F) with NH3 have been studied with complete active space self-consistent field (CASSCF), multireference configuration interaction (MR-SDCI), and multireference many body perturbation theory (MRMP) wave functions, using both effective core potential and all-electron methods. Upon their initial approach, the reactants yield an ion–molecule complex, CoNH3+(3E,5A2,5A1), with retention of C3ν symmetry. The Co+=NH3 binding energies are estimated to be 49 (triplet) and 45 (quintet) kcal/mol. Subsequently, the N–H bond is activated, leading to an intermediate complex H–Co–NH2+ (C2ν symmetry), through a three-center transition state with an energy barrier of 56–60 (triplet) and 70–73 (quintet) kcal/mol. The energy of H–Co–NH2+, relative to that of CoNH3+, is estimated to be 60 to 61 (triplet) and 44 (quintet) kcal/mol. However, the highest levels of theory employed here (including dynamic correlation corrections) suggest that the triplet intermediate HCoNH2+ may not exist as a minimum on the potential energy surface. Following Co–N or H–Co bond cleavage, the complex H–Co–NH2+ leads to HCo++NH2 or H+CoNH2+. Both channels (triplet and quintet) are found to be endothermic by 54–64 kcal/mol.