Ligand exchange reactions in molecular hydrogen complexes of Osmium(II): A quantum chemical study using density functional theory

The energetics and mechanism of ligand exchange reactions in a range of molecular hydrogen complexes of Os(II), [Os(NH3)(4)L(z)(eta(2)-H-2)]((z + 2)+) (where L(z) = C5H5-N, CH3CN, NH2OH, NH3, (CH3)(2)CO, H2O CN-, CH3COO-, and Cl-), have been studied using quantum chemical methods. Density functional theory using the BLYP functional was employed to determine the gas phase equilibrium geometries and the binding energies of H-2 and the trans ligand L(z). The effects of solvation on the energetics were estimated using two variants of the self-consistent reaction field method. Thermal enthalpy and entropy contributions were also calculated, resulting in theoretical estimates of standard free energy changes for the ligand exchange reactions [Os(NH3)(4)H2O(eta(2)-H-2)](2+) + L(z) --> [Os(NH3)(4)L(z)(pi(2)-H-2)]((z + 2)+) + H2O in aqueous solution which were compared with the available experimental data. A reasonable level of qualitative to semiquantitative agreement between theory and experiment is demonstrated, especially when L(z) = Cl-, CH3COO- and (CH3)(2)CO. In agreement with experiment, theory also predicts that [Os(NH3)(4)CN(eta(2)-H-2)](+) will hydrolyze, with H2O replacing H-2 as a ligand with the evolution of H-2 gas. The theoretical studies also suggest that ligand exchange in these systems takes place via an S(N)1 type mechanism, e.g., with the formation of a loosely associated [Os(NH3)(4)-(eta(2)-H-2)](2+) and H2O as transition state. The computed free energy changes of activation are consistent with the experimentally deduced values.