A density functional study of the dimerization mechanisms of molybdenum(vi) in aqueous solution

Abstract

The dimerization of MoO₂(OH)(H₂O)₃⁺ to form Mo₂O₅(H₂O)₆²⁺ has been studied by means of density functional calculations at the B3LYP/CEP-31G* level of theory, both in the gas-phase and in the solvent using a polarizable continuum model. Dissociative and concerted mechanisms were investigated, while it appeared that the associative pathway could be ruled out due the intrinsic instability of the ML₆–O–ML₅ intermediate that must be present in such process. From the two different monomer models used to study the dimerization reaction paths, the most stable reactant and product were obtained with two ML₆ units in which the three H₂O ligands describe a face of each octahedron. In this case, the dissociative path is preferred with respect to the concerted mechanism, both in vacuo and in solution. It was shown that solvent effects must be taken into account in order to obtain a product that is more stable than two non-interacting monomers, in agreement with experimental data. The reaction energy for the preferred mechanism in solution was computed to be within 1.5 kcal mol⁻¹ of the experimental value.