In this work dimeric complexes, dioxo-mu-oxo and oxo-bis (mu-oxo) of molybdenum, were theoretically modeled to try to understand reactivity for hydrocarbon oxidation. The first complex, dioxo (mu-oxo) thiocyanatomolybdenum (VI) bearing a 4,4'-di-ter-butyl-2,2-bipyridine ligand (complex 1) in trans conformation exhibits a high oxidizing ability, giving as product a dimeric complex oxo-bis ( mu-oxo) thiocyanatomolybdenum (V) with the same ligands (complex 2) in cis conformation. Calculations were performed by using CATIVIC, a parametric quantum chemistry method for catalytic reactions. Furthermore, new geometries were created, using the CATIVIC graphic interface, starting with complex 1 to transform it to cis conformation (complex 3). Molecular parameters for Mo were estimated from diatomic molecules and molecular properties, such as charges, interatomic distances, bond orders, and diatomic energies of selected bonds, and atoms were calculated for these complexes. In addition, LUMO characteristics were evaluated and discussed. Results suggest the feasibility of complex 1 transformation through a bridge Mo-O bond rotation to form complex 3 that is energetically less stable than complex 1 in 21 kcal/ mol. Results of bond strength indicate a high lability of the bipyridine ligand and a high LUMO density of states in cis conformation. Preliminary steps of the mechanism for the formation of a highly active site that explains the oxidizing ability of complex 1 is given by considering the rotation through a bridging Mo-O bond to form a cis complex 3 and then a Mo-N bond breaking. Vacancy creation could be considered as a previous step to the formation of active sites for catalytic oxidation. (c) 2005 Elsevier B.V. All rights reserved.