radicals, which is a contrary behavior presented by others vanadium complexes that may act as O2- producers or scavengers. For Haber–Weiss reactions, the analysis of the Gibbs free energy predict the following ordering: H2O>NH3>Cl->OH->C2O42-, Cl->C2O42-, NH3>oxo, H2O>oxo, NH3>oxo, Cl-, for ligands with vanadium V and IV in the formation of reactive oxygen species (ROS); which allow for the first time a criteria to predict ROS activity in vanadium complexes. All the results were obtained by using the Density Functional Theory, in its Kohn–Sham version, with four different exchange–correlation functionals (PBE, PBE0, B3LYP and M06-2X) coupled with the aug-cc-pVDZ basis set. It is well-known that transition metals sometimes exhibit a multireference character, for that reason several tests were considered over the vanadium complexes reported here. In particular, T1 test, multiconfiguration self-consistent field and coupled-clusters calculations were performed in order to test the feasibility of our results. In general, we can say that the M06-2X exchange–correlation functional exhibits the best performance to study theoretically vanadium complexes.