The enthalpy of the oxotransfer reaction of [Bu4N]2[WO2(mnt)2] (where mnt2- is maleonitriledithiolate) with PPh3 in an inert atmosphere in an acetonitrile solution was determined by calorimetry. The obtained enthalpy value (−93 ± 5) kJ mol-1 differs from the enthalpy value of the reaction carried out by us earlier under aerobic conditions by (16 ± 9) kJ mol-1. The obtained results indicate the participation of atmospheric oxygen in the catalytic process. DFT calculations of the structures [MOx(LL)2]2- (where M = Mo or W, x = 1 or 2, and LL is dithiolene ligand) were carried out. DFT calculations of the thermodynamic potential values for the oxygen transfer reaction in the system [MIVO2(LL)2]2- - [MVIO(LL)2]2- were also carried out. It was shown that the determining contribution to these processes is made by the enthalpy changes. The enthalpies of these processes depend on the donor-acceptor characteristics of the dithiolene ligands. In the case of good acceptors, the oxidation process is more exothermic. The opposite picture is observed for the reduction processes of the complex. The M═O bond length can be used as a quantitative criterion for donor-acceptor properties of the ligands. Also, for molybdenum complexes, reduction processes are more favorable, while for tungsten complexes, oxidation processes are more favorable. The calculations agree well with experimental data that prove the validity of the model used for DFT calculations.