| Summary: | The influence of the cobalt cation geometric environment on catalytic activity, namely, oxygen adsorption and its activation, was investigated by exploring two groups of systems. The first group was formed by cobalt cation complexes, in which the Co<sup>2+</sup> was surrounded by water-H<sub>2</sub>O or acetonitrile-CH<sub>3</sub>CN solvent molecules. This represents heteropolyacids salts (Co<sub>n</sub>H<sub>3-n</sub>PW(Mo)<sub>12</sub>O<sub>40</sub>), where the Co<sup>2+</sup> acts as a cation that compensates for the negative charge of the Keggin anion and is typically surrounded by solvent molecules in that system. The second group consisted of tungsten or molybdenum Keggin anions (H<sub>5</sub>PW<sub>11</sub>CoO<sub>39</sub> and H<sub>5</sub>PMo<sub>11</sub>CoO<sub>39</sub>), having the Co<sup>2+</sup> cation incorporated into the anion framework, in the position of one addenda atom. Detailed NOCV (Natural Orbitals for Chemical Valence) analysis showed that, for all studied systems, the σ-donation and σ-backdonation active channels of the electron transfer were responsible for the creation of a single Co-OO bond. Depending on the chemical/geometrical environment of the Co<sup>2+</sup> cation, the different quantities of electrons were flown from the Co<sup>2+</sup> <i>3d</i> orbital to the <i>π*</i> antibonding molecular orbitals of the oxygen ligand, as well as in the opposite direction. In molybdenum and tungsten heteropolyacids, modified by Co<sup>2+</sup> in the position of the addenda atom, activation of O<sub>2</sub> was supported by a π-polarization process. Calculated data show that the oxygen molecule activation changed in the following order: H<sub>5</sub>PMo<sub>11</sub>CoO<sub>39</sub> = H<sub>5</sub>PW<sub>11</sub>CoO<sub>39</sub> > Co(CH<sub>3</sub>CN)<sub>5</sub><sup>2+</sup> > Co(H<sub>2</sub>O)<sub>5</sub><sup>2+</sup>.
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