A mechanism of water oxidation catalyzed by the carbon-free tetra-Co containing polyoxometalates [Co-4(H2O)(2)(PW9O34)(2)](10-) (PCo4) and [Co-4(H2O)(2)(VW9O34)(2)](10-) (VCo4) is elucidated by DFT calculations. Computational analysis for PCo4 suggests that a first PCET step may proceed via a sequential electron then -proton transfer (ET + PT) pathway and leads to one electron oxidize species S-1 (POM-Co-III-OH). In contrast, the second PCET, which controls the potential required to form POM-Co-III-0' active species S-2 is clearly a concerted process. The overall S-0 -> S-2 transformation is estimated to require less than 1.48 V and 1.62 V applied potential at pH = 8 for PCo4 and VCo4 anions, respectively. At operando conditions, with the presence of a buffer and with an applied potential above the threshold potential the two H-atom removal could take place via concerted pathways. These steps represent rapid pre-equilibria before the rate determining step, which corresponds to the O-O bond formation. The key chemical step occurs via nucleophilic attack of an external water molecule to intermediate S-2. We assume that this step governs the kinetics of the reaction. Comparison of the calculated energetics and electronic structures of intermediate species in the PCo4 and VCo4 catalyzed water oxidation cycle shows that coupling of d orbitals of V and Co atoms in VCo4 increases the oxidation potential of the Co-center. The orbital coupling is also responsible for the higher catalytic activity of VCo4 because it increases the electrophilicity of Co-III-O moiety in the key S-2 species. (C) 2017 Elsevier Inc. All rights reserved.