In this work, the mechanism of water oxidation catalyzed by a tricopper-containing polyoxometalate (POM) [(SbW9O33)(2)Cu-3(II)(H2O)(3)](12) ([POM-(Cu2CuII)-Cu-II-OH2](12)), including (i) the deprotonation oxidation and (ii) O-O bond formation steps, was theoretically investigated for the first time. Calculations suggest that the Cu center is redox-inert and remains bivalence throughout the catalytic reaction. This is significantly different from the Ru-, Co- and Mn-containing POM-based water oxidation catalysts in which the metal centers (Ru, Co, Mn) are sequentially oxidized and reduced during catalytic reaction. In deprotonation oxidation step, two hydrogen atoms are removed from one water ligand. Therefore, an unusual metal-oxyl-radical species [POM-(Cu2CuII)-Cu-II-O-center dot center dot](12) is obtained, in which two unpaired electrons are assigned to the active O center. A monocopper-containing keggin POM [Cu-II(O-center dot center dot)SbW11O39](5) ([POM-Cu-II-O center dot center dot](5)) was selected as the model to explore the O-O bond formation mediated by [POM-(Cu2CuII)-Cu-II-O-center dot center dot](12). The O-O bond formation was proposed to proceed via the single electron transfer-water nucleophilic attack (SET-WNA) mechanism. Two electrons from the incoming water molecule is step by step transferred to the oxyl-radical ligand of [POM-Cu-II-O-center dot center dot](5). Furthermore, the SET-WNA process is characterized by two-state reactivity and the observed spin inversions between two potential energy surfaces effectively reduce the activation barrier. Therefore, the present work would provide the promising information for further designing water oxidation catalysts (WOCs). (C) 2019 Elsevier Inc. All rights reserved.