In this work, the key O-O bond formation step in water oxidation catalyzed by a polyoxometalate-based complex [Mn-3(H2O)(3)(Sbw(9)O(33))(2)](12-) was theoretically investigated. The Mn-V-oxo species rather than Mn-IV-oxo species, is computed to be responsible for O-O bond formation via water nucleophilic attack mechanism. During O-O bond formation based on Mn-V-oxo species, two-state reactivity is found and the energy profile switches from the 13et state to the 15et state through a minimum energy crossing point, which significantly reduces the activation barrier. The observed spin inversion phenomenon in O-O bond formation is attributed to the exchange-enhanced reactivity for 15et state, but not for 13et state. (C) 2019 Elsevier Inc. All rights reserved.