The formation of the O-O bond in the water oxidation catalyzed by the iron hydroxides was modeled with the Fe4O4(OH)(4) and Fe2O2(OH)(4) complexes by means of the density functional theory. Consideration of the local minima and transition states for various routes of the association of oxo- and hydroxo-centers reveals a determinative role of the oxyl radical center Fe-III-O center dot- in facilitating this process. The preferable scenario comprises two steps: (1) the abstraction of the electron and proton from the initial complex leaving behind a ferryl FeO group, and (2) the incorporation of ferryl oxygen into the tetramer edge to form peroxo group. (C) 2014 Elsevier B.V. All rights reserved.