The electron structure of ferryl ion [FeO](2+) active in oxidation of hydrocarbons is studied using CASSCF theory followed by ISA-MRMP correction. [FeO](2+) is modeled by the OFe(OH)(2) species. The (5)A(1) and B-5(2) states are predicted to be low-lying states for the [FeO](2+) unit. At the decreased (H)O-Fe-O(H) angles B-5(2) becomes the ground state. The B-5(2) state appears to be a mixture of configurations with quite large weight of the split-electron-pair configuration. This state can be considered as anion-radical state of oxygen which seems to be responsible for the activity of the [FeO](2+) moiety toward the activation of the C-H bond. (c) 2005 Elsevier B.V. All rights reserved.