Reactivity of the [FeO](2+) group in the abstraction of hydrogen from methane is determined by metastable oxyl state Fe-III-O-center dot causing the negative spin polarization of the methyl moiety as was shown by quantum-chemical means with the use of model iron hydroxide species FeO(OH)(2), Fe2O(OH)(5), and Fe4O5(OH)(3) as an example. The energy of the gap between the ground-state ferryl configuration Fe-IV=O and the oxyl state correlates with the energy barrier of the H-abstraction from methane. (C) 2017 Elsevier B.V. All rights reserved.