We have performed hybrid density functional theory (DFT) calculations on the geometric and electronic structures of low-lying doublet and quartet ferryl-oxo [Fe(IV)=O] oxidants and a doublet perferryl-oxo [Fe(V)=O] oxidant in Cytochrome P450. Fully optimized structures of compound I models have been determined, and the proper symmetry of wave functions has been restored by the spin-projection technique. The results show that the perferryl-oxo species is relatively low lying, as compared with the excited state of the ferryl-oxo species, if the iron-oxo bond is properly described as the mixing of several appropriate excited electronic configurations to minimize electron repulsion. This means that the perferryl-oxo species is virtually in a mixed-valent resonance state, up arrow Fe(V)=O <-> up arrow Fe(IV)center dot up arrow-down arrow center dot O, containing a highly reactive p pi atomic oxygen radical. The anionic thiolate ligand acts as a Lewis sigma base and functions to achieve the stability of the perferryl-oxo complex and to activate the oxo ligand trans to it by asymmetric bond distortion along the O-Fe-S axis by lengthening the Fe-O bond and shortening the Fe-S bond, prior to the hydrogen-atom abstraction from the substrate.