Density functional theory calculations have been carried out to elucidate the mechanism of cyclohexane hydroxylation by three possible isomers of [FeIV(O)(N-R-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine)]2+ (R is methyl or benzyl) (Klinker et al. in Angew Chem Int Ed 44:3690–3694, 2005). The calculations offer a mechanistic view and reveal the following features: (a) all the three isomers possess triplet ground states and low-lying quintet excited states, (b) the relative stability follows the order isomer A > isomer B > isomer C, in agreement with the conclusions of Klinker et al., (c) the theoretical investigations provide a rationale to explain the interconversion of the three isomers, (d) the reaction pathways of the C–H hydroxylation are initiated by a hydrogen-abstraction step, and (e) the three isomers react with cyclohexane via two-state-reactivity patterns on competing triplet and quintet spin-state surfaces. As such, in the gas phase, the relative reactivity exhibits the trend isomer B > isomer A, while at the highest level, B2//B1 with zero point energy and solvation corrections, the relative reactivity follows the order isomer B > isomer A > isomer C. Thus, the calculated reaction pathway shows that pyridine rings perpendicular to the Fe–O axis result in more reactive species, and a pyridine ring coordinated trans to the oxygen atom leads to the least reactive isomer.