An Iron(IV)-oxo heme(+center dot) complex (Compound I, Cpd I) is the proposed active species of heme enzymes such as the cytochromes P450 and is elusive; therefore, biomimetic studies on active site mimics give valuable insight into the fundamental properties of heme active species. In this work we present density functional theory (DFT) calculations on substrate hydroxylation by a Compound I mimic [Fe-IV = O(Por(+center dot))Cl] and its one-electron reduced form [Fe-Iv = O(Por)-Cl](-). Thus, recent experimental studies showed that [Fe-IV = O(Por)Cl](-) is able to react with substrates via hydride transfer reactions (Jeong, Y. J.; Kang, Y.; Han, A.-R.; Lee, Y.-M.; Kotani, H.; Fukuzumi, S.; Nam, W. Angew, Chem., Int. Ed. 2008, 47, 7321-7324]. By contrast, theoretical studies on camphor hydroxylation by these two oxidants concluded that the one-electron reduced form of Compound I is a sluggish oxidant of hydroxylation reactions [Altun, A.; Shaik, S.; Thiel, W. J. Am. Chem. Soc. 2007, 129, 8978-89871. To resolve the question why the one-electron reduced Compound I is an oxidant in one case and a sluggish oxidant in other cases, we have performed a DFT study on 10-methyl-9,10-dihydro acridine (AcrH(2)) hydroxylation by [Fe-IV = O(Por(+center dot))Cl] and [Fe-IV = O(Por)Cl](-). The calculations presented in this work show that both [Fe-IV = O(Por(+center dot))Cl] and [Fe-IV = O(Por)Cl](-) are plausible oxidants, but [Fe-IV = O(Por(+center dot))Cl] reacts via much lower reaction barriers. Moreover, [Fe-IV = O(Por(+center dot))Cl] reacts via hydride transfer, while [Fe-IV = O(Por)Cl](-) by hydrogen abstraction. The differences between hydride and hydrogen atom transfer reactions have been rationalized with thermodynamic cycles and shown to be the result of differences in electron abstraction abilities of the two oxidants. Thus, the calculations predict that [Fe-IV = O(Por)Cl](-) is only able to hydroxylate weak C-H bonds, whereas [Fe-IV = O(Por(+center dot))Cl] is more versatile.