Complexes 1-OH and 1-F are related complexes that share similar [X-Fe-III-O-Fe-IV = O](3+) core structures with a total spin S of 1/2, which arises from antiferromagnetic coupling of an S = 5/2 Fe-III-X site and an S = 2 Fe-IV = O site. EXAFS analysis shows that 1-F has a nearly linear Fe-III-O-Fe-IV core compared to that of 1-OH, which has an Fe-O-Fe angle of similar to 130 degrees due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C-H bond cleavage than 2, a related complex with a [OH-Fe-IV-O-Fe-IV = O](4+) core having individual S = 1 Fe-IV units. Interestingly, 1-F is 10-fold more reactive than 1-OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1-OH and 1-F strongly suggest that the H-bond in 1-OH does not significantly change the electrophilicity of the reactive Fe-IV = O unit and that the lower reactivity of 1-OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety.