We report that a novel use of 35 GHz H-1-ENDOR spectroscopy establishes the presence in 1 of an Fe-IV=O center dot center dot center dot H-O-Fe-III hydrogen bond predicted by density functional theory computations to generate a six-membered-ring core for 1. The hydrogen bond rationalizes the difference in the C-H bond cleavage reactivity between 1 and 4(OCH3) (where a CH3O group has replaced the HO on the Fe-III site). This result substantiates the seemingly paradoxical conclusion that the nonheme Fe-IV=O unit of 1 not only has the electrophilic character required for H-atom abstraction but also retains sufficient nucleophilic character to accept a hydrogen bond from the Fe-III-OH unit.