The geometric and electronic structures and reactivity of an S = 5/2 (HS) mononuclear nonheme (TMC)FeIII-OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of S = 1/2 (LS) FeIII-OOH complexes to understand parallels and differences in mechanisms of O-O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)FeIII-OOH]2+ complex is found to involve axial ligand coordination and a crossing to the LS surface for O-O bond homolysis. Both HS and LS FeIII-OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS FeIII-OOH, the transition state is late in O-O and early in C-H coordinates. However, for the HS FeIII-OOH, the transition state is early in O-O and further along in the C-H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS FeIII-OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS FeIII-OOH, the H-atom abstraction reactivity of HS FeIII-OOH is found to be highly dependent on both the ionization potential and the C-H bond strength of the substrate. LS FeIII-OOH is found to be more effective in H-atom abstraction for strong C-H bonds, while the higher reduction potential of HS FeIII-OOH allows it to be active in electrophilic reactions without the requirement of O-O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS FeIII-OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds.