Systematic investigations on H atom transfer (HAT) thermodynamics of metal O 2 adducts is of fundamental importance for the design of transition metal catalysts for substrate oxidation and/or oxygenation directly using O 2 . Such work should help elucidate underlying electronic-structure features that govern the OO–H bond dissociation free energies (BDFEs) of metal-hydroperoxo species, which can be used to quantitatively appraise the HAT activity of the corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous Co III - and Mn III -hydroperoxo complexes, 3-Co and 3-Mn , were calculated to be 79.3 and 81.5 kcal/mol, respectively, employing the Bordwell relationship based on experimentally determined p K a values and redox potentials of the one-electron-oxidized forms, 4-Co and 4-Mn . To further verify these values, we tested the HAT capability of their superoxo congeners, 2-Co and 2-Mn , toward three different substrates possessing varying O–H BDFEs. Specifically, both metal-superoxo species are capable of activating the O–H bond of 4-oxo-TEMPOH with an O–H BDFE of 68.9 kcal/mol, only 2-Mn is able to abstract a H atom from 2,4-di- tert -butylphenol with an O–H BDFE of 80.9 kcal/mol, and neither of them can react with 3,5-dimethylphenol with an O–H BDFE of 85.6 kcal/mol. Further computational investigations suggested that it is the high spin state of the Mn III center in 3-Mn that renders its OO–H BDFE higher than that of 3-Co , which features a low-spin Co III center. The present work underscores the role of the metal spin state being as crucial as the oxidation state in modulating BDFEs.