High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O-2 activation (e.g., by Fe porphyrin in Cytochrome P450) and O-2 production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M-IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn-IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn-V(O)(ttppc) (4) and Mn-III(H2O)(ttppc) (1 center dot H2O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di-tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k(2) = 2.73(12) X 10(4) M-1 s(-1), which is similar to 3 orders of magnitude larger than 4 (k(2) = 17.4(1) M-1 s(-1)). Reaction of 3 with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k(2) = 510(10)-36(1.4) M-1 s(-1) and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn-IV(OH) complex is a much more powerful H atom abstractor than the highervalent Mn-V(O) complex, or even some Fe-IV(O) complexes.