Metal-superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe-, Co-, and Ni-superoxo complexes have been reported to date, well-defined Mn-superoxo complexes remain rather rare. Herein, we report two mononuclear Mn-III-superoxo species, Mn-(BDPP)(O-2(center dot-)) (2, H2BDPP= 2,6-bis((2-(S)-diphenylhy-droxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn-((BDPP)-P-Br)(O-2(center dot-)) (2', H-2 (BDPP)-P-Br = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)-pyridine), synthesized by bubbling O-2 into solutions of their Mn(II )precursors, Mn(BDPP) (1) and Mn((BDPP)-P-Br) (1'), at -80 degrees C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin Mn-III center (S-Mn = 2) antiferromagnetically coupled to a superoxo radical ligand (S-oo center dot = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2' were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn-III -hydroperoxo species, Mn(BDPP)(OOH) (5) and Mn((BDPP)-P-Br)(OOH) (5'). Complexes 5 and 5' can be independently prepared by the reactions of the isolated Mn-III-aqua complexes, [Mn(BDPP)(H2O)]OTf (6) and [Mn((BDPP)-P-Br)(H2O)]OTf (6'), with H2O2 in the presence of NEt3. The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5'.