A superoxide-bridged dicopper(II) complex, [Cu-2(II)(XYLO) (O-2(center dot-))](2+) (1) (XYLO = binucleating m-xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [Cu-2(II) (XYLO)(O-2(2-))](+) (2), using ferrocenium (Fc(+)) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 degrees C. Using Me(10)Fc(+), a 1 reversible arrow 2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 +/- 0.01 V vs Fc(+/0). Addition of 1 equiv of strong acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [Cu-2(II)(XYLO)(OOH)](2+) (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pK(a) of 3 was thus determined to be 24 +/- 0.6 in MeTHF at -125 degrees C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O-H bond dissociation free energy (BDFE) was calculated as 81.8 +/- 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [Cu-2(II)(XYLO)(O-2(center dot-))](2+) (1), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 degrees C) = 5.6 M-1 s(-1). Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O-2-derived reduced species, such as pK(a), reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.