The nature of the ligand is an important aspect of controlling structure and reactivity in coordination chemistry. In connection with our study of heme/copper/oxygen reactivity relevant to cytochrome c oxidase O 2 -reduction chemistry, we compare the molecular and electronic structure of two highspin heme-peroxo-copper [Fe III -O 2 2--Cu II ] + complexes containing N 4 -tetradentate (1) or N 3 -tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to DFT calculations we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish μ-(O 2 2-) sideon to the Fe III and end-on to Cu II (μ-η 2 :η 1 ) binding for the complex 1 but side-on/side-on (μ-η 2 :η 2 ) μ-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh 3 , acid and phenols. The comparison of a new X-ray structure of μ-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two μ-oxo complexes (∠Fe-O-Cu = 178.2° in 1a, ∠Fe-O-Cu = 149.5° in 2a). The reaction of 2 with one equivalent of exogenous N-donor axial base leads to the formation of a distinctive low-temperature stable, low-spin heme-O 2 -Cu complex (2b), but under the same conditions the addition of an axial base to 1 leads to the dissociation of the hemeperoxo-Cu assembly and the release of O 2 . 2b reacts with phenols performing hydrogen-atom (e -+ H + ) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [Fe IV =O] complex (2c). The nature of 2c was confirmed by comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the hydrogen-atom abstraction was either 1) directly detected by EPR spectroscopy using phenols that produce stable radicals or 2) indirectly by detection of the coupling product of two phenoxyl radicals.karlin@jhu.edu, edward.solomon@stanford.edu. Supporting Information Available. UV-visible spectra of the reaction of (2b) with 1 equiv. of 2,4-di-tertbutylphenol ( Figure S1), UVvis spectra of the reaction of [(F 8 )Fe III -(O 2 2-)-Cu II (TMPA)] + (1) with DMAP ( Figure S2), EPR spectra of (2b) reaction with 1 equiv.of 2,4-di-tert-butylphenol ( Figure S3), GC-MS trace of the oxidative coupling of 2,4-di-tert-butylphenol in presence of (2b) ( Figure S4), ORTEP diagram ( Figure S5) and crystal data and structure refinement for [(F 8 )Fe III -(O 2-)-Cu II (AN)] + (2a) X-ray structure (Table S1), XAS spectra and computational data.NIH Public Access