The nature of the ligand is an important aspect of controlling the structure and reactivity in coordination chemistry. In connection with our study of heme−copper−oxygen reactivity relevant to cytochrome c oxidase dioxygen-reduction chemistry, we compare the molecular and electronic structures of two high-spin heme−peroxo−copper [Fe^IIIO₂²⁻Cu^II]⁺ complexes containing N₄ tetradentate (1) or N₃ tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to density functional theory 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-(O₂²⁻) side-on to the Fe^III and end-on to Cu^II (μ-η²:η¹) binding for the complex 1 but side-on/side-on (μ-η²:η²) 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, PPh₃, 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 and ∠Fe−O−Cu = 149.5° in 2a). The reaction of 2 with 1 equiv of an exogenous nitrogen-donor axial base leads to the formation of a distinctive low-temperature-stable, low-spin heme−dioxygen−copper complex (2b), but under the same conditions, the addition of an axial base to 1 leads to the dissociation of the heme−peroxo−copper assembly and the release of O₂. 2b reacts with phenols performing H-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 a comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the H-atom abstraction was either (1) directly detected by electron paramagnetic resonance spectroscopy using phenols that produce stable radicals or (2) indirectly detected by the coupling product of two phenoxyl radicals.