Prominent recent advances in synthetic modeling chemistry of cytochrome c oxidase include the generation and characterization of high-spin heme-peroxo-copper species derived from Fe-II/Cu-I/O-2 reactions. Comparison of spectroscopic properties of a series of related complexes leads to a finding that the coordination nature of the peroxo bridging group binding between heme and copper can be dramatically altered by copper ligand environment. Intrinsic copper/O-2 interactions seem to be applicable to the chemistry of heme/Cu/O-2 systems.