The electronic structures of ore-and peroxo-bridged binuclear copper compounds analogous to the active site of oxyhemocyanin an analyzed in terms of their framework electron counts with the help of density functional and extended Huckel calculations. Through-ring bonding in the Cu2O2 framework is discussed by means of a topological analysis of the electron density for the model compounds [(NH3)(3)Cu(mu-eta(2):eta(2)-O-2)Cu(NH3)(3)](2+), [(NH3)(3)Cu(mu-O)(2)Cu(NH3)(3)](2+), and [(PH3)(2)Cu(mu-H)(2)Cu(PH3)(2)]. The existence of isomeric peroxo-and bis(oxo)-bridged Cu complexes can be rationalized in light of the framework electron counting rules by taking into account that two electrons can be localized in the metal 3d orbitals in the former but delocalized through framework bonding molecular orbitals in the latter. An analysis of the theoretical and experimental structural data indicates that a reorganization of the Cu coordination sphere that can be affected by the nature of the terminal ligands is important for the relative stability of the two isomeric forms. In particular, the peroxo-bridged structure is favored by tridentate ligands, whereas the ore-bridged isomer is favored by bidentate ones. The stability of the two isomers is also compared for analogous complexes with different metal or bridging atoms for which only one isomeric form is known.