Certain side-on peroxo-dicopper(II) species with particularly low nu(O-O) (710-730 cm(-1)) have been found in equilibrium with their bis-mu-oxo-dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for O-O cleavage. In a previous study (Liang, H.-C., et al. J. Am. Chem. Soc. 2002, 124, 4170), we showed that oxygenation of the three-coordinate complex [Cu-I(MeAN)](+) (MeAN = N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{Cu-II (meAN)}(2)(mu-eta(2):eta(2)-O-2(2-))](2+) peroxo species with low nu(O-O) (721 cm(-1)), as characterized by UV-vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF6- salt, and an X-ray structure indicates the presence of an unusually long O-O bond (1.540(5) angstrom) consistent with the low nu(O-O). Extended X-ray absorption fine structure. and rR spectroscopic and reactivity studies indicate the exclusive formation of [{Cu-II(MeAN)}(2)(mu-eta(2):eta(2)-O-2(2-))](2+) without any bis-mu-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo-dicopper(II) species with a significantly long and weak O-O bond. OFT calculations show that the weak O-O bond results from strong sigma donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak O-O bond does not reflect an increase in backbonding into the sigma* orbital of the peroxide. Thus, although the O-O bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-mu-oxo dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity.