A series of side-on peroxide-bridged binuclear copper complexes [CU2(NnPY2)(O-2)](2+) (where n = 3-5), which have been proposed to adopt a butterfly Cu2O2 geometry due to the constraints placed on the Cu-Cu distance by the alkyl chain linker of length -(CH2)(n)- have been studied using absorption and resonance Raman spectroscopy and theoretical techniques. The four components of the peroxide to copper(II) charge-transfer transitions have been identified for the first time in the [Cu-2(NnPY2)(O-2)](2+) (where n = 3-5) complexes. The observed shift of the peroxide O-O stretch by 25 cm(-1) to higher energy and the changes observed in the energy and intensity of absorption bands including the presence of an additional band at 23 800-20 400 cm(-1) (420-490 nm) (not seen in planar Side-on peroxide-bridged dicopper cores) are correlated to the butterfly structure using transition dipole vector coupling and valence bond configuration interaction models. The identification of an absorption band at 23 800-20 400 cm(-1) (420-490 nm) associated with the butterfly side-on peroxide-bridged dicopper core is important since the isomeric, bis(mu-oxo) core is also characterized by an absorption band in this region. The changes in bonding associated with a butterfly distortion of the Cu2O2 core are defined, and the reactivity of the butterfly core with respect to electrophilic aromatic substitution and H atom abstraction reactions is compared with that of the planar side-on peroxide-bridged dicopper core using the frontier molecular orbital description.