The effect of endogenous donor strength on Cu2O2 bonds was studied by electronically perturbing [{(R-TMPA)Cu-II}(2)(O-2)](2+) and [{(R-MePY2)Cu}(2)(O-2)](2+) (R = H, MeO, Me2N), which form the end-on mu-1,2 bound peroxide and an equilibrium mixture of side-on peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) isomers, respectively. For [{(R-TMPA)Cu-II}(2)(O-2)](2+), v(o-o) shifts from 827 to 822 to 812 cm(-1) and v(Cu-O(sym)) shifts from 561 to 557 to 551 cm(-1), respectively, as R- varies from H to MeO to Me2N. Thus, increasing the N-donor strength to the copper decreases peroxide pi*(sigma), donation to the copper, weakening the Cu-O and O-O bonds. A decrease in v(Cu-O) of the bis-mu-oxo-dicopper(III) complex was also observed with increasing N-donor strength for the R-MePY2 ligand system. However, no change was observed for v(O-O) of the side-on peroxo. This is attributed to a reduced charge donation from the peroxide pi*(sigma), orbital with increased N-donor strength, which increases the negative charge on the peroxide and adversely affects the back-bonding from the Cu to the peroxide sigma* orbital. However, an increase in the bis-mu-oxo-dicopper(III) isomer relative to side-on peroxo-dicopper(II) species is observed for R-MePY2 with R = H < MeO < Me2N. This effect is attributed to the thermodynamic stabilization of the bis-mu-oxo-dicopper(III) isomer relative to the side-on peroxo-dicopper(II) isomer by strong donor ligands. Thus, the side-on peroxo-dicopper(II)/bis-mu-oxo-dicopper(III) equilibrium can be controlled by electronic as well as steric effects.