The variation of ligand para substituents on pyridyl donor groups of tridentate amine copper(l) complexes was carried out in order to probe electronic effects on the equilibrium between mu-eta(2):eta(2-)(side-on)-peroxo [Cu(II)2(O-2(2-))](2+) and bis(mu-oxo) [Cu-2(III) (O2-)(2)] species formed upon reaction with O-2. [Cu-I(R-PYAN)(MeCN)(n)]B(C6F5)(4) (R-PYAN = N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N',N'-trimethyl-propane-1,3-diamine, R = NMe2, OMe, H, and Cl) (1(R)) vary over a narrow range in their Cu-II/Cu-I redox potentials (E-1/2 vs Fe(cp)(2)(+/0) = -0.40 V for 1(NMe2), -0.38 V for 1(OMe), -0.33 V for 1(H), and -0.32 V for 1(Cl)) and in C-O stretching frequencies of their carbonyl adducts, 1(R)-CO: nu(C-O) = 2080, 2086, 2088, and 2090 cm(-1) for R = NMe2, OMe, H, and Cl, respectively. However, within this range of electronic properties for 1 R, dioxygen reactivity is significantly affected. The reaction of 1(Cl) or 1(H) with O-2 at -78 degrees C in CH2Cl2 gives UV-vis and resonance Raman spectra indicative of a mu-eta(2):eta(2)-(side-on)-peroxo dicopper(II) adduct (2(R)). Compound 1(OMe) reacts with O-2, yielding equilibrium mixtures of side-on peroxo (2(OMe)) and bis(mu-oxo) (3(OMe)) species. Oxygenation of 1(NMe2) leads to the sole generation of the bis(mu-oxo) dicopper(III) complex (3(NMe2)). A solvent effect was also observed; in acetone or THF, increased ratios of bis(mu-oxo) relative to side-on peroxo complex are observed. Thus, the equilibrium between a dicopper side-on peroxo and bis(mu-oxo) species can be tuned by ligand design-specifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis(mu-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations concerning the reactivity of the dioxygen adducts 2(H) and 3(NMe2) toward external substrates are also presented.