Copper(I) complexes of the new tripodal ligands tris[2-(1,4-diisopropylimidazolyl)]phosphine (Pim(iPr2)) and tris-[2-(1-isopropyl-4-tert-butylimidazolyl)]phosphine (Pim(iPr,tBu)) have been prepared and characterized by H-1 NMR and elemental analysis. The dioxygen reactivity of the pseudotetrahedral complexes [Cu(Pim(iPr2))CH3CN]BF4 and [Cu(Pim(iPr,tBu))CH3CN]BF4 has been examined. The former complex forms a violet peroxo-dicopper(II) species upon reaction with dioxygen at low temperature; the electronic absorption spectrum (lambda = 343 nm, epsilon = 19 500 M(-1) cm(-1); lambda = 549 nm, epsilon = 790 M(-1) cm(-1)) and O-O stretching frequency (v(O-O) = 750 cm(-1)) of this adduct approximate those of oxyhemocyanin. Warning a mixture of the peroxide complex and triphenylphosphine to room temperature affords [Cu(Pim(iPr2))PPh(3)]BF4 and produces no O=PPh(3). In methanol at -78 degrees C, the peroxide complex is unreactive toward acid, carbon dioxide, 1-hexene, and triphenylphosphine. Thermal decomposition of the dioxygen adduct in the presence of water yields a bis(hydroxide)-bridged dicopper(II) dimer, [CU(Pim(iPr2))](2)(OH)(2)(BF4)(2), which further reacts with atmospheric CO2 to produce the carbonate-bridged complex [CU(Pim(iPr2))]2CO3(BF4)(2). X-ray crystallography has been used to characterize the carbonate complex. Crystal data : orthorhombic, space group Pbca, with a = 16.8705(17) Angstrom, b = 17.4731(14) Angstrom, c = 51.358(4) Angstrom, V = 15139.4(22) Angstrom(3), and Z = 8. The structure of a prototypical mononuclear 5-coordinate complex, [Cu(Pim(iPr2))-OAc]BF4, has also been determined. Crystal data : monoclinic, space group P2(1)/n, with a = 9.657(3) Angstrom, b = 17.001(5) Angstrom, c = 24.642(6) Angstrom, beta = 100.987(23)degrees, V = 3971.4(19) Angstrom(3), and Z = 4. The geometry about copper(II) in both structures is approximately square pyramidal, indicating that Pim(iPr2) can accommodate side-on binding of dioxygen to copper. The more sterically hindered copper(I) complex [Cu(Pim(iPr,tBu))CH3CN]BF4 is inert to O-2, but it reacts readily with CO to form the isolable adduct [Cu(Pim(iPr,tBu))CO]BF4 with nu(CO) = 2086 cm(-1). In accord with its lack of dioxygen reactivity, [Cu(Pim(iPr,tBu))CH3CN]BF4 undergoes a quasi-reversible redox process at very high potential, with E(1/2) (in CH3CN) of + 1.08 V vs Ag/AgCl.