Complexes of the type [Cu-I(T1Et4RIP)]X (T1Et4RIP = tris(1-ethyl-4-R-imidazolyl)phosphine (R = methyl (Me), isopropyl (iPr)); X = PF6, ClO4, CF3SO3), in which T1Et4RIP functions as a tridentate nitrogen donor ligand, were prepared by Lewis acid-base reactions between [Cu-I(CH3CN)(4)]X and T1Et4RIP in dry, degassed solvents. The colorless acetonitrile adducts, [Cu-I(T1Et4RIP)(CH3CN)]X, were obtained by crystallization from acetontrile. The complex with R = iPr and X = PF6 crystallizes in the space group I2(1)3 with a = b = c 18.912(2) Angstrom and Z = 8. The refinement converged with a final R (R(w)) value of 0.075 (0.098) for 988 reflections with F > 3 sigma(F-o). The CUI is coordinated in a trigonally distorted tetrahedral fashion to three imidazolyl nitrogen atoms at 2.08 Angstrom and one acetonitrile nitrogen atom at 1.90 Angstrom. Exposure of dry solutions of these Cu-I complexes to O-2 below -60 degrees C resulted in an isolable purple species with spectroscopic and magnetic properties characteristic of (mu-eta(2):eta(2)-peroxo)dicopper(II) complexes. The representative complex analyzing for {[Cu(T1Et4MeIP)](2)(O-2)}(PF6)(2) has the following approximate values : diamagnetic; UV-vis, 338 nm (epsilon = 19 000 M(-1) cm(-1)), 521 nm (epsilon = 1000 M(-1) cm(-1)); upsilon-(O-O), 740 cm(-1). The Cu X-ray absorption fine structure spectrum of this complex with X = ClO4 was best fit with a Cu scatterer at 3.48 Angstrom, two O (or N) scatterers at 1.94 Angstrom, and two sets of N (or O) scatterers : two at 2.05 Angstrom and one at 2.30 Angstrom. These spectroscopic, magnetic, and structural properties are characteristic of (mu-eta(2):eta(2)-peroxo)dicopper(II) complexes having a planar Cu2O2 core and terminal nitrogen donor ligands, including the dicopper site in oxyhemocyanin. The [Cu-I(T1Et4RIP)(CH3CN)]X complexes with R = iPr react with O-2 at -78 degrees C to produce dioxygen adducts with spectroscopic and magnetic properties very similar to those listed above for R = Me. The complexes with R = iPr, upon repeated thermal cycling between -78 and approximate to-20 degrees C in methanol, exhibit reversible binding of dioxygen. This process can be readily monitored by the characteristic visible absorption spectrum of the dioxygen adduct. Thus, copper complexes of tris(imidazolyl)phosphines with 1,4-disubstituted imidazolyl groups have been successfully used to model spectroscopic, magnetic, structural, and functional properties of the dicopper site in hemocyanin.