The red copper(I) title compound {[Cu^I(imep)]⁺} is apparently the first synthetic copper complex to functionally mimic hemocyanin in its capacity to reversibly bind O₂ in solution under ambient conditions (1 atm O₂; room temperature) with a reversibility factor of ca. 80% per oxy/deoxy cycle. Manometry has established the O₂ : Cu reaction stoicheiometry to be the same (1 : 2) as for the oxygenation of hemocyanin. The green oxy form of [Cu^I(imep)]⁺ is proposed to be a µ-dioxygen species with each CuN₅O centre of the binuclear unit being six-co-ordinate and having two imidazole, one pyridine, and two imine nitrogen-donor atoms. Like oxyhemocyanin, the oxy form of [Cu^I(imep)]⁺ is essentially e.s.r. silent at 100 K, indicating a fully coupled S= 0 ground state if the binuclear centre is formally viewed as [Cu^II–O₂^–2–Cu^II]. However, unlike hemocyanin, the [Cu^I(imep)]⁺ cation does not form an adduct with CO under ambient conditions. If the imidazole moieties are replaced by pyridine groups. [Cu^I(pyep)]⁺, the copper (I) centre no longer reversibly bind O₂. This result is consistent with cyclic voltammetric data showing the Cu^I state for [Cu^I(pyep)]⁺ to be stabilized by 130 mV relative to that of [Cu^I(imep)]⁺. Cyclic voltammetry has also been used as a probe for observing and following what is presumed to be the reversible oxygenation process of [Cu^I(imep)]⁺ and to demonstrate the lack of reactivity with CO. Finally, the [M(imep)][ClO₄]₂ and [M(pyep)][ClO₄]₂(M = Cu^II or Zn^II) compounds have been isolated and characterized for purposes of comparison with their Cu^I analogue.