A Cu(I) complex, LCu(CH 3CN), was prepared and characterized, where L - is a sterically unencumbered β-diketiminate ligand, the deprotonated version of 4-(2,2-dimethylhydrazino)dimethylhydrazone-3-penten-2- one (LH). Analysis of FTIR spectra of the products of the reaction of LCu(CH 3CN) with CO indicate that L - is strongly electron donating, and support an equilibrium in solution between monomeric and dimeric forms with terminal and bridging CO ligands, respectively. Low temperature oxygenation of LCu(CH 3CN) generated a bis(μ-oxo)tricopper complex with a S = 1 [Cu 3O 2] 3+ core that was identified on the basis of UV-vis (λ max (ε, M -1 cm -1 per Cu) = 328 (10700), 420 (1500), 590 (835) nm) and X-band electron paramagnetic resonance (EPR) spectroscopy (Δm s = 2 transition at 1500 G), electrospray ionization (ESI) mass spectrometry, and spectrophotometric titration (0.35(2) equiv of O 2 per copper atom), magnetic susceptibility (μ eff = 2.8(1) BM), and H 2O 2 detection experiments (no H 2O 2 evolved upon acidification). Unlike other reported variants supported by neutral N-donor ligands, L 3Cu 3O 2 is not reduced by ferrocene, does not abstract H-atoms from phenols or 1,2-dihydroanthracene, oxidizes PPh 3 to Ph 3P=O, and generates carbonate species upon exposure to CO 2. This unique reactivity for a [Cu 3O 2] 3+ complex may be traced to the anionic charge and strong electron donating characteristics of L -.