A Cu(I) complex, LCu(CH3CN), was prepared and characterized, where L-is a sterically unencumbered beta-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(CH3CN) 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(CH3CN) generated a bis(mu-oxo)tricopper complex with a S = 1 [Cu3O2](3+) core that was identified on the basis of UV-vis (lambda(max) (epsilon, M-1 cm(-1) per Cu) = 328 (10700), 420 (1500), 590 (835) nm) and X-band electron paramagnetic resonance (EPR) spectroscopy (Delta m(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 (mu(eff) = 2.8(1) BM), and H2O2 detection experiments (no H2O2 evolved upon acidification). Unlike other reported variants supported by neutral N-donor ligands, L3Cu3O2 is not reduced by ferrocene, does not abstract H-atoms from phenols or 1,2-dihydroanthracene, oxidizes PPh3 to Ph3P=O, and generates carbonate species upon exposure to CO2. This unique reactivity for a [Cu3O2](3+) complex may be traced to the anionic charge and strong electron donating characteristics of L-.