A description of the structure and bonding of novel bis(mu-oxo)dicopper complexes and their bis(mu-hydroxo)dicopper decomposition products was derived from combined X-ray crystallographic, spectroscopic, and ab initio theoretical studies. The compounds [(LCu)(2)(mu-O)(2)]X(2) were generated from the reaction of solutions of [LCu-(CH3CN)]X with O-2 at -80 degrees C (L = 1,4,7-tribenzyl-1,4,7-triazacyclononane, L(Bn3); 1,4,7-triisopropyl-1,4,7-triazacyclononane, L(ipr3); or 1-benzyl-4,7-diisopropyl-1,4,7-triazacyclononane, L(iPr2Bn); X = variety of anions). The geometry of the [Cu-2(mu-O)(2)](2+) core was defined by X-ray crystallography for [(d(21)-L(Bu3)Cu)(2)(mu-O)(2)](SbF6)(2) and by EXAFS spectroscopy for the complexes capped by L(Bn3) and L(iPr3); notable dimensions include short Cu-O (similar to 1.80 Angstrom) and Cu ... Cu (similar to 2.80 Angstrom) distances like those reported for analogous M(2)(mu-O)(2) (M = Fe or Mn) rhombs.The core geometry is contracted compared to those of the bis(mu-hydroxo)dicopper(II) compounds that result from decomposition of the bis(mu-oxo) complexes upon warming. X-ray structures of the decomposition products [(L(Bn3)Cu)(L(Bn2H)Cu)- (mu-OH)(2)](O3SCF3)(2) . 2CH(3)CO, [(L(iPr2H)Cu)2(C)(mu-OH)(2)](BPh(4))(2) . 2THF, and [(L(iPrBn)Cu)(2)(mu-OH)(2)](O3SCF3)(2) showed that they arise from N-dealkylation of the original capping macrocycles. Manometric, electrospray mass spectrometric, and W-vis, EPR, NMR, and resonance Raman spectroscopic data for the bis(mu-oxo)dicopper complexes in solution revealed important topological and electronic structural features of the intact [Cu-2(mu-O)(2)](2+) core. The bis(mu-oxo)-dicopper unit is diamagnetic, undergoes a rapid fluxional process involving interchange of equatorial and axial N-donor ligand environments, and exhibits a diagnostic similar to 600 cm(-1) O-18-sensitive feature in Raman spectra. Ab initio calculations on a model system, {[(NH3)(3)Cu](2)(mu-O)(2)}(2+) predicted a closed-shell singlet ground-state structure that agrees well with the bis(mu-oxo)dicopper geometry determined by experiment and helps to rationalize many of its physicochemical properties. On the basis of an analysis of the theoretical and experimental results (including a bond valence sum analysis), a formal oxidation level assignment for the core is suggested to be [Cu-2(III)(mu-O2-)(2)](2+), although a more complete molecular orbital description indicates that the oxygen and copper fragment orbitals are significantly mixed (i.e., there is a high degree of covalency).