A series of diiron(II) complexes, [Fe-2(mu-L)(mu-O2CR)(O2CR)(N)(2)], where L is a dinucleating bis(carboxylate) ligand based on m-xylylenediamine bis(Kemp's triacid imide) and N is a pyridine- or imidazole-derived ligand, were prepared; as models for the: carboxylate-bridged non-heme diiron cores of the O-2-activating enzymes, soluble methane monooxygenase-hydroxylase (MMOH), and the R2 subunit of ribonucleotide reductase (RNR-R2). X-ray crystallographic studies revealed differences in the coordination geometry of the bridging monocarboxylate ligand, which shifts from monodentate to syn,syn-bidentate bonding modes. The extent of this carboxylate shift depends on both the steric bulk of the monocarboxylate and the basicity of the ancillary N-donor ligands. Exposure of these diiron(II) complexes to O-2 at -77 degrees C in nonpolar solvents (CH2Cl2, THF, toluene) yielded deep blue solutions (lambda(max) approximate to 580 nm, epsilon approximate to 1200 M-1 cm(-1)), consistent with the generation of diiron(III) peroxo species. This reaction was irreversible, and its stoichiometry was determined by manometry to be 1:1 in diiron(Il) complex and O-2. The diiron(III) peroxo complexes exhibited oxygen isotope-sensitive resonance Raman bands at similar to 860 cm(-1), which are assigned to the O-O stretching frequency of a mu-1,2-peroxodiiron(III) core. Fe-57 Mossbauer spectroscopy confirmed the assignment of the diiron(III) oxidation level and indicated that the two iron sites have inequivalent environments (delta(1) approximate to 0.47 mm s(-1), Delta E-Q1 approximate to 0.88 mm s(-1); delta(2) approximate to 0.63 mm s(-1), Delta E-Q2 approximate to 1.20 mm s(-1)). Kinetics experiments provided rate constants for the reaction and revealed it to be first order in both diiron(II) complex and O-2. The factors controlling the rate of formation of the blue species and its stability are discussed.