Dioxygen adducts of [Fe2L(O2CC6H5)]X2, where L represents the dinucleating ligands HPTB (anion of N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane), its N-ethyl analogue, and its tetrakis(pyridine) analogue, HPTP, can form and serve as models for the putative oxygenated intermediates of methane monooxygenase and ribonucleotide reductase. [Fe2(N-Et-HPTB)(O2CC6H5)J(BF4)2 (1) crystallizes in the triclinic space group with cell constants a = 13.04 (1) Å, b = 14.248 (7) Å, c = 18.09 (1) Å, α = 73.56 (6)°, β = 78.22 (7)°, γ = 67.71 (6)°, V= 2963 (9) Å3, Z = 2; R = 0.069, and Rw = 0.085. The Fe(II) sites are bridged by the alkoxide of the dinucleating ligand and a benzoate, affording a diiron core with an Fe-µ-O-Fe angle of 124.0 (3)° and an Fe-Fe distance of 3.473 (7) Å. Both Fe(II) centers have trigonal bypyramidal geometry, and NMR studies show that the remaining coordination sites are accessible to ligands such as DMSO and Ph3PO. The iron centers are antiferromagnetically coupled with J ∼ 20–26 cm−1 (JS1•S2). Irreversible dioxygen adducts form upon exposure of the diferrous complexes to O2 at low temperatures. The 1/O2 adduct and its HPTB analogue, 2/O2, are stable indefinitely in CH2Cl2 at −60 °C but decompose upon warming; the addition of DMSO or other polar aprotic solvents further stabilizes the adducts, allowing them to persist for short periods even at ambient temperature. The adduct of the pyridine analogue, 3/O2, on the other hand, is not observed at −80 °C unless a polar aprotic solvent is added to the CH2Cl2 solution. The adducts exhibit visible absorption maxima near 600 nm and resonance Raman features at ∼470 cm−1 (v(Fe-O)) and ∼890–900 cm−1 (v(O-O)). The latter is characteristic of a µ-1,2-peroxo species; in support, the NMR properties of the HPTB adducts indicate the presence of a moderately strong antiferromagnetic coupling interaction (J ∼ 140 cm−1). Carboxylate substitution on 1 effects a shift of the absorption maximum of the adduct, indicating that the carboxylate remains coordinated in the adduct. Thus, the adducts are proposed to have tribridged (µ-1,2-peroxo) (µ-carboxylato) (µ-alkoxo)diferric cores. The differing stabilities of the dioxygen adducts are also reflected in differences in reactivity. The addition of 2,4-di-tert-butylphenol or Ph3P does not affect the 1/O2 adduct at −50 °C but does accelerate the decomposition of the 3/O2 adduct, affording 0.5–0.6 equiv of the corresponding biphenol or OPPh3, respectively. The one-electron oxidation of a phenol by 3/O2 suggests that such an oxygenated species may be involved in the mechanism of the tyrosyl radical formation in ribonucleotide reductase; however, some further activation step is likely to be required for such a species to participate in the alkane hydroxylation mechanism of methane monooxygenase.