In order to model the syn disposition of histidine residues in carboxylate-bridged non-heme diiron enzymes, we prepared a new dinucleating ligand, H[subscript 2]BPG[subscript 2]DEV, that provides this geometric feature. The ligand incorporates biologically relevant carboxylate functionalities, which have not been explored as extensively as nitrogen-only analogues. Three novel oxo-bridged diiron(III) complexes, [Fe[subscript 2](μ-O)(H[subscript 2]O)2(BPG[subscript 2]DEV)](ClO[subscript 4])[subscript 2] (6), [Fe[subscript 2](μ-O)(μ-O[subscript 2]CAr[superscript iPrO])(BPG[subscript 2]DEV)](ClO[subscript 4]) (7), and [Fe[subscript 2](μ-O)(μ-CO[subscript 3])(BPG[subscript 2]DEV)] (8), were prepared. Single-crystal X-ray structural characterization confirms that two pyridyl groups are bound syn with respect to the Fe−Fe vector in these compounds. The carbonato-bridged complex 8 forms quantitatively from 6 in a rapid reaction with gaseous CO[subscript 2] in organic solvents. A common maroon-colored intermediate (λ[subscript max] = 490 nm; ε = 1500 M[superscript −1] cm[superscript −1]) forms in reactions of 6, 7, or 8 with H[subscript 2]O[subscript 2] and NEt[subscript 3] in CH[subscript 3]CN/H[subscript 2]O solutions. Mass spectrometric analyses of this species, formed using [superscript 18]O-labeled H[subscript 2]O[subscript 2], indicate the presence of a peroxide ligand bound to the oxo-bridged diiron(III) center. The Mössbauer spectrum at 90 K of the EPR-silent intermediate exhibits a quadrupole doublet with δ = 0.58 mm/s and ΔE[subscript Q] = 0.58 mm/s. The isomer shift is typical for a peroxodiiron(III) species, but the quadrupole splitting parameter is unusually small compared to those of related complexes. These Mössbauer parameters are comparable to those observed for a peroxo intermediate formed in the reaction of reduced toluene/o-xylene monooxygenase hydroxylase with dioxygen. Resonance Raman studies reveal an unusually low-energy O−O stretching mode in the peroxo intermediate that is consistent with a short diiron distance. Although peroxodiiron(III) intermediates generated from 6, 7, and 8 are poor O-atom-transfer catalysts, they display highly efficient catalase activity, with turnover numbers up to 10 000. In contrast to hydrogen peroxide reactions of diiron(III) complexes that lack a dinucleating ligand, the intermediates generated here could be re-formed in significant quantities after a second addition of H[subscript 2]O[subscript 2], as observed spectroscopically and by mass spectrometry.National Institute of General Medical Sciences (U.S.) (Grant GM032134)