Two [Fe-2(II)(N-EtHPTB)(mu-O2X)](2+) complexes, where N-EtHPTB is the anion of N,N,N' N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O2X is O2PPh2 (1 center dot O2PPh2) or O2AsMe2 (1 center dot O2AsMe2), have been synthesized. Their crystal structures both show interiron distances of 3.54 angstrom that arise from a (mu-alkoxo)diiron(II) core supported by an O2X bridge. These diiron(II) complexes react with O-2 at low temperatures in MeCN (-40 degrees C) and CH2Cl2 (-60 degrees C) to form long-lived O-2 adducts that are best described as (mu-eta(1):eta(1)-peroxo)diiron(III) species (2 center dot O2X) with nu(O-O) similar to 850 cm(-1). Upon warming to -30 degrees C, 2 center dot O2PPh2 converts irreversibly to a second (mu-eta(1):eta(1)-peroxo)diiron(III) intermediate (3 center dot O2PPh2) with nu(O-O) similar to 900 cm(-1), a value which matches that reported for [Fe-2(N-EtHPTB)(O-2)(O2CPh)](2+) (3 center dot O2CPh) (Dong et al. J. Am. Chem. Soc. 1993, 115,1851-1859). Mossbauer spectra of 2 center dot O2PPh2 and 3 center dot O2PPh2 indicate that the iron centers within each species are antiferromagnetically coupled with J similar to 60 cm(-1), while extended X-ray absorption fine structure analysis reveals interiron distances of 3.25 and 3.47 angstrom for 2 center dot O2PPh2 and 3 center dot O2PPh2, respectively. A similarly short interiron distance (3.27 angstrom) is found for 2 center dot O2AsMe2. The shorter interiron distance associated with 2 center dot O2PPh2 and 2 center dot O2AsMe2 is proposed to derive from a triply bridged diiron(III) species with alkoxo (from N-EtHPTB), 1,2-peroxo, and 1,3-O2X bridges, while the longer distance associated with 3 center dot O2PPh2 results from the shift of the O2PPh2 bridge to a terminal position on one iron. The differences in nu(O-O) are also consistent with the different interiron distances. It is suggested that the O center dot center dot center dot O bite distance of the O2X moiety affects the thermal stability of 2 center dot O2X, with the O2X having the largest bite distance (O2AsMe2) favoring the 2 center dot O2X adduct and the O2X having the smallest bite distance (O2CPh) favoring the 3 center dot O2X adduct. Interestingly, neither 3 center dot O2AsMe2 nor the benzoate analog of 2 center dot O2X (2 center dot O(2)Bz) are observed.