Two [FeII2(N-EtHPTB)(μ-O2X)] 2+ complexes, where N-EtHPTB is the anion of N, N,N N-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O 2X is O2PPh2 (1.O2PPh2) or O 2AsMe2 (1-O2AsMe2), have been synthesized. Their crystal structures both show interiron distances of 3.54 À that arise from a (μ-alkoxo)diiron(II) core supported by an O 2X bridge. These diiron(II) complexes react with O2 at low temperatures in MeCN (-40 °C) and CH2Cl2 (-60 °C) to form long-lived O2 adducts that are best described as (μ-η1: ηpexoxo)diiron(III) species (2-O2X) with ν0-0 ~ 850 cm-1. Upon warming to -30 °C, 2-O 2PPh2 converts irreversibly to a second (μ- η1:η1 -peroxo)diiron(III) intermediate (3-O 2PPh2) with v0-o ~ 900 cm-1 , a value which matches that reported for [Fe2(NEtHPTB)(O 2)(O2CPh)]2+ (3 • O2CPh) (Dong et al. J. Am. Chem. Soc. 1993, 115, 1851 -1859). Mössbauer spectra of 2-O2PPh2 and 3-O2PPh2 indicate that the iron centers within each species are antiferromagnetically coupled with J~ 60 cm-1, while extended X-ray absorption fine structure analysis reveals interiron distances of 3.25 and 3.47 Á for 2-O 2PPh2 and 3-O2PPh2, respectively. A similarly short interiron distance (3.27 Á) is found for 2-O 2AsMe2. The shorter interiron distance associated with 2-O2PPh2 and 2-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.O2PPh2 results from the shift of the O2PPh2 bridge to a terminal position on one iron. The differences in v0-o are also consistent with the different interiron distances. It is suggested that the O - O bite distance of the O2X moiety affects the thermal stability of 2-O2X, with the O 2X having the largest bite distance (O2AsMe2) favoring the 2-O2X adduct and the O2X having the smallest bite distance (O2CPh) favoring the 3-O2X adduct. Interestingly, neither 3-O2AsMe2 nor the benzoate analog of 2-O 2X (2-O2Bz) are observed.