Several [Fe-2(II)(N-EtHPTB)(mu-O2X)](2+) complexes (1 center dot O2X) have been synthesized, where N-EtHPTB is the anion of N,N,N'N'-tetrakis (2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O2X is an oxyanion bridge. Crystal structures reveal five-coordinate (mu-alkoxo)diiron(II) cores. These diiron(II) complexes react with O-2 at low temperatures in CH2Cl2 (-90 degrees C) to form blue-green O-2 adducts that are best described as triply bridged (mu-eta(1):eta(1)-peroxo)diiron(III) species (2 center dot O2X). With one exception, all 2 center dot O2X intermediates convert irreversibly to doubly bridged, blue (mu-eta(1):eta(1)-peroxo)diiron(III) species (3 center dot O2X). Where possible, 2 center dot O2X and 3 center dot O2X intermediates were characterized using resonance Raman spectroscopy, showing respective nu(O-O) values of similar to 850 and similar to 900 cm(-1). How the steric and electronic properties of O2X affect conversion of 2 center dot O2X to 3 center dot O2X was examined. Stopped-flow analysis reveals that oxygenation kinetics of 1 center dot O2X is unaffected by the nature of O2X, and for the first time, the benzoate analog of 2 center dot O2X (2 center dot O2CPh) is observed.