Two tetracarboxylate diiron(II) complexes, [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(C5H5N)(2)] (1a) and [Fe-2(mu-O2CArTol)4((4)-(BuC5H4N)-Bu-t)(2)] (2a), where (ArCO2-)-C-Tol = 2,6-di(p-tolyl)benzoate, react with O-2 in CH2Cl2 at -78 degreesC to afford dark green intermediates 1b (lambda(max) congruent to660 nm; epsilon = 1600 M-1 cm(-1)) and 2b (lambda(max) congruent to 670 nm; epsilon = 1700 M-1 cm(-1)), respectively. Upon warming to room temperature, the solutions turn yellow, ultimately converting to isolable diiron(III) compounds (Fe-2(mu-OH)(2)(mu-O2CArTol)(2)(O2CArTol)(2)L-2] (L = C5H5N (1c), 4-(BuC5H4N)-Bu-t (2c)), EPR and Mossbauer spectroscopic studies revealed the presence of equimolar amounts of valence-delocalized (FeFeIII)-Fe-II and valence-trapped (FeFeIV)-Fe-III species as major components of solution 2b. The spectroscopic and reactivity properties of the (FeFeIV)-Fe-III species are similar to those of the intermediate X in the RNR-R2 catalytic cycle. EPR kinetic studies revealed that the processes leading to the formation of these two distinctive paramagnetic components are coupled to one another. A mechanism for this reaction is proposed and compared with those of other synthetic and biological systems, in which electron transfer occurs from a low-valent starting material to putative high-valent dioxygen adduct(s).