The reactivity between a thiolate-ligated five-coordinate complex [Fe-II((SN4)-N-Me2(tren))](+) (1) and dioxygen is examined in order to determine if O-2 activation, resembling that of the metalloenzyme cytochrome P450, can be promoted even when O-2 binds cis, as opposed to trans, to a thiolate. Previous work in our group showed that [Fe-II((SN4)-N-Me2(tren))](+) (1) reacts readily with superoxide (O-2) in the presence of a proton source to afford H2O2 via an (FeOOH)-O-III intermediate, thus providing a biomimetic model for the metalloenzyme superoxide reductase (SOR). Addition of O-2 to 1 affords binuclear mu-oxo-bridged [Fe-III((SN4)-N-Me2(tren))](2)(mu(2)-O)(PF6)(2).3MeCN (3). At low temperatures, in protic solvents, an intermediate is detected, the details of which will be the subject of a separate paper. Although the thiolate ligand does not appear to perturb the metrical parameters of the unsupported mu-oxo bridge (Fe-O = 1.807(8) Angstrom, and Fe-O-Fe = 155.3(5)degrees fall in the usual range), it decreases the magnetic coupling between the irons (J = -28 cm(-1)) and creates a rather basic oxo site. Protonation of this oxo using strong (HBF4, HCl) or weak (HOAc, NH4PF6, LutNHCl) acids results in bridge cleavage to cleanly afford the corresponding monomeric anion-ligated (OAc- (6), or Cl- (7)) or solvent-ligated (MeCN (4)) derivatives. Addition of OH-converts [Fe-III((SN4)-N-Me2(tren))(MeCN)](2+) (4) back to-mu-oxo 3. Thus, mu-oxo bridge cleavage is reversible. The protonated mu-hydroxo-bridged intermediate is not observed. In an attempt to prevent mu-oxo dimer formation, and facilitate the observation of O-2-bound intermediates, a bulkier tertiary amine ligand, tren-Et-4 = N-(2-amino-ethyl)-N-(2-diethylamino-ethyl)-N',N'-diethyl-ethane-1,2-diam ine, and the corresponding [Fe-II((SN4)-N-Me2(tren-Et-4))](+) (5) complex was synthesized and structurally characterized. Steric repulsive interactions create unusually long Fe-II-N(3,4) amine bonds in 5 (mean distance = 2.219(1) Angstrom). The [(tren-Et-4)N4SMe2](1-) ligand is unable to accommodate iron in the +3 oxidation state, and consequently, in contrast to most thiolate-ligated Fe(II) complexes, [Fe-II((SN4)-N-Me2(tren-Et-4))](+) (5) does not readily react with O-2. Oxidation of 5 is irreversible, and the potential (E-p(a) = +410 mV (vs SCE)) is anodically shifted relative to 1 (E-1/2 = -100 mV (vs SCE)).