A kinetic study of the reaction between a diiron(II) complex [Fe-2(II)(mu -OH)(2)(6-Me-3-TPA)(2)](2+) 1, where 6-Me-3-TPA = tris(6-methyl-2-pyridylmethyl)amine, and dioxygen is presented. A diiron(III) peroxo complex [Fe-2(III)(mu -O)(mu -O-2)(6-Me-3-TPA)(2)](2)+ 2 forms quantitatively in dichloromethane at temperatures from -80 to -40 degreesC. The reaction is first order in [Fe-2(II)] and [O-2], with the activation parameters DeltaH double dagger = 17 +/- 2 kJ mol(-1) and DeltaS double dagger = -175 +/- 20 J mol(-1) K-1. The reaction rate is not significantly influenced by the addition of H2O or D2O. The reaction proceeds faster in more polar solvents (acetone and acetonitrile), but the yield of 2 is not quantitative in these solvents. Complex 1 reacts with NO at a rate about 10(3) faster than with O-2. The mechanistic analysis suggests an associative rate-limiting step for the oxygenation of 1, similar to that for stearoyl-ACP Delta (9)-desaturase, but distinct from the probable dissociative pathway of methane monoxygenase. An eta (1)-superoxo (FeFeIII)-Fe-II species is a likely steady-state intermediate during the oxygenation of complex 1.