The kinetics of O2-uptake of five-coordinated Co2+/tren complexes (tren = 2,2′, 2″-tris(2-aminoethyl)amine) have been studied extensively. The kinetics of formation of (tren)Co(O2, OH)Co(tren)3+ exhibits two steps. The rate law of O2-addition, the first step, was of the form: rate = ( k