Kinetics and mechanisms of the reduction of µ-amido-µ-superoxo- and µ-amido-µ-peroxo-bis[bis(ethylenediamine)cobalt(III)] complexes by sulphite, nitrite, and arsenite in aqueous solutions: evidence for bonded and non-bonded activated complexes in the reduction of dioxygen-bridged complexes
Abstract
The µ-amido-µ-superoxo-complex (en)2Co·µ(NH2,O2)·Co(en)24+and the µ-amido-µ-peroxo-complex (en)2Co·µ(NH2,O2)·Co(en)23+both react with sulphite ions to give the µ-amido-µ-sulphato-complex(en)2Co·µ(NH2,SO4)·Co(en)23+. The µ-amido-µ-superoxo-reaction proceeds with the intermediate formation of the µ-amido-µ-peroxo-complex and the sulphite radical. Stoicheiometries at pH 1–3 are (complex : sulphite) 2 : 3 for the superoxo- and 1 : l for the peroxo-complex, and rate laws are of the form ks[superoxo][SO32–] and kp[peroxo][H2SO3] respectively, where ks= 3·80(±0·07)× 104 l mol–1 s–1 and kp= 55·4(±6·1) l mol–1 s–1, both at l·4 °C and I= 1·0M(LiClO4). The protonated µ-peroxo-complex may be the reactive species in the latter, in which case the rate law can be expressed in an alternative form. The complexes also react with nitrite ions to give the µ-amido-µ-nitrito-complex (en)2Co·µ(NH2,NO2)·Co(en)24+. With arsenite ions a different reaction path is operative, and the product is the µ-amido-µ-hydroxo-complex, (en)2Co·(NH2,OH)·Co(en)24+.