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 k_s[superoxo][SO₃^2–] and k_p[peroxo][H₂SO₃] respectively, where k_s= 3·80(±0·07)× 10⁴ l mol^–1 s^–1 and k_p= 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⁺.