Kinetics and mechanism of the oxidation of thiosulphate ions by copper(II) ions in aqueous ammonia solution

Copper(II) ions in aqueous ammonia solution oxidize thiosulphate ions initially to tetrathionate ions; the latter then undergo a subsequent disproportionation reaction to yield trithionate and thiosulphate ions. The detailed kinetics of the reaction suggest a mechanism which involves substitution of thiosulphate ion into the co-ordination sphere of a triamminecopper(II) complex in the rate-determining step. A one-equivalent electron transfer from the thiosulphate to the copper(II) ion, occurring in the intermediate triamminecopper(II)–thiosulphate complex, gives rise to copper(I) and S2O3– ions, the latter dimerizing to tetrathionate ions.