Oxidation of Formatopentaamminecobalt(III) Perchlorate by Silver(II)

Abstract

Formatopentaamminecobalt (III) perchlorate is oxidized relatively rapidly by AgO in acid conditions (1 M H₂SO₄, HNO₃ or HO₃SCF₃), with approximately 97% of the total cobalt retained in the cobalt(III) state as [Co(NH₃)₅(OH₂)]³⁺ and [Co(NH₃)₅X]ⁿ⁺ (X = SO₄^2- or NO₃^-). A combination of product distribution, silver(I) and silver(II) concentration dependence and ¹⁸O-labelling experiments indicate that the major pathway (97%) is oxidation of [Co(NH₃)₅(OCHO)]²⁺ directly to [(NH₃)₅Co(OCO)]³⁺ and [(NH₃)₅Co(O₂COH)]²⁺ by Ag^III (through the disproportionation 2Ag^II ↔ Ag^III + Ag^I ) in a relatively rapid hydride extraction. A minor path (3%) appears to involve a one-electron oxidation by Ag^II to the complexed radical anion [(NH₃)₅Co(OC^?O)]²⁺ which then decays intramolecularly to yield Co^II and CO₂. In 1 M H₂SO₄ the cobalt(III) intermediate, [(NH₃)₅Co(OCO)]³⁺, has been shown by ¹⁸O-labelling and competition experiments to decay by dissociation into [Co(NH₃)₅]³⁺ and CO₂. The pentacoordinate intermediate [Co(NH₃)₅]³⁺ immediately captures OH₂ and HSO₄^-.