Reaction of the Pentacyanocobaltate(II) Ion with Molecular Oxygen

Abstract

By spectrophotometric scanning of reacting solutions, we have studied the processes which can occur when Oz is admitted to aqueous solutions containing the Ion [Co^II(CN)₅]^3- in the concentration range 0.0002-0.4M, at 0°. A scheme is given to explain the formation of and inter-relations between [(CN)₅Co^III-O-O-Co^III(CN)₅]^6-; [(CN)₅Co^III-O-O-Co^IV(CN)₅]^5-; [(CN)₅Co^IIIOH₂]^2- ; [Co^III(CN)₆]^3-, and a complex believed to be [(CN)₅Co^III-O-OH]^3-, which absorbs at 272 mp Preparative methods are described for obtaining several of the oxidation products in the crystaline state. Hydrolysis of [(CN)₅Co^III-O-O-Co^III(CN)₅]^6- to [(CN)₅Co^IIIOH₂]^2- takes place via the 272 mμ complex, which is fairly stable in alkaline solution and is resistant to oxidation. The same 272 mμ complex can be formed more directly by oxygenation of the hydrido complex [(CN)₅Co^IIIH]^3-, which is present in aged or hydrogenated solutions of pentacyanocobaltate(II). All of the pentacyano oxidation products can be reduced by sodium borohydride to [(CN)₅Co^IIIH]^3-, which is further reduced by excess borohydride to an insoluble green compound and finally to metallic cobalt. We have briefly examined the reactions of ammonia and CN^- with [(CN)₅Co^IIIOH₂]^2-, and the reaction of [(CN)₅Co^III-O-O-Co^IV(CN)₅]^5- with reducing agents, including [Co^II(CN)5]^3-.