The initial step in the thermal decomposition of Co(NH3)63+ in acidic aqueous solution is the replacement of NH3 by H2O, which occurs by a hydrogen-ion independent path, first order in complex, with rate coefficient k1 = 7.9 × 10−5 s−1 (140.4°), ΔH* = 36.6 kcal mol−1 and ΔS* = 10.7 cal deg−1 mol−1 in 0.1 M HClO4. For Co(NH3)5OH23+, there is a similar initial aquation path with k1 = 12.6 × 10−5 s−1 (140.6°), ΔH* = 41.9 kcal mol−1 and ΔS* = 24 cal deg−1 mol−1 and also a path first order in complex but inverse first order in [H+] with k2′ = 6.2 × 10−1 M s−1 (140.6°), ΔH* = 43.5 kcal mol−1 and ΔS* = 26.7 cal deg−1 mol−1 in perchlorate media of ionic strength 1.0 M. The effects of electrolyte type and concentration on the rates of these reactions have been examined. Subsequent aquation steps are relatively rapid because of the predominance of inversely [H+]-dependent pathways and are followed by redox to Co(H2O)62+, NH4+, N2, N2O, and a minor amount of O2. A mechanism involving OH and NH2 radicals is proposed for the redox step.

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ANTHONY MARTIN NEWTON. Can. J . Chem. 64, 31 1 (1986).In acetic acid -sodium acetate buffer of pH 5.6 (25°C) the C O ( N H~)~S~O , + ion undergoes redox decomposition rather than aquation. First-order kinetic are observed and the reaction products c o 2 + , NH3, and S4O6'-are due to internal reduction of Co(II1) by coordinated S2032-. In dilute perchloric acid of pH <4 the rate is retarded, first-order plots are not linear, and S4062-is not a major product of the reaction. It is proposed that, in dilute HClO,, protonation of C O ( N H~)~S~O~+ depletes the concentration of the reactive complex and that decomposition of coordinated HS2O3-occurs. Conversion of 0-bonded ~~0 2 -to S-bonded ~~0 2 -in the reactive complex is also considered.ANTHONY MARTIN NEWTON. Can. J. Chem. 64, 311 (1986) Dans un tampon d'acide acitique -acetate de sodium, a pH 5,6 et a 25OC, l'ion C O ( N H~)~S~O~+ subit plutBt une reaction d'oxydo-reduction qu'une reaction d'aquation. On observe une cinktique du premier ordre et les produits de la reaction, soit le co2+, le NH3 et le S4062-, proviennent de la reduction interne du Co(II1) par l'ion S2O3'-coordonnk. L'acide perchlorique diluk, pH <4, retarde la vitesse alors que les courbes d'ordre un ne sont pas lineaires et que I'ion S4O6'-n'est pas un produit majoritaire de la reaction. On pense que, en presence de HC104 dilut, la protonation de l'ion C O ( N H~)~S~O~+ diminue la concentration du complexe reactif et que I'ion coordonte HS2O3-se decompose. On considere tgalement la transformation de l'ion S203'-lie par I'oxyg&ne en l'ion ~2 0 3~-lie par le soufre dans le complexe rtactif.[Traduit par le journal] i