Kinetic studies on the reaction of nitrous acid with the µ-superoxobis[penta-amminecobalt(III)] complex

Abstract

The stoicheiometry and products of the reaction of nitrous acid with the µ-superoxobis[penta-amminecobalt(III)] complex in perchloric acid solutions, µ= 0·5M(NaClO₄), are as in (i) with a large (125-fold) excess of nitrous acid. At lower (excess) nitrous acid concentrations as much as 75% of the initial cobalt(III) product is present as Co(NH₃)₅H₂O³⁺(with an equivalent amount of NO₃^–). The rate law is as in (ii), with initial reactant concentrations, (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺+ 2HNO₂ [graphic omitted] Co(NH₃)₅NO₃²⁺+ Co²⁺+ NO₃^–+ 5NH₄⁺(i), d In [(NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺]/dt=k_uncat=k_HNO2[HNO₂]²[H⁺]^–1(ii), [(NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺]= 2·5 × 10^–4—5·7 × 10^–4M, [HNO₂]= 0·43 × 10^–2—1·3 × 10^–2M and [H⁺]= 0·07—0·30M. At 25 °C k_HNO2(which is a composite term) is 1·04 ± 0·01 l mol^–1 s^–1. The reaction is catalysed by bromide ions (0·25 × 10^–2—1·25 × 10^–2M), when there is a further term in the rate law as in (iii). At 25 °C, k^Br_cat(s^–1)=k_BrK_IP[Br^–][HNO₂]//_{1 +KIP[Br^–]}(iii), [H⁺]= 0·07—0·25M and [HNO₂]= 0·24 × 10^–2—1·0 × 10^–2M, the equilibrium constant K_IP for the ion-pairing of Br^– to the complex is 37·9 ± 3·6 l mol^–1, and the rate constant k_Br for the reaction of the ion-pair with HNO₂ is 1·20 ± 0·09 l mol^–1(s^–1). The cobalt(III) product from the catalysed path appears to be exclusively Co(NH₃)₅H₂O³⁺. Chloride is much less effective as a catalyst, K_IP= 6·3 ± 1·2 l mol^–1 and k_Cl= 0·13 ± 0·02 l mol^–1 s^–1(25 °C).