Reactions of µ-hydroxo-dicobalt complexes. Part 1. The kinetics of the equilibration of the µ-amido-µ-hydroxo-bis[tetra-ammine cobalt(III)] complex with dilute nitric acid

Abstract

The extent of the reaction of nitric acid with the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex has been studied spectrophotometrically. A self-consistent interpretation is possible assuming that the equilibrium (i) in (NH₃)₄Co[graphic omitted]Co(NH₃)₄⁴⁺+ H⁺+ NO₃^– [graphic omitted] (NH₃)₄Co[graphic omitted]Co(NH₃)₄⁴⁺(i) which the aquo-nitrato-complex (ε= 318 at 528 nm.) is produced is predominant up to about 6M-HNO₃. At higher nitric acid concentrations there is evidence for a second nitrato-complex. This is probably the µ-amido-µ-nitrato-complex (ε > 380), although formation of the dinitrato-complex cannot be ruled out. The equilibrium constant K₁₂ for (i) is 0·022 ± 0·002 I.² mole^–2 at 25° in 2M-nitrate. Rate constants for the reaction of the aquo-nitrato-complex in (i) were obtained in most cases by diluting 6–9·6M-HNO₃ solutions of the µ-amido-µ-hydroxo-complex which had been allowed to equilibrate at ca. 0°. At 25° and µ= 2. 0M, after allowing for a nitrate medium effect, k₂= 7·96 × 10^–3 sec.^–1, ΔH₂‡= 19·4 (±0·3) kcal. mole^–1, and ΔS₂‡=–3·2 (±0·7) e.u. It is concluded that k₁ for the reaction of the µ-amido-µ-hydroxo-complex is 4·2 × 10^–4 l.² mole^–2 sec.^–1 at 25° and that activation parameters for this step are ΔH₁‡= 13·7 (±0·6) kcal. mole^–1 and ΔS₁‡=–28·4 (±2·0) e.u.