Kinetics of aquation and formation of the tetra-aqua(2,2′-bipyridyl)-chromium(III) ion

Abstract

Kinetics of aquation of the tetra-aqua(2,2′-bipyridyl)chromium(III) ion and of its formation from bipy and hexa-aquachromium(III) ion has been studied as a function of acidity and Cr³⁺ concentration at 1·0M ionic strength (NaClO₄). No build-up of the unidentate bipy intermediate has been observed and with an excess of Cr³⁺ ion present the first-order rate constant for the approach to equilibrium is given by k_obs.=k₁[Cr³⁺]+(k_–1k_–2/k₂). At 140·2 °C the second-order formation rate constant k₁=(2·0 ± 0·1)× 10^–3[H⁺]^–1+(7·0 ± 1·4)× 10^–6[H⁺]^–2 I mol^–1 s^–1 and the composite term k_–1k_–2/k₂=(9·5 ± 0·2)× 10^–5+(6·3 ± 0·3)× 10^–7[H⁺]^–1 s^–1 Speculative breakdown of the composite term yields the rate of ring closure k₂= 5·3 s^–1 and the rate of bond rupture k_–1′= 1·3 × 10^–2 s^–1 for the intermediate [Cr(OH₂)₅(bipy)]³⁺ ion, giving k₂k_–1′= 408.