Kinetics and mechanism of the acid hydrolysis of the nitritopenta-amminechromium(III) ion

Abstract

The acid-catalysed hydrolysis of the nitritopenta-amminechromium(III) ion has been studied spectrally by conventional and stopped-flow methods. In the range 0·01–0·10M-HClO₄, and constant ionic strength (µ= 1·0, NaClO₄), the rate law is: d[Cr(NH₃)₅OH₂³⁺]/dt=k[H⁺][Cr(NH₃)₅ONO²⁺]. At 25°, k= 0·37M^–1 sec.^–1, ΔH= 21·2 ± 0·3 kcal. mole^–1, and ΔS‡= 11 ± 1 cal. mole^–1 deg.^–1. Above 0·1M-HClO₄ this rate law is no longer applicable; at 10·0°, µ= 1·0, and in the presence of perchloric acid up to 1M, the pseudo-first-order rate constant, k′, varies according to the equation: 10³k′=(0·03 ± 0·08)+(43·2 ± 2·8)[H⁺]+(55·4 ± 5·2)[H⁺]² sec.^–1. In this range of acid concentration there is no spectral evidence for extensive protonation of the complex. Above 20°, side-reactions were observed in which the complex decomposed by loss of co-ordinated ammonia molecules.