Homolysis versus heterolysis in the decomposition of chromium(III) complexes in acidic aqueous solution

Abstract

The hydrolysis of 4-pyridiomethylpenta-aquochromium(III) ion (I) in oxygen-free, aqueous perchloric acid proceeds according to the rate law –d[(I)]/dt=k₁k₂[(I)]/k_–1[Cr²⁺], over a range of temperatures and electrolyte concentrations. A homolytic pre-equilibrium: ⁺HNC₅H₄CH₂Cr(H₂O)₅²⁺+ H₂O [graphic omitted] ⁺HNC₅H₄ĊH₂+ Cr(H₂O)₆²⁺ is indicated, this being followed by a rate-determining unimolecular rearrangement (k₂) of the organic radical prior to reduction by chromium(II). Without added chromium(II), this leads to a half-order dependence of –d[(I)]/dt on [(I)]. The activation energies for the k₁ path and the overall half-order reaction (30·0 and 16·8 kcal mol^–1 respectively) are such that k₁ can become rate-determining at lower temperatures (ca. 25°). At 55° in 0·03M-perchloric acid k₁= 3·25 × 10^–5 sec^–1, k_-1= 1·0 × 10^–2M^–1 sec^–1, and k₂= 3·9 × 10^–6 sec^–1. The overall reaction rate shows a marked medium dependence at electrolyte concentrations >0·1M. Hydrolysis of other [Cr^III(H₂O)₅X]^{(3 –n)+}species (other than X = PhCH₂^–) are demonstrably heterolytic.