Cis-Activation by Oxyanions Coordinated to Chromium(III): Aquation and Base Hydrolysis of Nitratopentaamminechromium(III) Ion

Abstract

The aquation of Cr(NH3)5NO32+ in acidic solutions yields not only Cr(NH3)5OH23+ (33%) but also more highly aquated species including cis-Cr(NH3)4(OH2)23+ and Cr(NH3)3(OH2)33+. In 0.1 M HClO4, several successive reactions of comparable rates are involved, but at pH 6 only the two competing initial reactions producing Cr(NH3)5OH2+ and cis-Cr(NH3)4(OH)NO3+ are rate-controlling, so that the overall reaction rate is first-order in substrate with the apparent parameters kA = 2.4 × 10−3 s−1(25°), ΔHA* = 20.4 kcal mol−1, and ΔSA* = −2.1 cal deg−1 mol−1. In alkaline solutions, the pseudo-first-order rate of hydrolysis at a given [OH−] is governed by kobs = kA + kOH [OH−], with kOH = 1.1 × 10−2 M−1 s−1 (25°), ΔHOH* = 25.2 kcal mol−1, and ΔSOH* = 17.0 cal deg−1 mol−1. The path characterized by kOH leads exclusively to Cr(NH3)5OH2+. In both aquation and base hydrolysis, it is the Cr—ONO2 bond that is broken. The production of more highly aquated species in the aquation reactions is attributed to transient chelation of NO3− at the expense of a ligand cis to it. The evidence for this mechanism, and for its operation when oxyanions other than nitrate are coordinated to Cr(III), is reviewed.