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

The aquation of Cr(NH 3 ) 5 NO 3 2+ in acidic solutions yields not only Cr(NH 3 ) 5 OH 2 3+ (33%) but also more highly aquated species including cis-Cr(NH 3 ) 4 (OH 2 ) 2 3+ and Cr(NH 3 ) 3 (OH 2 ) 3 3+ . In 0.1 M HClO 4 , several successive reactions of comparable rates are involved, but at pH 6 only the two competing initial reactions producing Cr(NH 3 ) 5 OH 2+ and cis-Cr(NH 3 ) 4 (OH)NO 3 + are rate-controlling, so that the overall reaction rate is first-order in substrate with the apparent parameters k A = 2.4 × 10 −3 s −1 (25°), ΔH A * = 20.4 kcal mol −1 , and ΔS A * = −2.1 cal deg −1 mol −1 . In alkaline solutions, the pseudo-first-order rate of hydrolysis at a given [OH − ] is governed by k obs = k A + k OH [OH − ], with k OH = 1.1 × 10 −2 M −1 s −1 (25°), ΔH OH * = 25.2 kcal mol −1 , and ΔS OH * = 17.0 cal deg −1 mol −1 . The path characterized by k OH leads exclusively to Cr(NH 3 ) 5 OH 2+ . In both aquation and base hydrolysis, it is the CrONO 2 bond that is broken. The production of more highly aquated species in the aquation reactions is attributed to transient chelation of NO 3 − 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.