Kinetic studies on 4d and 5d transition-metal complexes. Part VI. Kinetic study of the azidopenta-amminerhodium(III) ion in acidic aqueous solutions

Abstract

A spectrophotometric study of the kinetics of the spontaneous and acid-catalysed aquation reactions of the azidopenta-amminerhodium(III) ion is reported. At low acid concentrations the observed pseudo-first-order rate constants, at constant temperature, are reproduced by equations of the form k=ka+k2°[H+]104AI½/(1+I½) The Arrhenius equations are: ka= 4·78 × 1010 exp (–28,400 ± 950)/RT and k2°= 3·83 × 109 exp (–24,580 ± 230)/RT. At higher acid concentrations, ca. 9M, a limiting rate is reached. The kinetic data and independent spectrophotometric studies confirm the existence of an intermediate [Rh(NH3)5N3H]3+ in the acid-catalysed reaction. The pKa value for the reaction [Rh(NH3)5N3H]3+→[Rh(NH3)5N3]2++ H+ is estimated by two independent methods to be ca.–2·2 and appears to be virtually independent of temperature.