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=k_a+k₂°[H⁺]10^4AI½/(1+I½) The Arrhenius equations are: k_a= 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 pK_a 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.