Water–ammonia exchange at amminerhodium(III) complexes in aqueous ammoniacal solution

Abstract

The kinetics of water–ammonia exchange at trans-[Rh(NH₃)₄(OH)₂]⁺ and [Rh(NH₃)₅(OH)]²⁺ have been studied in aqueous ammoniacal solution. The reaction of trans-[Rh(NH₃)₄(OH)₂]⁺ with ammonia to form [Rh(NH₃)₅(OH)]²⁺ has been studied in the temperature range 68–90 °C. The pH dependence of the observed rate constant indicates H₂O to be the leaving group. The kinetics could be interpreted in terms of the usual Eigen–Wilkins interchange mechanism, with a more associative character than cobalt(III). The reaction of [Rh(NH₃)₅(OH)]²⁺ with ammonia has been studied in the temperature range 112–142 °C, where the reaction results in an equilibrium mixture of [Rh(NH₃)₅(OH)]²⁺ and [Rh(NH₃)₆]³⁺. The equilibrium constant has been determined spectrophotometrically and kinetically. The forward reaction is pH-independent. The reverse reaction, which gives a small contribution to the overall rate, is base-catalysed. These results can be explained by a direct interchange of OH^– and NH₃ or by the conjugate-base mechanism.