Kinetic studies on 4d and 5d transition-metal complexes. Part V. Kinetics of the reaction of iodopenta-ammineiridium(III) perchlorate with sodium hydroxide in aqueous solutions

Abstract

The kinetics of the reaction [Ir(NH₃)₅I]²⁺+ OH^–→[Ir(NH₃)₅OH]²⁺+ I^–1 have been studied over the range 76–97° under conditions which allow the calculation of second-order rate constants at zero ionic strength. The results are in agreement with the Brönsted–Bjerrum theory of the primary salt effect. The variation of the rate constant at zero ionic strength with temperature is reproduced by the equation k₂⁰(mole I.^–1 sec.^–1)= 18·54 exp –(36,320 ± 630)/RT. The activation energies of the reactions [M(NH₃)₅I]²⁺+ OH^–→[M(NH₃)₅OH]²⁺(M = Cr^III, Co^III, Rh^III, and Ir^III) increase linearly with ligand-field strengths. This suggests that these reactions proceed by a common mechanism. The magnitude of the ligand-field contribution to the observed activation energies is evidence against a displacement mechanism.