A kinetic study of the thiocyanatopentammine-chromium(III) and -cobalt(III) complex ions in acidic and basic solutions

Abstract

The kinetics of the aquation and hydroxide ion substitutions of thiocyanatopentammine-chromium(III) and -cobalt(III) complex ions have been investigated over a range of temperature. The rate of disappearance of complex ion and the rate of appearance of thiocyanate ion were measured spectrophotometrically. The aquations are of first order and are not acid-catalysed. The values of the pre-exponential terms, A, are, in general, of the order of magnitude expected for reactions between an ion and a polar molecule. The relationship between log₁₀A and the entropy of solvation for a series of formally similar reactions emphasises the importance of solvation effects.

The reactions with hydroxide ion are of first order in each reactant. The variation of rate with the ionic strength of the medium illustrates the importance of a bimolecular step in the reaction mechanism. The pre-exponential terms are similar in magnitude and are very high. The high pre-exponential terms of similar reactions of ammines, particularly cobaltammines, support mechanisms involving pre-equilibrium but do not, in themselves, distinguish between conjugate-base and ion-pair mechanisms.