Mechanism of octahedral substitutions in nonaqueous media. Part VII. Influence of acid and base on the replacement rates of chloride by thiocyanate in trans-dichloro- and trans-chloronitro-bis(ethylene-diamine)cobalt (III) ions in methanol and ethanol

Abstract

The rate of replacement of chloride by thiocyanate in the trans-dichloro-complex ion in methanol and ethanol is accelerated by addition of hydroxide ions and reduced by addition of perchloric acid. The rate of the analogous replacement in the chloronitro-complex ion in methanol is reduced by addition of hydroxide ions and unaffected by addition of the acid. In ethanol, neither base nor acid caused any effect on the replacement rate in the chloronitro-complex ion. These different effects support a previous claim that replacement in the complexes does not follow the same mechanism, and are explained in terms of an equilibrium between the complex and its conjugate base. The conjugate base is more reactive in a dissociation-type process, e.g. in the replacement in the dichloro-complex ion. In the chloronitro-complex ion the replacement requires ‘reagent assistance’, and the conjugate base can react faster or slower than the complex ion, depending on the importance of bond-making and -breaking in the transition state.