Mechanism of octahedral substitution reactions in non-aqueous solutions. Part VI. Kinetics of the displacement of chloride from the non-electrolyte cis-trichlorobis-(o-dimethylarsinophenyl)methylarsine-cobalt(III) by bromide and thiocyanate ions in mixed solvents

Abstract

The rates of replacement of the chloride in cis-[Co TTAS Cl₃][TTAS = bis-(o-dimethylarsinophenyl)methylarsine] by bromide and thiocyanate are independent of the nature and concentration of the anion but depend on the concentration of methanol in the mixed methanol–dichloromethane and methanol–nitromethane solvents. When the methanol concentration is expressed in mole-fraction units the rate constant does not depend greatly upon whether the other component of the solvent is dichloromethane or nitromethane. No reaction takes place in pure dichloromethane and only a slow reaction is observed in pure nitromethane with a rate that is independent of the anion concentration. The rate-determining step is thought to be a solvolytic interchange of unknown molecularity. Direct interchange between the non-electrolyte complex and the anion is difficult because there is no way in which the anion can be attracted into the inner solvation shell of the uncharged complex. The cationic solvento-intermediate is able to attract the anions and undergo a rapid anation.