Octahedral cobalt(III) complexes of the chloropentammine type. Part X. Effect of replacing chlorine by bromine on the reactivity of some cis-chloroaminebis(ethylenediamine)cobalt(III) cations

Abstract

Rate constants for aquation, and base hydrolysis of cis-[Co en₂(amine)Br]²⁺ where amine may be isoproylamine, n-propylamine, allylamine, prop-2-ynylamine, ethylamine, methylamine, or hydroxylamine, have been determined and Arrhenius parameters calculated. The results are compared with data for the corresponding chloro-complexes. For solvolytic aquation, the effect of replacing chlorine by bromine arises from a combination of activation energy and entropy, acting in opposition, while for base hydrolysis, the latter appears to be the major factor controlling the difference in reactivity between the bromo-complexes and their chloro-analogues thus confirming the formation of an ion-pair between the complex cation and hydroxide ion as a prerequisite. The transition states for these substitutions invlove varying degrees of participation from the incoming group, depending on the nature of the amine. For the isopropylamine complex, steric congestion may provide a sufficient driving force for the departure of the outgoing halogen, but increasing nucleophilic interaction is required on proceeding along the amine series. In the absence of either added acid or alkali, the cis-bromohydroxylaminebis(ethylenediamine)cobalt(III) complex undergoes a spontaneous redox reaction.