Kinetics of aquation and base hydrolysis of some trans-[Co(tn)2(B)Cl]n+(tn = 1,3-diaminopropane; B = OH, NCS, N3, NO2, NH3, or CN) complexes. The effect of ring size on reactivity

Abstract

The rate constants for the displacement of Cl^– from trans-[Co(tn)₂(B)Cl]ⁿ⁺(tn = 1,3-diaminopropane; B = OH, NCS, N₃, NO₂, NH₃, or CN) have been measured in water over a range of temperature and pH and the behaviour is compared to that of the analogous 1,2-diaminoethane complexes. For B = NCS, OH (and Cl), where substitution is accompanied by stereochemical change and a large positive ΔS^‡, the rate of uncatalysed aquation is very much enhanced. For B = CN, NO₂, NH₃(and SO₃), where the entropies of activation for uncatalysed aquation are smaller (even negative) and where substitution occurs with retention of configuration, the increase in reactivity is not much more than ten-fold. In the case of B = N₃, where the diaminoethane complex aquates with steric change but with a negative ΔS^‡ value, the reactivity increase is between the two limits and ΔS^‡=+39 J K^–1 mol^–1. The base hydrolysis of all complexes studied is only increased by a factor of 10–20 in spite of the common occurrence of stereochemical change.