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

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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.