Cyclic effects with medium ring triamines. Enthalpy and entropy contributions to the solution stability of copper(II) and zinc(II) complexes with 1,4,7-triazacyclononane

Thermodn. of ionization of Cu(I)2+ (II) and Zn(I)2+ (III) were detd. and compared with data for Cu(Q)2+ (IV, Q = diethylenetriamine) and Zn(Q)2+ (V). The similar stabilities of II and IV are explained by offsetting differences in enthalpy and entropy terms. Comparison of III and V indicates a cyclic effect enhancing the stability of III; this effect is attributed solely to the enthalpy differences, since the entropy differences would stabilize V