Equilibria of complex formation between several bivalent metal ions and macrocyclic tri- and penta-amines

Abstract

Values of log K_ML, ΔH, and ΔS(at / 0.2 mol dm^–3 and 25 °C) have been determined potentiometrically or polarographically for the 1 : 1 reactions between Zn^II, Cd^II, Hg^II, or Pb^II and macrocyclic polyamine ligands including 9- and 10-membered triamines and 15-, 16-, and 17-membered penta-amines. The resulting data are compared with those for the relevant linear polyamines. Whether in tri- or penta-amines, ligand cyclization has minor effects on the enthalpy change but has major effects on the entropy change on complex formation with these metal ions. The latter term is responsible for the increased stability of the macrocyclic complexes. This is opposite to the enthalpy-governed macrocyclic effect earlier reported for copper(II)–penta-amine complexes. Among the homologous macrocyclic ligands, the smallest ligands consistently show the highest affinity towards any of the metal ions tested, and this originates from the entropy term. A survey of the tri-, tetra-, and penta-amine systems indicates some correlation between large macrocyclic effects and the ratio of the size of the metal ion to the size of the macrocyclic cavity.