Thermodynamics of complex formation of aliphatic linear tetra-amines. ΔH and ΔS for the reactions of 1,5,9,13-tetra-azatridecane with protons and some bivalent transition-metal ions

Abstract

Complexes with aliphatic multidentate polyamine ligands, containing five-membered rings, are more stable than the corresponding complexes containing six-membered rings, for systems with separated and linked rings. This greater stability must be solely ascribed to a more exothermic heat of reaction. The heats of protonation and of formation of nickel(II), copper(II), and zinc(II) complexes of the quadridentate ligand 1,5,9,13-tetra-azatridecane (tt) have been determined calorimetrically at 25° in 0·1 M-sodium nitrate and used to calculate the entropies of protonation and complex formation. The complexes of such a ligand contain a system of three linked six-membered rings. In order to point out the different effect of steric constraints in five- and six-membered ring systems ΔH and ΔS of complex formation of several linear aliphatic polyamines have been reviewed. The comparison of appropriate thermodynamic functions brings the evidence for two different types of steric constraints: those present in an individual chelate ring and those arising from the progressive linking of chelate rings. The individual ring strain is greater in six-membered rings, while the cumulative ring strain has a greater destabilising effect on linked five-membered rings. Moreover in the case of complexes containing six-membered rings the standard molar entropies of metal complexes have been found to be linearly related to standard molar entropies of the free ligand.