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

Heats of protonation and formation of Ni(II), Cu(II), and Zn(II) complexes of the quadridentate ligand 1,5,9,13-tetraazatridecane were detd. calorimetrically at 25° in 0.1M NaNO3 and the corresponding entropies were calcd.; the complexes contain 3 linked 6-membered rings. Comparison of ΔH and ΔS of complex formation with those of other linear aliphatic polyamines which give 5- and 6-membered ring complexes showed that there are 2 different steric constraints, those present in an individual chelate ring, and those arising from progressive linking of chelate rings. The individual ring strain is greater in 6-membered rings, while the cumulative ring strain is more destabilizing in linked 5-membered rings. In complexes contg. 6-membered rings the std. molar entropies of metal complexes are linearly related to the std. molar entropies of the free ligands.