First potentiometric determination of stability constants of nickel(II) complexes with tetra-azacycloalkanes

Abstract

The stability constants of nickel(II) complexes with 1,4,7,10-tetra-azacyclotetradecane (L¹), 1,4,8,12-tetra-azacyclopentadecane (L²), and 1,5,9,13-tetra-azacyclohexadecane (L³) have been determined by a ‘batchwise’ potentiometric technique at 25 °C in 0.5 mol dm^–3 KNO₃. Ligand L² forms the strongest complex (log K= 18.38), L¹ the next strongest (log K= 14.81), and L³ the weakest of the three (log K= 13.23). By combining the standard free energies for complex formation with the standard enthalpies obtained previously, the entropy contributions have been calculated and are discussed. The three ligands all form monoprotonated complexes [Ni(HL)]³⁺ and their stability constants have been determined. The stabilities of the 1 : 1 complexes [NiL]²⁺ have been compared with those of the corresponding complexes with the analogous open-chain ligands. The macrocyclic complexes are more stable than the corresponding complexes with non-cyclic tetra-amines. This extra stability (‘macrocyclic effect’) is due to a favourable entropy contribution, the enthalpy contribution being unfavourable.