First potentiometric determination of stability constants of nickel(II) complexes with tetra-azacycloalkanes
- 1 January 1983
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 6,p. 1189-1191
- https://doi.org/10.1039/dt9830001189
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.Keywords
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