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Dissolution enthalpies of NaCl and Nal in aqueous solutions of butan-2-one and 1,2-dimethoxyethane have been measured and appropriate enthalpic pair interaction coefficients, h,,(electrolyte-non-electrolyte) in water have been determined. These results along with the other data concerning the NaCI-non-electrolyte and Nal-nonelectrolyte pairs taken from our earlier reports and from t h e literature, were analysed with respect to the effect of t h e non-electrolyte properties on variations of the h,, values in water. The h,, coefficients for both NaCI-nonelectrolyte and Nal-non-electrolyte systems are linearly correlated with the heat capacity of the non-electrolyte i n teract i on with solvent water.The group contributions illustrating the interactions between the electrolyte (NaCI or Nal) and CH,, OH, 0, CO, ' P e p ' and 'iPep' groups in t h e non-electrolyte molecules were calculated and discussed from the viewpoint of a competition between the solute-solute and solute-solvent interactions.

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When a bidentate ligand L-L is added to the square planar Ni(tmhd)(2) (tmhd = tetramethylhepanedionate), the octahedral complex Ni(tmhd)(2)L-L is formed. This reaction has been studied by vis spectroscopy in toluene at 25 degrees C and in the gas phase between 150 and 350 degrees C. It allows the comparison on one hand of the chelate effect of three ligands forming five-membered chelate rings: (i) the flexible N-N ligand tetramethylethylenediamine (TEME); (ii) the rigid N-N ligand 2,2-bipyridine (BPY); (iii) the flexible N-O ligand dimethylaminomethoxyethane (MAO). On the other hand, it allows the comparison of these ligands with the six-membered chelate ring-forming N-N ligand 1,3-tetramethylpropylenediamine (TEMP). From the temperature dependence of the gas-phase stability constants, enthalpies and entropies of the complex-forming reactions have been derived. As there are no solvation effects in the gas phase, the reaction enthalpies are the metal-ligand bond enthalpies. This is of particular interest for the hemilabile ligand MAO. For the N-N ligands, the stability of the metal-ligand bonds decreases in the order TEME > BPY > TEMP. The entropy of the complex formation with the two flexible ligands TEME and MAO is the same, while it is slightly more positive for the rigid BPY and a lot more positive for TEMP. Delta(form)G degrees (298) of the complexes is more negative in the gas phase than in solution because the solvation energy of the reactants is more negative than the solvation energy of the products. This is shown in detail for the formation of Ni(tmhd)(2)BPY where data of a complete thermodynamic cycle are presented.

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