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 degreesC and in the gas phase between 150 and 350 degreesC. 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.