Chelate effect in the gas phase. The complexes of Ni(2,2,6,6-tetramethyl-3,5-heptanedionate)(2) with bidentate ligands

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 form ed. This reaction has been studied by vis spectroscopy in toluene at 25 deg reesC and in the gas phase between 150 and 350 degreesC. It allows the comp arison on one hand of the chelate effect of three ligands forming five-memb ered 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 t he comparison of these ligands with the six-membered chelate ring-forming N -N ligand 1,3-tetramethylpropylenediamine (TEMP). From the temperature depe ndence of the gas-phase stability constants, enthalpies and entropies of th e complex-forming reactions have been derived. As there are no solvation ef fects in the gas phase, the reaction enthalpies are the metal-ligand bond e nthalpies. This is of particular interest for the hemilabile ligand MAO. Fo r 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 posit ive for the rigid BPY and a lot more positive for TEMP. Delta (form)G degre es (298) of the complexes is more negative in the gas phase than in solutio n because the solvation energy of the reactants is more negative than the s olvation 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 presente d.