THERMOCHEMICAL STUDIES OF 2,2,6,6-TETRAMETHYL-3,5-HEPTANEDIONE CHELATES OF SCANDIUM GROUP ELEMENTS

The standard molar enthalpies of formation of crystalline chelates M(t hd)(3), (M = Sc, Y, La; and thd = 2,2,6,6-tetramethyl-3,5-heptanediona te), determined by reaction-solution calorimetry at T = (298.15 +/- 0. 02) K, were found to be - (2317.83 +/- 11.7) kJ.mol(-1), - (2386.1 +/- 11.4)kJ.mol(-1), and - (2442.2 +/- 11.8)kJ.mol(-1), respectively. The corresponding molar enthalpies of sublimation, (79.6 +/- 2.4) kJ.mol( -1), 117 kJ.mol(-1), and (156.0 +/- 4.6) kJ.mol(-1), were obtained by differential scanning calorimetry or vapour pressure measurements, and enabled the calculation of the standard molar enthalpies of formation of the gaseous chelates. From the standard molar enthalpies of dissoc iation of the chelates (1575 +/- 62) kJ.mol(-1), (1652 +/- 62) kJ.mol( -1), and (1679 +/- 62) kJ.mol(-1), respectively, the mean metal-to-oxy gen bond enthalpies were also calculated to be (263 +/- 10)kJ.mol(-1), (275 +/- 10)kJ.mol(-1), and (280 + 10) kJ.mol(-1), by assuming the eq uivalence of all bonds in the chelates. This first sequence of values obtained for the scandium group elements showed a tendency towards an increase of values with the increase of the metal's atomic number in t his series. (C) 1997 Academic Press Limited.