THE MEAN ENTHALPY OF THE LANTHANIDE OXYGEN BOND IN 2,2,6,6-TETRAMETHYL-3,5-HEPTANEDIONE CHELATES OF PRASEODIMIUM AND HOLMIUM

The general thermochemical reaction LnCl3 . 6H2O(c) + 3Hthd(l) + 73.92H2O(l) = Ln(thd)3(c) + 3HCl . 26.64H2O(aq); DELTArHm (Ln = Pr, Ho and thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) was employed to determine through solution-reaction calorimetry at 298.15 K the standard molar enthalpies of formation of crystalline chelates, -2434.3 +/- 11.5 (Pr) and -2384.8 +/- 11.5 (Ho) kJ mol-1. These values and the corresponding molar enthalpies of sublimation enabled the determination of the standard molar enthalpies of chelates in the gaseous phase. From these values the mean enthalpies of the lanthanide-oxygen bond, 265 +/- 10 (Pr) and 253 +/- 10 (Ho) kJ mol-1 were calculated.4532332