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) + 73.92 H2O(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 determin e 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 correspondi ng molar enthalpies of sublimation enabled the determination of the st andard molar enthalpies of chelates in the gaseous phase. From these v alues the mean enthalpies of the lanthanide-oxygen bond, 265 +/- 10 (P r) and 253 +/- 10 (Ho) kJ mol-1 were calculated.