STANDARD MOLAR ENTHALPY OF FORMATION OF BIS(2,2,6,6-TETRAMETHYLHEPTANE-3,5-DIONATO) DIOXOURANIUM(VI) IN THE CRYSTALLINE AND GASEOUS STATES - THE MEAN U-O BOND-DISSOCIATION ENTHALPY

The standard (po = 0.1 MPa) molar enthalpy of formation, at the temperature 298.15 K, of crystalline bis(2,2,6,6-tetramethylheptane-3,5-dionato) dioxouranium(VI) {uranyl(VI) dipivaloylmethanate, UO2(DPM)2}, was determined by solution-reaction calorimetry as -(2169.9±7.6) kJ·mol-1. The vapour pressure of the crystal, as function of the temperature, was measured using the Knudsen mass-loss effusion technique and the standard molar enthalpy of sublimation, at the temperature 298.15 K, was derived as (156.9±1.9) kJ·mol-1. From these results, the standard molar enthalpy of formation of the complex, in the gaseous state, was derived and the mean uranium(VI)-oxygen bond-dissociation enthalpy for the binding of the ligand to the metal, 〈D〉(U-O), was calculated as (223±10) kJ·mol-1. © 1993 Academic Press Ltd.