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

Abstract

The standard ( p o = 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, UO 2(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.