Thermochemical properties of the rare earth complexes with pyromellitic acid

Fourteen rare earth complexes with pyromellitic acid were synthesized and characterized by means of chemical and elemental analysis, and TG–DTG. The constant-volume combustion energies of complexes, ΔcU  , were measured by a precise rotating-bomb calorimeter (RBC-type II). Their standard molar enthalpies of combustion, ΔcHmθ, and standard molar enthalpies of formation, ΔfHmθ, were calculated at T = 298.15 K. The relationship of ΔcHmθ and ΔfHmθ with the atomic numbers of the elements in the lanthanide series was examined. The results show that a certain amount of covalence is present in the chemical bond between rare earth cations and the ligand.