The thermodynamics of complex formation between uramildiacetic acid and the alkaline-earth metals

Abstract

Values for the thermodynamic functions (enthalpy and entropy changes) were determined for the complexation reactions of alkaline-earth metals by uramildiacetic acid (UDA) and for the complexation reactions of beryllium by nitrilotriacetic acid (NITA) and 1.2-diaminoethanetetracetic acid (EDTA) using the temperature coefficient method. For the reactions of uramildiacetic acid both enthalpy and entropy changes are favourable and may be correlated with the “effective” ion radii of the metals; the complexes seem to retain coordinated water molecules and to have a coordination number which exceeds the possibilities of the ligand. However, in the case of the beryllium complex, the values obtained suggest a different structure, possibly tetrahedral and more pronounced covalent interactions. These values contrast with those determined in the cases of the reactions of beryllium with NITA and EDTA, for which the entropy changes are higher and favourable and the enthalpy changes unfavourable, as happens with the complexation reactions of magnesium by the same ligands.