Thermodynamic Parameters and Coordination Behavior of Eu(III) Complexes with Tartrate and Oxalate Ligands: Study Using NMR and Potentiometry Methods
Issabekova S. Belgibayeva D. Amerkhanova S. Satayeva Z. Abilova G. Almuratova K. Aikenova N. Sharipova L.
February 2026Multidisciplinary Digital Publishing Institute (MDPI)
Inorganics
2026#14Issue 2
This work presents a detailed study of the coordination of Eu(III) with tartrate and oxalate ligands in aqueous solutions. The following techniques were employed: potentiometric titrations, 1D 1H, 13C multinuclear NMR spectroscopy, 2D NMR experiments (COSY, HMQC, HMBC), and UV-Vis spectroscopy. Overall (cumulative) formation constants (logβ) were determined at ionic strengths of 0.1, 0.5, and 1.0, M KNO3 over the temperature range 298–318 K. At 298 K, the oxalate complexes are significantly more stable (logβ = 7.63→15.70 as the ionic strength increases from 0.1 to 1.0 M) than the corresponding tartrate species (logβ = 5.11→8.87). Analysis of the temperature dependence of logβ shows that the Gibbs free energy change comprises both temperature-dependent terms and an approximately temperature-independent covalent contribution, the latter becoming strongly negative values in the tartrate system. The NMR data support a bidentate coordination mode involving deprotonated hydroxyl and carboxylate groups, whereas 17O NMR monitors the mechanism of water exchange within the Eu(III) hydration sphere. In the UV-Vis domain, a distinct blue shift in the absorption band is observed at 0.1 M KNO3, while at 1.0 M KNO3, the band shows a pronounced decrease in intensity, a hypochromic effect. This behavior can be attributed to increased structural distortion and a partial loss of coplanarity within the tartrate coordination environment. By contrast, the oxalate system behaves differently: the spectra, together with the thermodynamic data, support a more covalent Eu–O interaction, consistent with stabilization of Eu(III) by two dicarboxylate ligands adopting distinct coordination modes.
europium(III) complexes , hydration sphere , ionic strength , NMR spectroscopy , oxalate , tartrate , thermodynamic stability
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Department of Chemistry, Institute of Natural Sciences, L.N. Gumilyov Eurasian National University, Astana, 010000, Kazakhstan
Department of Technology of Food and Processing Industries, S. Seifullin Kazakh Agrotechnical University, 62 Zhenis Ave., Astana, 010000, Kazakhstan
Department of Chemistry and Food Technology, K. Zhubanov Aktobe Regional University, 34 Aliya Moldagulova Ave., Aktobe, 030000, Kazakhstan
Department of Chemistry and Oil-Gas Technology, Bukhara State University, 11M Iqbal Str., Bukhara, 200100, Uzbekistan
Department of Chemistry
Department of Technology of Food and Processing Industries
Department of Chemistry and Food Technology
Department of Chemistry and Oil-Gas Technology
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