Complexation of U(VI) with dipicolinic acid: thermodynamics and coordination modes
Chao Xu1, Guoxin Tian, Simon J Teat
1Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
Abstract:
Complexation of UO2(2+) with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO4. The stability constants (log β1 and log β2) for two successive complexes, UO2L and UO2L2(2-) where L(2-) stands for the deprotonated dipicolinate anion, were determined to be 10.7 ± 0.1 and 16.3 ± 0.1 by spectrophotometry. The enthalpies of complexation (ΔH1 and ΔH2) were measured to be -(6.9 ± 0.2) and -(28.9 ± 0.5) kJ·mol(-1) by microcalorimetry. The entropies of complexation (ΔS1 and ΔS2) were calculated accordingly to be (181 ± 3) and (215 ± 4) J·K(-1)·mol(-1). The strong complexation of UO2(2+) with DPA is driven by positive entropies as well as exothermic enthalpies. The crystal structure of Na2UO2L2(H2O)8(s) shows that, in the 1:2 UO2(2+)/DPA complex, the U atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to UO2(2+) via its equatorial plane in a tridentate mode. The structural information suggests that, due to the conjugated planar structure of DPA with the donor atoms (the pyridine nitrogen and two carboxylate oxygen atoms) arranged at optimal positions to coordinate with UO2(2+), little energy is required for the preorganization of the ligand, resulting in strong UO2(2+)/DPA complexation.
More Related Videos
Related Concept Videos
Valence Bond Theory
Complexation Equilibria: Overview
The equilibrium constant of the complexation reaction is represented as the formation constant...
Coordination Compounds and Nomenclature
Colors and Magnetism
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
Formation of Complex Ions
Complexation Equilibria: The Chelate Effect


