Thermodynamic and Structural Insights into the Complexation of U(VI) with NH2OH by Potentiometry, Raman Spectroscopy,
Shibo Zhang1, Suliang Yang1, Qian Liu1
1Department of Radiochemistry, China Institute of Atomic Energy, Beijing 102413, China.
None:
The complexation of NH2OH with U(VI) in aqueous solution is thermodynamically and structurally investigated by potentiometric and Raman spectral titrations. Four U(VI)-NH2OH species, UO2(NH2O)+, UO2(NH2O)2, UO2(NH2O)3-, and (UO2)2(NH2O)4(OH)22-, have been identified and their stability constants are determined to be 11.45 ± 0.09, 22.68 ± 0.12, 27.53 ± 0.09, and 27.74 ± 0.14, respectively, at room temperature. Two coordination modes of NH2O- anions, side-on bidentate mode and head-on monodentate mode, are revealed in the complexes. A single crystal grown under high-pH conditions unexpectedly afforded a dinuclear complex, Na2(UO2)2(NH2O)4(OH)2(H2O)5. Single-crystal X-ray diffraction analysis reveals that the two uranyl units are bridged by hydroxide ligands. This solid-state structure suggests the possible presence of a corresponding dinuclear species, (UO2)2(NH2O)4(OH)22-, in solution. Because this species shares the same proton stoichiometry as the mononuclear species UO2(NH2O)3-, potentiometric titration cannot discriminate between them; in contrast, Raman spectroscopic titration, by identifying a characteristic shift of the uranyl symmetric stretching vibration at 777.4 cm-1, confirms its presence in solution.
Related Concept Videos
Valence Bond Theory
Crystal Field Theory - Octahedral Complexes
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...
Ladder Diagrams: Complexation Equilibria
The formation constant, K1, for the formation of Cd(NH3)2+ complex from cadmium and ammonia is 3.55 × 102. Log K1 (i.e. pNH3) is 2.55, and...
Formation of Complex Ions
Complexometric Titration: Ligands
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.


