Chemical Speciation of the Uranyl Ion under Highly Alkaline Conditions. Synthesis, Structures, and Oxo Ligand Exchange Dynamics

The tetramethylammonium counterion was used to suppress formation of insoluble uranate salts, M2U2O7, and allow for a detailed structural and spectroscopic characterization of UO2(OH)n2-n (n = 4, 5) under highly alkaline aqueous solution conditions. Single crystals of [Co(NH3)6]2[UO2(OH)4]3·H2O were obtained by cooling a dilute solution of Co(NH3)6Cl3 and UO2(NO3)2·6H2O in 3.5 M (Me4N)OH to 5 °C. The asymmetric unit contains three distinct UO2(OH)42- ions, each displaying a pseudo-octahedral coordination geometry with trans oxo ligands. The three independent UO2(OH)42- ions in the unit cell give average UO and U−OH distances of 1.82(1) and 2.26(2) A, respectively. EXAFS data on solid [Co(NH3)6]2[UO2(OH)4]3·H2O and aqueous UO22+ in 3.5 M (Me4N)OH solution were collected at the U LIII edge, and the resulting radial distribution function shows a single asymmetric peak. For the solid and solution, curve fitting reveals two near neighbors. For the crystalline solid, the first shell was fit with two O atoms at ...