Crystal Structure of Uranyl Carboxylates
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Crystal Structure of Uranyl Carboxylates Paul A. Giesting, Peter C. Burns, and Nathan J. Porter Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, U.S.A. ABSTRACT Uranyl-organic complexation in geologic fluids can have a profound impact upon uranium solubility and transport. Studies of uranyl organometallic crystal structures provide a basis for understanding complexation of the uranyl ion in solution. The crystal structures of several novel uranyl oxalates, synthesized under mild hydrothermal conditions, have been determined. These structures demonstrate new features little seen or not previously known in this chemical system, in particular polymerization into infinite sheets and direct linkage of uranyl polyhedra. Further work on the chemistry of this and other systems of hexavalent uranium and low molecular weight carboxylic acids, especially formic acid, is likely to turn up new insights. Although a hierarchical scheme exists for classifying inorganic uranyl compounds [1], no similar work has been done for organic compounds. Such a hierarchy would have practical benefits, in particular making structural information more accessible and understandable to workers studying related problems such as the environmental transport of hexavalent uranium as dissolved organic complexes. We offer a simple scheme that classifies uranyl oxalate structures by analyzing the long-range structural features and the coordination environments of uranyl ions, which leads to a structural symbol that can be used to easily identify uranyl oxalates with common structural features. This system is equally applicable to other carboxylate complexes with the uranyl ion, and could be extended to apply to any organic complex of the uranyl ion. INTRODUCTION Uranium, which is both the chief fuel and the chief waste product of the nuclear industry, is a major pollutant in the environment and demands a variety of cleanup responses. In the oxygenated, hydrated near-surface environment, hexavalent uranium dominates and occurs as the uranyl ion. The uranyl ion forms strong soluble complexes with both inorganic and organic ligands. The complexation chemistry of the uranyl ion has been extensively, although not thoroughly, studied. Some 368 structures of inorganic uranyl compounds are known, including 89 mineral structures [2]. 583 structures of uranyl complexes with organic ligands and/or urea were found in the Cambridge Structure Database (CSD) and the Inorganic Crystal Structure Database (ICSD) as of August 2005; this number includes solely crystal structures with organic ligands bonded to uranium and linear dioxouranium groups. The two uranyl oxygens of the uranyl ion tend strongly to remain in a straight line with the uranium atom. Four, five, or six more atoms from coordinating ligands in almost all cases surround the uranium atom. The uranyl oxygens repel these, so they cluster in a plane perpendicular to the axis of the ion. As noted by Burns et al. [1], inorganic uranyl complex
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