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Concept
Uranyl
Summary
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula UO22+. It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in nuclear fuel reprocessing.
The uranyl ion is linear and symmetrical, with both U–O bond lengths of about 180 pm. The bond lengths are indicative of the presence of multiple bonding between the uranium and oxygen atoms. Since uranium(VI) has the electronic configuration of the preceding noble gas, radon, the electrons used in forming the U–O bonds are supplied by the oxygen atoms. The electrons are donated into empty atomic orbitals on the uranium atom. The empty orbitals of lowest energy are 7s, 5f and 6d. In terms of valence bond theory, the sigma bonds may be formed using dz2 and fz3 to construct sd, sf and df hybrid orbitals (the z-axis passes through the oxygen atoms). (dxz, dyz) and (fxz2 and fyz2) may be used to form pi bonds. Since the pair of d or f orbitals used in bonding are doubly degenerate, this equates to an overall bond order of three.
The uranyl ion is always associated with other ligands. The most common arrangement is for the so-called equatorial ligands to lie in a plane perpendicular to the O–U–O line and passing through the uranium atom. With four ligands, as in [UO2Cl4]2−, the uranium has a distorted octahedral environment. In many cases more than four ligands occupy the equator.
In uranyl fluoride, UO2F2, the uranium atom achieves a coordination number of 8 by forming a layer structure with two oxygen atoms in a uranyl configuration and six fluoride ions bridging between uranyl groups.
Official source
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