Uranyl - Structure and Bonding

Structure and Bonding

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(+6) has the electronic configuration, 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 and to construct sd, sf and df hybrid orbitals (the z axis passes through the oxygen atoms). (, ) and ( and ) 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 2- the uranium has a distorted octahedral environment. In many cases there are more than four equatorial ligands. The presence of the equatorial ligands lowers the symmetry of the uranyl ion from point group D_∞h for the isolated ion to, for example, D_4h in a distorted octahedral complex; this permits the involvement of d and f orbitals other than those used in U-O bonds.

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. A similar structure is found in α-uranium trioxide, with oxygen in place of fluoride, except that in that case the layers are connected by sharing oxygen atom from "uranyl groups", which are identified by having relatively short U-O distances. A similar structure occurs in some uranates, such as calcium uranate, CaUO₄, which may be written as Ca(UO₂)O₂ even though the structure does not contain isolated uranyl groups.