*2.3. Coordination of U and Se*

The crystal structures of all the natural and synthetic compounds described herein are based on the chained or layered substructural units built by the linkage of U- and Se-centered coordination polyhedra. U(VI) atoms form approximately linear UO2 <sup>2</sup><sup>+</sup> uranyl ions (*Ur*) with two short U6<sup>+</sup>≡O2<sup>−</sup> bonds. *Ur* cation is coordinated in the equatorial plane by other four to six oxygen atoms, to form a tetra-, penta-, or hexagonal bipyramid, respectively, as a coordination polyhedron of U6<sup>+</sup> atoms. The selenite group has a configuration of a trigonal pyramid with its apical vertex occupied by the Se4<sup>+</sup> cation possessing a stereochemically active lone-electron pair. In the crystal structures of number of synthetic uranyl selenium compounds, there are also Se(VI) species that form [SeO4] <sup>2</sup><sup>−</sup> tetrahedra.
