*2.4. Graphical Representation and Anion Topologies*

For topological analysis, the theory of graphical (nodal) representation of crystal structures [16] and the anion topology method [17] were used along with the classification suggested in [18]. Anion topologies were used to describe the layered complexes having edge-sharing polymerization of uranyl coordination polyhedra. For the rest of the structures, graphical representation was used. Each graph has a special index *cc*D–U:Se–#, where *cc* means "cation-centered", D indicates dimensionality (1—chains; and 2—sheets), U:Se ratio, # is the registration number of the unit. Each anion topology is indicated by a, so called, ring symbol, *p*<sup>1</sup> *<sup>r</sup>*1*p*<sup>2</sup> *<sup>r</sup>*<sup>2</sup> ... , where *p* is the sum of vertices in a topological cycle, and *r* is the number of the respective cycles in the reduced section of the layer.

Three-connected selenate tetrahedra, sharing three of its corners with adjacent uranyl bipyramids, and 2- or 3-connected selenite pyramids, can possess the fourth non-shared corner or lone electron pair, respectively, oriented either *up*, *down*, or disordered relative to the plane of the chain, layer, or, in particular, to the equatorial plane of the uranyl bipyramid. Such ambiguity gives rise to geometric isomerism with various orientations of the Se-centered polyhedra. To distinguish the isomers, their orientation matrices were assigned using symbols **u** (*up*), **d** (*down*), **m** (orientation *up*-*down* topologically equivalent), or -(white vertex, Se-centered polyhedron, is missing in the graph).
