Crystal-chemical functions of [Hg-2](2+) and [Hg-3](4+) groups in inorganic structures

About 20 structures of natural and synthetic inorganic compounds containing a [Hg-2] (dumbell) pair (Hg-Hg = 2.47-2.56 Angstrom) or a [Hg-3] triangle (Hg-Hg-av = 2.7 Angstrom) are analyzed. From crystal-chemical viewpoint, it is reasonable to consider these groups as a single cation with coordinates of its geometrical center. In all cases, the coordination polyhedra are co nvex polyedra are convex polyhedra with 8-14 vertices and with distances fr om the center of the vertex of 3.00-3.91 Angstrom, which makes the [Hg-2](2 +) and [Hg-3](4+) cations analogs of Cs. Examples are given to demonstrate the advantage of the suggested analysis of structures with a [Hg-n] group. this approach extends classical crystal chemistry to compounds with nonunif orm chemical bonds, in particular, to compounds with pseudocluster and clus ter groups inserted in the ionic structure; therefore, it is especially use ful for comparison and classification purposes and for analysis of structur e-forming factors.