Crystal-chemical functions of [Hg2]2+ and [Hg3]4+ groups in inorganic structures

About 20 structures of natural and synthetic inorganic compounds containing a H_g2] (dumbbell) pair (Hg-Hg = 2.47?2.56 å) or a H_g3] triangle (Hg-Hg_av = 2.7 å) 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 polvhedra are convex polxhedra with 8–14 vertices and with distances from the center to the vertex of 3.00?3.91 å, which makes the H_g2]²⁺ and Hg3]⁴⁺ cations analogs of Cs. Examples are given to demonstrate the advantages of the suggested analysis of structures with a Hg_n] group. This approach extends classical crystal chemistry to compounds with nonuniform chemical bonds, in particular, to compounds with pseudocluster and cluster groups inserted in the ionic structure; therefore, it is especially useful for comparison and classification purposes and for analysis of structure-foming factors.