The Structure of Hgcro4

The hgcro4 structure is a complex three-dimensional -Hg2+-N3– framework that is mainly interrupted and contains large interstices. It incorporates a small amount of [Hg+-Hg+]2+ dimer and additional anion species, O2-, (OH)- and (NH2)- that coordinate Hg2+.

Unlike the Cd(HgI2)2(HgII)3O4(CrO4)2 and Zn(HgII)4O4(CrO4)2 structures, which contain two identical -Hg2+-N3– network, the hgcro4 structure has two separate structural subunits: a mercury-oxygen network (with tetrahedral coordination of N3- by Hg2+) and cadmium/zinc cations bound directly to [CrO4]2- anions in an acid/base relationship. It also contains oxygen atoms that do not belong to any of the chromate anion species, but are exclusively bonded to metal cations.

This arrangement is characterised by a series of long, more or less linear Hg-O distances between 2.01 and 2.05 A and O-Hg-O angles of 164-177deg. The O-Hg-O tetrahedral bonds are in an octahedral environment due to the presence of the Cd2+ cation and the Zn2+ cation.

The oxygen-centred polyhedra are connected to the tetrahedral [CrO4] groups by sharing edges and vertices into a three-dimensional framework that is completed through additional M-O bonds. This arrangement is reminiscent of the similar set-up of the M(HgI2)2(HgII)3O6(CrO4)2 and Zn(HgII)4O4(CrO4)2 crystal structures described in this article. This is a direct proof of the structure-forming role of packing of halogen atoms in some Hg3 Y 2 X 2 chalcohalides. The symmetry of these crystal structures is a combination of a rigid atomic group symmetry and a space group symmetry that partly predetermines the distorted octahedral structure.