hgcro4 is the new Mercurial command line tool for managing large collections of files in the same repository. It is designed to be used alongside the existing Mercurial client applications, rather than as a replacement for them. hgcro4 provides additional features such as the ability to create and manage repositories, work with a remote repository via SSH, and the ability to synchronize changes to multiple servers.

The three title compounds Cd(HgII)4O4(CrO4), Zn(HgII)4O4(CrO4) and the previously known monohydrate b-hgcro4 adopt unique structure types that show characteristic crystal-chemical features of the respective metal cations. Both cadmium and zinc adopt a distorted octahedral coordination of the chromate anions CrO 42- in the oxochromate(VI) anions, whereas mercury adopts linear (or nearly linear) coordination.

A common feature of all three structures is the presence of oxygen atoms bound exclusively to the mercury, cadmium or zinc cations. The atoms are linked by sharing vertices and edges in oxygen-centred polyhedra that fuse into a three-dimensional framework. The octahedral [CrO4] tetrahedra of the oxygen-centred polyhedra are connected by additional M-O bonds to form a strong cohesion network.

The cadmium and zinc compounds are similar in structural terms, but differ in the symmetry of the space group: Cd(HgII)4O4(CrO4) has orthorhombic symmetry (space group Pbca, eight formula units), while Zn(HgII)4O4(CrO4) is triclinic in space group P1-(two formula units). Both structures display two types of Hg-O chains with short distances of 2.001(8) and 2.016(8) A and more or less linear Hg-O-Hg angles ranging from 164 to 177deg. The similarities between the two compounds also extend to their oxygen-centred polyhedra, which are linked through shared vertices and edges to complete the cohesion network.