Guanidinium tetrabromidomercurate(II)

The Hg atoms in the crystal structure of the title compound, (CH6N3)2[HgBr4], are tetrahedrally coordinated by four Br atoms and the resulting [HgBr4]2− tetrahedral ions are linked to the [C(NH2)3]+ ions by bromine–hydrogen bonds, forming a three-dimensional network. In the structure, the anions are located on special positions. The two different Hg⋯Br distances of 2.664 (1) and 2.559 (1) Å observed in the tetrabromidomercurate unit are due to the connection of Br atoms to different number of H atoms.

The Hg atoms in the crystal structure of the title compound, (CH 6 N 3 ) 2 [HgBr 4 ], are tetrahedrally coordinated by four Br atoms and the resulting [HgBr 4 ] 2À tetrahedral ions are linked to the [C(NH 2 ) 3 ] + ions by bromine-hydrogen bonds, forming a three-dimensional network. In the structure, the anions are located on special positions. The two different HgÁ Á ÁBr distances of 2.664 (1) and 2.559 (1) Å observed in the tetrabromidomercurate unit are due to the connection of Br atoms to different number of H atoms.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BT2874).

Comment
The guanidium ion, [C(NH 2 ) 3 ] + is interesting due to its ability of making hydrogen bonds and its unique planar shape (Terao et al., 2000). Further, the guanidium ions tend to undergo reorientation motions about their (pseudo) C 3 axes in the crystals. Due to the soft nature of the Hg atom amenable to polarization, the Hg-halogen bonds are sensitive to the intermolecular interactions such as hydrogen bonding (Ishihara et al., 2002). This was evident in the halogen NQR of the Hg compounds in which the resonance lines are widely spread in frequency (Furukawa et al., 2005). Thus we are interested in studying the structure and bonding in this class of compounds. As a part of our study, we report herein the crystal structure of Guanidinium tetrabromidomercurate(II). In the structure, mercury atoms are tetrahedrally coordinated by four bromine atoms and the resulting HgBr 4 tetrahedra are interconnected to the [C(NH 2 ) 3 ] + ions by bromine-hydrogen bonds (Fig. 1) forming a three-dimensional network. In the tetrabromidomercurate unit, two different Hg-Br distances were observed: Hg-Br1 = 2.664 (1) Å and Hg-Br2 = 2.559 (1) Å. The shorter distance of the latter is due to its connection with two hydrogen atoms, whereas the Br1 is connected to four different hydrogen atoms, which elongate the Hg-Br bond (Fig.2).
The C(NH 2 ) 3 moity (Fig. 3) itself is planar where the N-H bonds are somewhat elongated (1.01 (2) Å) to form the network bonds to the bromine atoms of the HgBr 4 tetrahedra.

Experimental
Guanidinium tetrabromidomercurate(II) was prepared by slow concentration of methanolic solution containing mercuric bromide (0.01 mole) and guanidium bromide (0.02 mole) in 1:2 molar ratio. The purity of the compound was checked by elemental analysis and characterized by its NMR and NQR spectra (Furukawa et al., 2005). The single crystals used in X-ray diffraction studies were grown in methanolic solution by a slow evaporation at room temperature.

Refinement
The N-H distances were restrained to 0.87 (1)Å and the coordinates of the H atoms were refined with isotropic displacement parameters set to 1.2 times of the U eq of the parent atom.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.