Redetermination of the crystal structure of K2Hg(SCN)4

The redetermination of the crystal structure of potassium tetrathiocyanatomercurate(II) reveals all atoms tombe located and shows much higher precision and accuracy in comparison with the previously determined structure.

Single crystals of K 2 Hg(SCN) 4 [dipotassium tetrathiocyanatomercurate(II)] were grown from aqueous solutions of potassium thiocyanate and mercury(II) thiocyanate and studied by single-crystal X-ray diffraction. In comparison with the previously reported structure model [Zvonkova (1952). Zh. Fiz. Khim. 26, 1798Khim. 26, -1803, all atoms in the crystal structure were located, with lattice parameters and fractional coordinates determined to a much higher precision. In the (crystal) structure, the Hg II atom is located on a twofold rotation axis and is coordinated in the form of a distorted tetrahedron by four S atoms of the thiocyanate anions. The K + cation shows a coordination number of eight.

Chemical context
In search for suitable educts for fluorination we thought that K 2 Hg(SCN) 4 would be a well-suited candidate. Once we had obtained the compound, we noticed that the original structure determination (Zvonkova, 1952) was of low precision with the light atoms (C and N) not determined, so we redetermined the crystal structure to much higher precision and accuracy.
As may be expected, the two unique SCN À anions are almost linear [178.0 (3), 178.2 (3) ], and the angles are comparable with those reported for Hg(SCN) 2 [177.5 (13) ; Beauchamp & Goutier, 1972]  The K + cation shows a coordination number of eight, with disparate bond lengths that can be associated with a [4 + 3 + 1] coordination. Four K-N distances are in the range 2.816 (4)-3.031 (5) Å , three K-S distances are in the range 3.4466 (11)-3.5315 (12) Å and there is one very long K-N distance of 3.793 (5) Å . Therefore, the resulting coordination polyhedron is of an odd shape. The K + cation is coordinated in total by five [Hg(SCN) 4 ] 2À units, three of these in a monodentate manner (two via N atoms and one via the S atom of the thiocyanate anions) and the other two in a bidentate mode (via the N and S atoms of neighboring thiocyanate anions). Overall, a complex three-dimensional framework results. The crystal structure of the title compound is shown in Fig. 2.

Synthesis and crystallization
Potassium tetrathiocyanatomercurate(II) was synthesized by slowly adding a potassium thiocyanate solution (2.076 g, 21.36 mmol in 10 ml H 2 O) to a boiling solution of mercury(II) thiocyanate (3.176 g, 10.03 mmol in 10 ml H 2 O). After the formed mercury sulfide had been filtered off through a Bü chner funnel, the solution was concentrated on a hot plate until crystallization set in. The crystallized product was collected on a Bü chner funnel and the filtrate was allowed to stand at room temperature until crystals of much better quality were obtained. A selected colorless single crystal was investigated by X-ray diffraction. Mercury(II) thiocyanate was prepared as reported previously (Hermes, 1866) using mercury(II) nitrate and potasium thiocyanate and was recrystallized out of ethanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. As a starting model for the structure refinement, the atomic coordinates of the previously reported K 2 Hg(SCN) 4 structure model were used (Zvonkova,  The crystal structure of K 2 Hg(SCN) 4 viewed along [110]. Displacement ellipsoids are shown at the 70% probability level at 293 K. Bonds involving the K + cation are omitted for clarity. 1952). The positions of the C and N atoms were located from a difference-Fourier map.  SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2015); software used to prepare material for publication: publCIF (Westrip, 2010).

Dipotassium tetrathiocyanatomercurate(II)
Crystal data 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.