Crystal structure of dipotassium dibromo-bis(thiocyanato)mercurate(II), K2[HgBr2(SCN)2]

Abstract

C 2Br2HgK2N2S2, hexagonal, P63/m (no. 176), a = 12.896(1) Å, c = 11.293(1) Å, V = 1626.5 Å 3 , Z = 6, Rgt(F) = 0.037, wRref(F 2 ) = 0.064, T = 293 K. Source of materialK2HgBr2(SCN)2 was prepared from 1 mmol of Hg(SCN)2 which was dissolved in 50 mL of water with the stoichiometric amount of KBr. The mixture was refluxed during one hour and the resulting clear solution was slowly evaporated at room temperature, yielding millimeter sized, transparent prisms of K2HgBr2(SCN)2. Crystals were filtered off, washed with a small amount of methanol and dried at room temperature. DiscussionThe mercury atom is coordinated to two bromine atoms and two sulfur atoms (figure, top). The thiocyanate bonding is coherent with an HSAB explanation (Hard and Soft Acids and Bases). The metal coordination environment can be described as a distorted tetrahedron, with S-Hg-S and Br-Hg-Br angles equal to 127.3(1)°and 97.19(4)°, respectively. Among mercury compounds, the present environment is not unusual. In the thiocyanate chemistry, we already observed it in Hg3CdCl2(SCN)6 [1]. Thus, the structure could be described as a packing of [HgBr2(SCN)2] 2-complex anions and potassium cations. However, due to the coordination of the SCN -group, a 3D description of the crystal structure taking the potassium coordination into account seems to be a better one. The SCN -group is coordinated to mercury through the sulphur atom and to three potassium atoms through nitrogen. This scheme is denoted IVc [2]. To our knowledge, the present tetradentate mode had never been observed. The K1 environment is a trigonal prism formed by four nitrogen and two bromine atoms. The K2 environment is an octahedron formed by three nitrogen and three bromine atoms. Finally, the K3 surrounding is an octahedron, elongated along the c axis, formed by six bromine atoms. The figure, bottom, shows the 3D interconnection of the metal cations. The most simple description is as follows. The K2 octahedra are connected through three Br bridges forming dimers. These dimers are linked through three K2-(NCS)Hg(SCN)-K2 bridges, a connection becoming very rigid through K1-N bonds. This leads to chains parallel to the c axis:The linking K3-[Br2]3-K3 unit is also parallel to c. These two parallel chains are connected directly through Hg-Br2 bonds and indirectly through Br1-Hg, K1-Br2 bonds, resulting in a very compact 3D structure.