Crystal structures of two new octochlorotrimercurate(II) RbNH4Hg3Cl8·2H2O and (NH4)2Hg3Cl8·2H2O

The crystal structures of RbNH4Hg3Cl8·2H2O and (NH4)2Hg3Cl8·2H2O compounds at room temperature were determined by single crystal X-ray diffraction. The first one crystallizes in the monoclinic system, space group Cm (No. 8), with a = 21.8170(12) Å; b = 4.2800(8) Å; c = 9.028(3) Å; β = 107.017(11)°; V = 806.1(3) Å3 and Z = 2. The second mercurate (NH4)2Hg3Cl8·2H2O crystallizes also in the monoclinic system, space group P21/m (No. 11), with: a = 21.086(2) Å; b = 4.2587(6) Å; c = 9.0360(9) Å; β = 97.291(8)°; V = 804.87(15) Å3 and Z = 2. The salt structures consist of two-dimensional network [HgCl4]n2n− parallel to the plane formed by b-axis and [1 0 1] direction, constituted of deformed HgCl6 octahedra sharing four corners. These sheets are themselves formed of HgCl6 octahedra triples chains, connected through common Cl(2)–Cl(2) and Cl(3)–Cl(3) edges. In each compound, the monovalent cations and water molecules are located between the sheets ensuring the stability of the structure by hydrogen bonding contacts: N–H⋯Cl and O–H⋯Cl. Differential scanning calorimetry reveals three anomalies at 356, 391 and 419 K for RbNH4Hg3Cl8·2H2O and 373, 388 and 413 K for (NH4)2Hg3Cl8·2H2O. The first peak for each compound can be attributed to an orientational disorder of ammonium groups. IR spectroscopy shows that these materials lose crystallization water gradually in the temperature range 370–490 K.