Crystal structure of Tutton’s salts K2[MII(H2O)6](SeO4)2, MII = Co, Ni, Zn and refinement of the crystal structure of potassium hexaaquamagnesium( II) selenate, K2[Mg(H2O)6](SeO4)2

H12K2MgO14Se2, monoclinic, P121/a1 (no. 14), a = 9.264(1) Å, b = 12.421(1) Å, c = 6.2514(8) Å, / = 104.29(1)°, V = 697.1 Å, Z = 2, Rgt(F) = 0.017, wRref(F ) = 0.035, T = 295 K. H12CoK2O14Se2, monoclinic, P121/a1 (no. 14), a = 9.226(1) Å, b = 12.375(1) Å, c = 6.2768(7) Å, / = 104.210(9)°, V = 694.8 Å, Z = 2, Rgt(F) = 0.029, wRref(F ) = 0.074, T = 295 K. H12K2NiO14Se2, monoclinic, P121/a1 (no. 14), a = 9.179(2) Å, b = 12.367(3) Å, c = 6.254(2) Å, / = 104.46(2)°, V = 687.5 Å, Z = 2, Rgt(F) = 0.020, wRref(F ) = 0.052, T = 295 K. H12K2O14Se2Zn, monoclinic, P121/a1 (no. 14), a = 9.212(1) Å, b = 12.391(1) Å, c = 6.2780(8) Å, / = 104.20(1)°, V = 694.7 Å, Z = 2, Rgt(F) = 0.016, wRref(F ) = 0.034, T = 295 K. Source of material Tutton’s salts of the most probably isomorphous series K2[M(H2O)6](SeO4)2 were prepared by dissolution of equimolar amounts of potassium selenate K2SeO4 and M selenate hexahydrate MSeO4. ·. 6H2O in hot destilled water and ensuing evaporation of the solvent. Differently coloured, mostly ideomorphous single crystals of dimensions up to 10 mm were obtained. Experimental details To reduce errors introduced by insufficient corrections of absorption effects, it was attempted to grind a crystal sphere in order to apply a spherical absorption correction in the course of data processing. The grinding produced an ellipsoid, whose deviation from spherical is characterized by the given max, mid and min crystal dimensions. Discussion Studying crystal chemistry, metal-water interaction and system of hydrogen bonds in different series of Tutton salts (Rb sulfates [1], Rb selenates [2], Cs sulfates [3], Cs selenates [4], K sulfates [5]) by single crystal and powder diffraction experiments, it is essential to conduct these experiments as similar as possible in order to reduce errors and obtain truly comparable results. Therefore, we redetermined the known structure of the potassium hexaaquamagnesium(II) selenate [6] with single crystal X-ray diffraction, resulting in a better agreement factor R1 and determined the so far unknown crystal structures of the other compounds of the potassium selenate series. These are isotypic. The Tutton salt structure is characterized by irregular MO8 polyhedral (here with M = K), d(K—O) = 2.740(3) 3.450(2) Å, and M(H2O)6 octahedra, which are linked to the SeO4 groups by a system of hydrogen bonds. Compared to the earlier studied isotypic Rb selenates [2], one finds for the present compounds: 1. Except for Mg, the lattice parameters a, b, c of the M compounds decrease on average by 1.5 %, 2.1 %, 1.2 %, respectively. This follows from K being smaller than Rb and hence from the smaller volume of the KO8 polyhedron [7,8]. The unit cell angle / decreasing for all corresponding compounds by about 1.0°, shows a 5 6 times more negative slope in the linear correlation between / and the M cation radius. The volume of the unit cell decreases by about 4.5 %. With respect to the exchange of M, the linear correlations between the lattice parameters and the cation radii exhibit for a and c, almost the same slope than in the Rb selenates, but for b, a 4 5 times smaller one. 2. The volumes of the KO8 coordination polyhedra in the K selenate salts are on average 9.9 % smaller than those of the RbO8 moieties. This corresponds to the 3rd power of the ratio mean K—O and Rb—O distances, (d(K—O)/d(Rb—O))= 0.906. 3. The volumes of the MO6 coordination octahedra agree with those in the corresponding Rb selenates. 4. In agreement with chemical expectation, the selenate group is practically invariant against the M cation exchange in the hexahydrate complexes. The volumes of the MO6 coordination polyhedra increase as the cation radius R(M) [7] increases. Z. Kristallogr. NCS 224 (2009) 351-354 / DOI 10.1524/ncrs.2009. 0156 351 © by Oldenbourg Wissenschaftsverlag, München