Refinement of the crystal structure of potassium hexaaquairon(II) sulfate, K2[Fe(H2O)6](SO4)2 and potassium hexaaquazinc(II) sulfate, K2[Zn(H2O)6](SO4)2

FeHi2K20i4S2, monoclinic, P\l\la\ (no. 14), a = 9.065(1) Â, b = 12.252(1) À, c = 6.1646(5) λ, β = 104.536(7)°, V = 662.8 Â, Ζ = 2, Rgt(F) = 0.020, wR^F) = 0.055, Γ =295 Κ. H12K2O14S2Z11, monoclinic, P\2\la\ (no. 14), a = 9.029(1) Â, b = 12.204(1) À, c = 6.1470(5) λ, β = 104.795(9)°, V = 654.8 Â, Ζ = 2, Rgt(F) = 0.022, wR^F) = 0.058, T= 295 K. Source of material Both representatives of the most probably isomorphous Tutton's salts series K2[M(H20)6](S04)2 were prepared by dissolution of equimolar amounts of potassium sulfate K2SO4 and M sulfate heptahydrate MSC>4 • 7H2O in hot destilled water and ensuing evaporation of the solvent. Differently coloured, mostly ideomorphous single crystals of dimensions up to 10 mm were obtained. * Correspondence author (e-mail: h.euler@um-bonn.de) Discussion Studying crystal chemistry, metal water interaction and the system of hydrogen bonds in different series of Tutton salts (Rb sulfates [1], Rb selenates [2], Cs sulfates [3], Cs selenates [4]) by single crystal and powder diffraction, it is essential to conduct the experiments as similar as possible in order to reduce errors and to obtain truly comparable results. Therefore, we redetermined the known structures of the Κ sulfates [5-10] with X-ray diffraction under similar conditions and confirmed, that all compounds are isotypic with space group Plila (no. 14), Ζ = 2 [11]. For the present paper, only crystal structure determinations of two more Tutton salts K2[M(H20)6](S04)2, M = Fe, Zn with improved /{-values are presented, but for the following considerations, all quoted results were used [11]. The Tutton salt crystal structure is characterized by irregular Mk)8 polyhedra (here with M = K), d(K—O) = 2.717(1) 3.288(1) Â, and M"(H20)6 octahedra, which are linked to the SO4 groups by a system of hydrogen bonds. According to expectations, the most distorted Μ(Η2θ)β octahedron is obtained for M = Cu (Jahn-Teller distortion) with d(M—O) = 1.940(1) 2.227(1) Â. Compared to the earlier studied isotypic Rb sulfates [1], one finds for the present compounds: 1. Except for Mg and Cu, the lattice parameters a, b, c of all the different M compounds decrease on average by 1.9 %, 2.2 %, 1.5 %, respectively. This follows from the smaller K ion and hence to the smaller volume of the KOs-polyhedron [12,13]. The unit cell angle β decreasing for all compounds, except for M = Cu, by about 1.1°, shows a more negative slope in the linear correlation betweenβ and the M cation radius. With respect to the exchange of Μ , the linear correlations between the lattice parameters and the cation radii exhibit i) for a, the same slope as in the Rb sulfates, but ii) for b, a significantly smaller and iii) for c, a significantly larger slope, respectively. 2. The volumes of the various KOs coordination polyhedra in the K-salts is on average 10.5 % smaller. This corresponds to the 3rd power of the ratio of the mean Κ—O and Rb—O distances, (d(K—0)/S(Rb—0))= 0.892. 3. The volumes of the MOe coordination octahedra agree with those of the corresponding Rb sulfates. 4. While in agreement with chemical expectation, the sulfate group is practically invariant against the M cation exchange in the hexahydrate-complex, the volumes of both the MOe and the KOg coordination polyhedra increase as the cation radius /? (M) [13] increases. For V(KOgi, the increase is more than three times smaller than that for V(M Oe) and it is also significantly smaller than that for the analog Rb sulfates. 5. As in the Rb sulfates, the a parameter of the Mg and Cu compounds deviate significantly from the a vs RÇM ) relation. The 172 K 2 [Fe(H 2 0)6](S0 4 )2 , K2[Zn(H 20)6](S0 4)2 same holds for the b parameters of the M g and Cu salts, respectively, the former being larger, the latter smaller. The finding for M g can be attributed to the larger M-O electronegativity difference causing somewhat weaker K-O interactions as reflected by the increased distances, d(K—05) = 3.140( 1 ) Â and d ( K — 0 6 ) = 2 . 994(1) Â, as compared to the other M n compounds of the same series. The shape of the CuOe octahedron is caused by the JahnTeller effect which is the reason for the exceptional behavior of