Ba3[Sn(OH)6][SeO4]2·3H2O, a hydrated 1:2 double salt of barium hexahydroxidostannate(IV) and barium selenate(VI)

The title compound, Ba3[Sn(OH)6][SeO4]2·3H2O, was prepared by mimicking geochemical crystallization processes, and the hexagonal crystal structure determined. Its hierarchical structure comes from four different primary building units, setting up secondary building units that are arranged into layers via their {BaO9} coordination polyhedra and cross-linked with the SBUs of adjacent layers via common [Sn(OH)6]2− ions.

ions, all three of point group symmetry C 3 , and a mono-capped {BaO 9 }-square antiprism of point group symmetry C 1 . The secondary building units result from three of the barium coordination polyhedra linked together via common edges. While one of the two tetrahedral voids formed from these trimeric units is filled by one bidentate, chelating 2 -selenate ion, the other one remains unoccupied as the corresponding second selenate ion only acts as a monodentate, 1 -ligand. SBUs are completed by hexahydroxidostannate(IV) ions sharing adjacent edges on the uncapped faces of the three, mono-capped square antiprisms. These SBUs are arranged into layers via common edges on the uncapped, square faces of the {BaO 9 } coordination polyhedra in a way that the hexahydroxidostannate(IV) ions act as linkage between two neighboring layers.

Chemical context
The hexahydroxidostannate(IV) ion, [Sn(OH) 6 ] 2À , is a well established tin(IV) anion in chemistry (Scholder, 1981), mineralogy (Strunz & Nickel, 1998) and even archaeology (Basciano et al. 1998), although the number of well defined and structurally described compounds is rather low, especially in case of two-valent cations as these compounds are only slightly soluble. In a former paper (Kamaha & Reuter, 2009), we demonstrated strategies for how to circumvent these difficulties by combining slow anion formation with slow crystallization, mimicking to some extent geochemical crystal formation processes.
Here we present our results on experiments where we offered selenate(VI) anions parallel to the slow formation of hexahydroxidostannate(IV) ions, as possible co-anions during crystallization. In a typical experiment we exposed BaSnO 3 pellets to a Na 2 SeO 4 -solution, resulting after a long period in the formation of colorless, hexagonal prisms of Ba 3 [Sn(OH) 6 ][SeO 4 ] 2 Á3H 2 O, a hydrated 1:2 double salt of barium hexahydroxidostannate(IV), Ba[Sn(OH) 6 ], and barium selenate(VI), Ba [SeO 4 ]. From both compounds, only the structure of the selenate has been described in the literature (Andara et al., 2005).

Structural commentary
The title compound crystallizes in the non-centrosymmetric, hexagonal space group P6 3 and was refined as an inversion twin, giving a Flack parameter of 0.037 (11). With two formula units in the unit cell, the asymmetric unit consists of 1/3 formula unit: a Ba 2+ ion and a water molecule, both in general positions, and two crystallographically independent [SeO 4 ] 2À ions and one [Sn(OH) 6 ] 2À ion, all three having the point group C 3 . In addition to the {BaO 9 }-coordination polyhedron, these complex anions represent the primary building units, PBUs.
The two crystallographically different Sn-O distances within the hexahydroxidostannate(IV) anion ( Fig. 1, Table 1) are identical within standard deviations [d(Sn1-O1) = 2.052 (2) Å and d(Sn1-O2) = 2.054 (2) Å ]. The mean value of 2.053 (2) Å is somewhat shorter than the mean value of 2.060 (10) Å observed in other hexahydroxidostannates (Kamaha & Reuter, 2009), but lies in the observed range of 2.039-2.075 Å . Deviations from the geometry of a regular octahedron are better expressed in terms of the bond angles, best described by the non-linearity of the octahedron axes, which show bond angles of 178.7 (1) . All oxygen atoms of the [Sn(OH) 6 ] 2À ion coordinate to two different Ba atoms in a 2coordination mode, while the hydrogen atoms are involved in hydrogen bonds (Table 2) with the oxygen atoms O3 and O7 of the two different [SeO 4 ] 2À ions.
As a result of their C 3 symmetry, the structural parameters (Table 3) (Kolitsch, 2002)] the individual Se-O distances differ significantly, reflecting the different functionality of both kind of oxygen atoms. Bonds to the apical oxygen atoms are considerably longer [1.654 (4), 1.648 (4) Å ] than those to the basal ones [1.634 (2), 1.633 (2) Å ]. In the first selenate ion, the apical oxygen atom acts as acceptor of three hydrogen bonds, while the corresponding oxygen atom of the second selenate ion coordinates to three barium ions. On the other hand, the three basal oxygen atoms act as acceptor of one hydrogen bond and also perform coordinative bonds, each to a different barium ion, in the first selenate ion while those of the second selenate ion accept two hydrogen bonds. Besides bond-length differences, deviations from the geometry of a regular tetrahedron result in both selenate ions having bond angles widening between the basal oxygen atoms, giving them the shape of slightly flattened trigonal pyramids rather than strict tetrahedra (Fig. 2).
The coordination sphere of the barium ion consists of nine oxygen atoms: two from water molecules, four from two [Sn(OH) 6 ] 2À ions, one from the first selenate ion and two,  Table 1 Selected geometric parameters (Å , ).

Figure 1
Ball-and-stick model of the [Sn(OH) 6 ] 2À ion with atom numbering of the asymmetric unit and orientation of the threefold rotation axis (blue). With exception of the hydrogen atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn as displacement ellipsoids at the 40% level. Bonds between oxygen and barium are indicated as shortened, two-colored sticks while hydrogen bonds are visualized as dashed lines (red). Table 3 Bond lengths (Å ) within the mono-capped {BaO 9 } square antiprism.
Symmetry codes: (i) y, Àx + y + 1, z + 1 respectively, from the second selenate ion (Fig. 3). In summary, } coordination sphere has the shape of a mono-capped square antiprism. The uncapped face of this coordination polyhedron only is built up from the oxygen atoms of two hexahydroxidostannate ions. Its shape is almost square [maximal angle deviations from rectangular: AE0.6 (1) , maximal deviation from planarity: AE0.0132 Å , side lengths: 2.7705 (2)-3.1720 (2) Å ]. In contrast, the capped face of the square antiprism consists of oxygen atoms from two water molecules and from basal oxygen atoms of the two different selenate ions. Its shape [maximal deviation from planarity: AE0.0022 (2) Å ] is much better described as an acute trapezoid with a longer/shorter base of 4.4606 (2)/3.7164 (2) Å , legs of 3.4108 (2)/3.2331 (2) Å and angles between 103.01 (1) and 78.42 (1) . The dihedral angle between these planes is 5.64 (1) . These deviations from a regular square antiprism are mainly caused by coordination to the selenate ions, as the apical oxygen atom of the second one constitutes the cap of the {BaO 9 } coordination polyhedron, giving rise to a bidentate-chelating coordination mode of this selenate ion while the first selenate ion only acts as monodentate ligand. Ba-O bond lengths (Table 3)

Supramolecular features
The interaction of the four different PBUs is visualized in Ball-and-stick model of the mono-capped, square-prismatic {BaO 9 } coordination polyhedron. Atom colors and bond design as in Fig. 2. Symmetry codes: (1) y, 1 À x + y, 1 2 + z; (2) 2 À x, 1 À y, 1 2 + z; (3) 1 À y, x À y, z; (4) 1 À x + y, 2 À x, z; (5) 1 + x À y, x, 1 2 + z.  Ball-and-stick models of the two crystallographically independent [SeO 4 ] 2À ions with atom numbering of the asymmetric unit and orientation of the threefold rotation axis (blue). With exception of the hydrogen atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn as displacement ellipsoids at the 40% level. Coordination bonds between oxygen and barium are drawn as shortened, twocolored sticks, hydrogen bonds between oxygen and hydrogen atoms of water molecules and hydroxyl groups are drawn as dashed lines (red).
in Wyckoff position b. These three PBUs are linked together via common edges, each of them composed of the coordinated water molecule and the apical oxygen atom of the second selenate ion. In addition, this selenate ion shares its remaining three, basal oxygen atoms with the three surrounding barium ions, thus filling the tetrahedral void between the three {BaO 9 } coordination polyhedra. On the other hand, the opposite void of the trimeric {BaO 9 } unit is empty, as the first selenate ion only shares its three basal oxygen atoms with the three {BaO 9 } coordination polyhedra but not the apical one. Each SBU is completed by a hexahydroxidostannate(IV) ion sharing one edge with the uncapped face of the mono-capped {BaO 9 } antiprisms. These secondary building units are linked together with three others, each via a common edge of the uncapped square of the {BaO 9 } coordination polyhedra. In this way, each {BaO 9 } coordination polyhedron shares two opposite edges of its square faces with two neighboring barium coordination polyhedra, resulting in a trigonal-prismatic void between the three interconnected SBUs. All corners of these voids consist of hydroxyl groups from [Sn(OH) 6 ] 2À ions with the tin atoms of these PBUs situated on threefold rotation axes in Wyckoff position a.
In summary, the SBUs are arranged in layers perpendicular to the c axis direction (Fig. 5). The pores within these layers are occupied by selenate ions (Se2) of adjacent layers. These selenate ions are connected with the layer via hydrogen bonds (Table 2) to the water molecules and hydroxyl groups of the [Sn(OH) 6 ] 2À ions, while the latter cross-link adjacent layers.
Adjacent layers are rotated through 120 against each other, resulting in a compact crystal packing without any accessible holes, channels or pores (Fig. 6) & Taylor, 1971;Effenberger et al., 1983), also crystallizing in space group P6 3 . In contrast to the title compound, the coordination polyhedron of the earth metal in this mineral is reduced from nine to eight and may be described as a one-face distorted square antiprism. While the hexahydroxidosilicate adopt a similar position as the hexahydroxidostannate ion, the two other complex anions of thaumasite are only linked via hydrogen bonds to the trimeric units of {CaO 8 } polyhedra as these secondary building units are not cross-linked into layers.   (Sheldrick, 2015), DIAMOND (Brandenburg, 2006), Mercury (Macrae et al. (2020) and publCIF (Westrip, 2010).

Figure 5
Polyhedral model showing the aufbau principle of the layers (top view above, side view below) as result of the interconnection of the SBUs. Polyhedra colors according to Fig. 1 to 3. Isolated selenate ions (Se2) in the three-sided pores belong to adjacent layers. The corresponding hydrogen bonds are omitted for clarity.

Synthesis and crystallization
Single crystals of the title compound were obtained in a longduration experiment exposing a BaSnO 3 (Celest) pellet prepared by heating equimolar amounts of SnO 2 and BaO for 40 h at 1688 K to 10 ml of a solution of Na 2 SeO 4 (Fluka) in a rolled rim glass vessel closed with a snap-on lid. Colorless, hexagonal prisms occurred after several months in the sludge of the mouldered BaSnO 3 pellet.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were clearly identified in difference Fourier syntheses. Their positions were modeled with respect to a common O-H distance of 0.96 Å and a bond angle of 104.9 in case of the water molecule before they were fixed and allowed to ride on the corresponding oxygen atoms. Refinement of two common isotropic temperature factors, one for the hydrogen atoms of the hydroxyl groups and one for the hydrogen atoms of the water molecule, allowed us to check the reliability of their positions.