Dilithium hexahydroxidostannate(IV) dihydrate, a second monoclinic modification with a layer structure

The title compound, Li2[Sn(OH)6]·2H2O, is dimorphic. As for the previously described α-modification, the title β-modification crystallizes in the monoclinic system and contains the same primary building units, viz [Sn(OH)6]2− octahedra and [Li(μ2-OH)3(H2O)] tetrahedra. In contrast to the Sn—O bond lengths that are very similar in both modifications, the Li—O bond lengths differ significantly, in particular those involving the water molecule. In the new β-modification, the primary building units are linked into layers parallel to (010). The [Sn(OH)6]2− octahedra (-1 symmetry) form hexagonal nets and the [Li(μ2-OH)3(H2O)] tetrahedra are situated in between, with their apices in an alternating fashion up and down. O—H⋯O hydrogen bonds between OH groups and water molecules exist within the layers as well as between them.


D-HÁ
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008 In combination with bivalent cations, the anion is found in some rare tin minerals of natural and anthropogenic (Basciano et al., 1998) (Jacobs & Stahl, 2000). Only from lithium, the structures of the anhydrous compound (Yang et al., 2001) as well as that of the dihydrate (Reuter & Bargon, 1997) are known. The last one crystallizes in the monoclinic space group P2 1 /n with [Sn(OH) 6 ] 2octahedra and [Li(µ 2 -OH) 3 (H 2 O)] tetrahedra linked in a three-dimensional way. By changing the crystallization conditions, we were able to isolate a new modification (in the following called β) of the dihydrate. This polymorph likewise crystallizes in the same space group and consists of the same building units as the known modification (in the following called α), but these primary units are In addition to the strong covalent and electrostatic interactions between cations, anions and water molecules, hydrogen bonds ( Fig. 4) are important and fall into two categories: hydrogen bonds within the layers and those between the layers.
The first ones are dominated by the water molecules that are coplanar with the layer plane and act as donors of two hydrogen bonds to two OH-groups of two different neighboring octahedra as well as acceptors of a hydrogen bond of an hydroxyl group of a third octahedron. The water molecules are also involved in the interlayer hydrogen bonds as acceptors resulting in an overall trigonal-bipyramidal coordination mode at the oxygen atom of the water molecule.

Experimental
In a typical experiment, equimolar amounts of freshly prepared K 2 [Sn(OH) 6 ] and LiNO 3 were dissolved as far as possible in 15 ml H 2 O 2 (15% wt ). Undissolved reagents were removed by centrifugation before the solvent was allowed to evaporate slowly. Compact, colorless single crystals of the title compound were formed during some weeks.

Refinement
Hydrogen atoms were clearly identified in difference Fourier syntheses. Their positions were refined with respect to a common O-H distance of 0.96 Å and for the water molecule an H-O-H angle of 104.9° before they were fixed and allowed to ride on the corresponding oxygen atoms. One common isotropic displacement factor was refined for all Hatoms. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. A suitable single-crystal was selected under a polarization microscope and mounted on a 50 µm MicroMesh MiTeGen Micromount TM using FROMBLIN Y perfluoropolyether (LVAC 16/6, Aldrich). Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.