[SnF(bipy)(H2O)]2[SnF6], a mixed-valent inorganic tin(II)–tin(IV) compound

The title compound represents a ionic mixed-valent tin(II)–tin(IV) compound with the bivalent tin atom as central atom of the cation and the tetravalent tin atom as central atom of the anion. Regarding the first and second coordination sphere, the bivalent tin atom is fourfold, seesaw and fivefold, trapezoid–pyramidal coordinated while the tetravalent tin atom exhibits an octahedral coordination.

In the title compound, bis[aqua(2,2 0 -bipyridine)fluoridotin(II)] hexafluoridotin(IV), [SnF(C 10 H 8 N 2 )(H 2 O)] 2 [SnF 6 ], an ionic mixed-valent tin(II)-tin(IV) compound, the bivalent tin atom is the center atom of the cation and the tetravalent tin atom is the center atom of the anion.With respect to the first coordination sphere, the cation is monomeric, with the tin(II) atom having a fourfold seesaw coordination with a fluorine atom in an equatorial position, a water molecule in an axial position and the two nitrogen atoms of the chelating 2,2 0 -bipyridine ligand in the remaining axial and equatorial positions.The bond lengths and angles of this hypervalent first coordination sphere are described by 2c-2e and 3c-4e bonds, respectively, all of which are based on the orthogonal 5p orbitals of the tin atom.In the second coordination sphere, which is based on an additional, very long tin-fluorine bond that leads to dimerization of the cation, the tin atom is trapezoidal-pyramidally coordinated.The tetravalent tin atom of the centrosymmetric anion has an octahedral coordination.The differences in its tin-fluorine bond lengths are attributed to hydrogen bonding, as the two of the four fluorine atoms are each involved in two hydrogen bonds, linking anions and cations together to form strands.

Chemical context
Mixed-valent tin(II)-tin(IV) compounds are most often discovered by chance as a by-product of reactions in which tin(II) compounds are used as starting compounds.Two processes are discussed as potential sources of tetravalent tin (Gurnani et al., 2013): the oxidation of divalent tin by atmospheric oxygen and the disproportionation of tin(II) into elemental tin and tin(IV).However, there is no evidence that the reaction proceeds in favor of either process, as the amount of the mixed-valent compound is usually limited to a few crystals.For these reasons, elemental tin is often added to the reaction mixture or an inert gas atmosphere is used.

Structural commentary
The title compound crystallizes in the triclinic space group P1 with half a formula unit in the asymmetric unit resulting in a centrosymmetric [Sn IV F 6 ] 2À anion and a [Sn II F(H 2 O)(bipy)] + cation in a general position (Fig. 1).
In the cation, the first coordination sphere of the bivalent tin atom (Fig. 2) consists of one fluorine atom, the oxygen atom of the water molecule and both nitrogen atoms of the 2,2 0 -bipyridine ligand.In this seesaw coordination, the fluorine atom [d(Sn-F) = 2.022 (1) A ˚] adopts an equatorial (eq) position and the water molecule [d(Sn-O = 2.408 (2) A ˚] an axial (ax) one, while the nitrogen atoms of the chelating 2,2 0 -bipyridine ligand occupy the two other axial and equatorial positions.According with this arrangement, both tin-nitrogen distances differ [0.025 (1) A ˚] with the shorter one to N eq [2.344 (2) A ˚] and the longer one to N ax [2.369 (2) A ˚].
Within the equatorial plane, the N eq -Sn-F bond angle is 91.5 (1) � , whereas the axis is clearly bent [136.2 (1) � ].Among the axial-to-equatorial bond angles, the bond angle between the nitrogen atoms is very acute [68.5 (1) � ] due to the steric restrictions of the ligand.Conspicuously, the bond angle trans to this chelating bond angle is also quite acute [73.4 (1) � ] while the other two are much more obtuse [78.1 (1) � , 79.6 (1) � ].
In the VB concept with its localized 2c-2e bonds, the bond ratios within the cation are difficult to describe because the tin atom with its non-bonding 5s electron pair and the eight electrons of the four donor atoms (F À ,O,N,N) exceeds the electron octet of a main group element.In contrast, the MO theory according to Pimentel and Rundle (Pimentel, 1951;Rundle, 1949) provides a simple and logical explanation for this hyper-valence in the form of a 3c-4e bond with the exclusive participation of p orbitals on all three atoms.In a seesaw-shaped coordination geometry, this 3c-4e bond is usually found in the axially arranged atoms, while the atoms in the equatorial plane are bonded via 2c-2e bonds through the other p orbitals of the tin atom.This bonding concept not only explains the different tin-nitrogen bond lengths, but also the bond angles around or smaller than 90 � as a result of the orthogonality of the tin p-orbitals.It also explains the remarkable long [2.408 (2) A ˚] tin-oxygen distance to the water molecule, for which bond lengths between 2.207 (2) A ånd 2.226 (2) A ˚[mean value: 2.226 (17) A ˚] are observed (Kleeberg et al., 2022) and Reuter � [SnF(C 10 H 8 N 2 )(H 2 O)] 2 [SnF 6 ]   Acta Cryst. (2024). E80, 921-925   Figure 1 Ball-and-stick model of the ions found in the crystal of the title compound, showing the atom numbering of the asymmetric unit.With the exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn with displacement ellipsoids at the 50% probability level.

Figure 2
Ball-and-stick model (left: side view; right: front view) of the the first, seesaw coordination sphere of the bivalent tin atom Sn2 of the cation, highlighting selected bond lengths (A ˚) and angles ( � ).With the exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn with displacement ellipsoids at the 50% probability level.For clarity, only the carbon atoms between the two nitrogen atoms of the bipyridine ligand are shown, the position of all other carbon atoms are indicated as shortened sticks.Axial bonds are drawn as sticks of reduced thickness in order to underline the presence of a 3c-4e bond.

Figure 3
Ball-and-stick model of the second, trapezoid-pyramidal coordination sphere of Sn2, highlighting selected bond lengths (A ˚) and angles ( � ), viewed down the fluorine-tin bond.With the exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn with displacement ellipsoids at the 50% probability level.For clarity, only the carbon atoms between the two nitrogen atoms of the bipyridine ligand are shown, the position of all other carbon atoms are indicated as shortened sticks.Axial bonds are drawn as sticks of reduced thickness in order to underline the presence of a 3c-4e bond, as is the additional asymmetric 3c-4e bond shown with a dashed line.Symmetry code: (i) À x, À y, 1 À z. trigonal pyramid and the three oxygen atoms of the water molecules at its base.
If only the first coordination sphere is taken as a basis, the cation is monomeric.However, there is a second monomer in its immediate vicinity, whereby both are in contact with each other via a long tin-fluorine bridge [d(Sn-F) = 2.763 (1) A ˚] resulting from a center of symmetry (Fig. 3).This additional, weak bond is in the trans position [h(N-Sn-F) = 148.4(1) � ] to the original, equatorial tin-nitrogen bond and extends the seesaw-shaped, fourfold coordination of the two tin atoms to fivefold, pyramidal coordinations, in which the tin atoms are each below [� least-squares = À 0.6035 (1) A ˚] the trapezoidal, strongly uneven [�� max , least-squares = À 0.147 (1)/0.138( 1

Figure 6
Ball-and-stick model showing in detail the hydrogen-bonding scheme between cation and anion (dashed red lines).With the exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn with displacement ellipsoids at the 50% probability level.The outof-plane deflections (A ˚) of the hydrogen atoms are given in square brackets, i = center of symmetry.Symmetry codes: and the fluorine atoms of the anion play a central role in the formation of the crystal structure.Both hydrogen atoms of the water molecule but only one fluorine atom of the anion are involved in these hydrogen bonds whereby a centrosymmetric, almost planar, eight-membered -O-H� � �F ring is formed (Fig. 6).The geometries of the two crystallographic independent hydrogen bonds are given in Table 1.The out-of-plane deflections of the H atoms are +0.011A ˚for H101 and À 0.001 A ˚for H102.As a result of these hydrogen bonds, the anions and cations are arranged into strands that expand in the a-axis direction (Fig. 7).
Isolated monomeric or dimeric cations of the type [Sn II F(LB 0 ) 3 ] + or [Sn II F(LB 0 ) 2 ] + with LB 0 = neutral Lewis base have not yet been described in the literature, but a comparable mixed-valent tin(II)-tin(IV) compound also from SnF 2 and 2,2 0 -bipyridine was previously described by Gurnani et al. (2013)

Synthesis and crystallization
A mixture of approximately 157 mg (1 mmol) of SnF 2 (Aldrich) and 156 mg (1 mmol) of 2,2 0 -bipyridine (Sigma Aldrich) was placed in a Petri dish and 5 ml of N,N-dimethylformamide (Sigma Aldrich) were added via a syringe.No elemental tin was added, nor was oxygen from the air excluded.The following processes were observed from day to day with a light microscope (Stemi 1000, Zeiss, Oberkochen, Germany).After two days small, colorless, block-like crystals research communications 924 Ro ¨wekamp-Krugley and Reuter

Figure 7
Stick model of the crystal packing looking down the crystallographic c axis and showing the strands resulting from the hydrogen bonds (dashed red lines) between anions and cations.Color code as in Fig. 4.  of the title compound appeared, which, after two additional days, reached a size that was sufficient for a single crystal X-ray determination.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.The positions of all H atoms were clearly identified in difference-Fourier maps.Those of the organic ligand were refined with calculated positions (C-H = 0.93 A ˚) and a common isotropic displacement parameter.The positions of the H atoms of the water molecule were refined with a fixed O-H distance of 0.96 A ˚and a bond angle of 104.95 � before they were fixed and allowed to ride on the parent O atom with an isotropic displacement parameter.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes. Fractional in electron-precise tin(II) compounds of the type [Sn(OH 2 ) 3 ] 2+ with the tin atom at the apex of a research communications 922 Ro ¨wekamp-Krugley ) A ˚] base while the shortest bonds [d(Sn-F) = 2.022 (1) A ˚] point in the direction of the apical fluorine atoms.This dimerization leads to a bridging angle of 111.8 (1) � at the fluorine atoms and a four-membered, centrosymmetric and therefore exactly planar tin-fluorine ring.Its rhomboidal shape is characterized by acute [68.2 (1) � ] angles at the tin atoms and obtuse [111.8 (1) � ] ones at the fluorine atoms.Extending the Pimentel-Rundle concept,Musher (1969) suggests that such additional, very weak bonds result from a p orbital of the central atom, that is involved in a classical 2c-2e bond on the one hand and a 3c-4e bond on the other.As result, in such 'asymmetric' 3c-4e bonds, one ligand is much more strongly bound to the central atom than the trans-ligand.Despite this additional bond, the coordination sphere of the tin atom (Fig.4) remains hemi-directed(Shimoni-Livny et al., 1998).The octahedral shape of the centrosymmetric [SnF 6 ] 2À anion is reflected in bond angles around 90 � [88.4 (1)-91.6 (1) � ] and very similar tin-fluorine bond lengths.While four of the bonds (F12, F13) are almost identical in length [1.949 (1)/1.942(2) A ˚], two bonds (F11) are somewhat longer [1.993 (1) A ˚] because the fluorine atoms undergo hydrogen bonding with the hydrogen atoms of two different water molecules (Fig.5).
Figure 4Space-filling model of the [SnF(bipy)(H 2 O)] + cation looking down the apical tin-fluorine bond and visualizing the hemi-directed coordination of the bivalent tin atom Sn2.Atoms whose spheres are penetrated by other atoms are visualized as truncated two-colored spheres, the hydrogen bond is indicated by a dashed red line.Atom colors and van der Waals radii (A ˚) are as follows: F = green/1.47,H = white/1.10,C = gray/1.70,O = red/1.52,N = blue/1.55and Sn = brass/2.17.Symmetry code: (i) À x, À y, 1 À z.

Figure 5
Figure 5Polyhedron and ball-and-stick model of the octahedral, centrosymmetric [SnF 6 ] 2À anion with bond lengths (A ˚) and hydrogen bonds indicated by dashed red lines.All atoms are drawn with displacement ellipsoids at the 50% probability level.

Table 2
Experimental details.