Crystal structure of [NiHg(SCN)4(CH3OH)2]n

The crystal structure of [NiHg(SCN)4(CH3OH)2]n is made up of HgS4 tetrahedra and trans-NiN4O2 octahedra, linked together by thiocyanato bridges. The methanol molecules point to the cavities of the resulting framework.

The title compound, catena-poly [[bis(methanol-O)nickel(II)]-di--thiocyanato-4 N:S-mercurate(II)-di--thiocyanato-4 N:S], was obtained from a gel-growth method using tetramethoxysilane as gelling agent. The crystal structure is composed of rather regular HgS 4 tetrahedra (point group symmetry .2.) and trans-NiN 4 O 2 octahedra (point group symmetry 2..) that are linked through thiocyanato bridges into a three-dimensional framework. The methanol molecules coordinate via the O atom to the Ni 2+ cations and point into the voids of this arrangement while a weak O-HÁ Á ÁS hydrogen bond to an adjacent S atom stabilizes it.

Chemical context
Compounds of the type MHg(SCN) 4 (M is a divalent transition metal) exhibit interesting physical properties. For example, CoHg(SCN) 4 is a calibrant for magnetic susceptibility measurements using the Faraday method (Brown et al., 1977), and representatives with M = Fe, Mn, Zn and Cd show second-order non-linear optical (NLO) properties (Bergman et al., 1970;Yan et al., 1999).
In an attempt to grow crystals of the desired compound NiHg(SCN) 4 using a gel-growth method (Henisch, 1996), starting from TMOS (tetramethoxysilane) as gelling agent, we obtained the title compound, [NiHg(SCN) 4 (CH 3 OH) 2 ] n viz. a methanol-containing phase, instead. Methanol is generated during the gelling process of the silicate-based material according to the idealized reaction (H 3 CO) 4 Si + 4 H 2 O ! 4 H 4 SiO 4 + 4 H 3 COH and then becomes part of the crystal structure.

Structural commentary
The basic structure units of [NiHg(SCN) 4 (CH 3 OH) 2 ] n are HgS 4 tetrahedra (point group symmetry .2.) and trans-NiN 4 O 2 octahedra (point group symmetry 2..) that are linked through the bridging thiocyanate anions into a three-dimensional framework structure (Fig. 1). The Hg-S bond lengths [mean 2.552 (3) Å ; Table 1] are in very good agreement compared with those of HgS 4 tetrahedra in the above-mentioned solvent-free MHg(SCN) 4 structures, which have a mean of 2.57 (5) Å . The trans-NiN 4 O 2 octahedra are defined by four N atoms belonging to four bridging thiocyanate anions and by two O atoms of isolated methanol molecules. The displacement parameters of the methanol molecule are rather high. The methanol molecule has relatively much space for libration, because it is not part of the framework structure and points into the remaining free space. Thus the displacement ellipsoids of the methanol O and especially of the C atom are enlarged (Fig. 1). Moreover, there is only a weak hydrogenbonding interaction to an adjacent S atom that stabilizes this arrangement (Table 2).
[NiHg(SCN) 4 (CH 3 OH) 2 ] n and [NiHg(SCN) 4 (H 2 O) 2 ] n have a similar composition. Although the basic structure units (HgS 4 tetrahedra and trans-NiN 4 O 2 octahedra linked by thiocyanate bridges) are the same, the corresponding crystal structures are markedly different. The methanol-containing structure has tetragonal symmetry and is non-centrosymmetric, the water-containing structure has monoclinic symmetry and is centrosymmetric (space group C2/c). Whereas in the water-containing structure the HgS 4 and NiN 4 O 2 polyhedra are alternately arranged in layers parallel to (001) (Fig. 2), the arrangement in the methanol-containing compound is markedly different (Fig. 1).
The common structural motif in the above-mentioned MHg(SCN) 4 compounds is the linkage of MN 4 units (planar configuration for Cu and tetrahedral for all other M members) and tetrahedral HgS 4 units through thiocyanate bridges. It seems that a coordination number of four is not favoured for structures with M = Ni. In the structures of [NiHg(SCN) 4 -(CH 3 OH) 2 ] n , [NiHg(SCN) 4 (H 2 O) 2 ] n and NiHg 2 (SCN) 6 , the Ni 2+ ions all have coordination numbers of six, which is probably the reason why a compound with composition NiHg(SCN) 4 (most probably requiring a [4]-coordination for Ni 2+ ) has not yet been isolated.
The colourless precipitate was filtered off, washed with water and dried. For the gel-growth experiment, 1.2 g Ni(NO 3 ) 2 Á6H 2 O and 1.2 g NH 4 SCN were dissolved in 20 ml water. To this solution, 0.5 g freshly prepared Hg(SCN) 2 was slowly added until complete dissolution. Then 2 ml TMOS was added dropwise under stirring. Gelling time was about 3 h. After one week, blue single crystals of the title compound up to 5 mm in length had formed in the gel matrix.

Refinement
The H atom of the methanol hydroxy group was located from a difference map and was refined with a distance restraint of 0.90 (1) Å . The H atoms associated with the methyl group of the methanol molecule could not be located from difference Fourier maps. As a result of the high libration of this molecule, it seems probable that the methyl H atoms are disordered and were therefore refined with two positions with half-occupancy and rotated by 60 degrees. U eq of these H atoms were set 1.5U iso of the parent C atom. The remaining maximum and minimum electron densities are found 0.36 and 0.06 Å , respectively, from atom O1. Reflection (011) was affected by the beamstop and was discarded from the refinement. Experimental details are given in Table 3  Computer programs: APEX2 and SAINT (Bruker, 2008), SHELXS97 and SHELXL97 (Sheldrick, 2008) and ATOMS for Windows (Dowty, 2006

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. 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.