Crystal structure of the Anderson-type heteropolyoxometalate; K2[H7CrIIIMo6O24]·8H2O: a redetermination revealing the position of the extra H atom in the polyanion

K2[H7CrIIIMo6O24]·8H2O contains a symmetric hydrogen bond in which the H atom does not lie on a crystallographic centre of symmetry. The structure has been redetermined in order to locate the position of the seventh H atom.


Chemical context
This redetermined structure of a typical Anderson-type heteropolyoxometalate (Anderson, 1937) 2À polyanion. This has not only an extra H atom but this atom also forms a very short hydrogen bond [2.461 (3) Å ]; however, the H atom that contributes to the short hydrogen bond does not lie on a crystallographic centre of symmetry.
An example of a relatively short hydrogen bond in which the H atom does lie on a crystallographic centre of symmetry in an Anderson-type polyanion was reported in the polyoxometalate, viz. K 7 [H 4.5 -PtMo 6 O 24 ] 2 Á11H 2 O (Lee et al., 2010). In this compound, the two polyanions form a dimer, viz.
[ (H 4.5 PtMo 6 O 24 ) 2 ] 7À via seven hydrogen bonds, viz. four 3 -O-HÁ Á Á 1 -O (terminal Mo O atom), two 2 -O-HÁ Á Á 2 -O and one central/symmetric 3 -OÁ Á ÁHÁ Á Á 3 -O. The H atom of the central hydrogen bond lies on a crystallographic centre of symmetry (space group P1: 1 2 , 0, 1 2 ) with a 3 -OÁ Á ÁHÁ Á Á 3 -O (1) distance of 2.553 (3) Å . In this way, the hydrogen bond is symmetric, OÁ Á ÁHÁ Á ÁO, and the donor and acceptor cannot be distinguished. The dimerization of the polyanion by these hydrogen bonds is possible because the 3 -O atoms in the polyanion are only partially protonated. The location of the H atom in the central 3 -OÁ Á ÁHÁ Á Á 3 -O unit was determined from a centrosymmetric electron density map around the H-atom position. This centrosymmetric inter-pretation of the hydrogen bond is strongly supported by the bond-valence sums (BVS; Brown & Altermatt, 1985;Brese & O'Keeffe, 1991). The sum around the strongly bonded 3 -O atom is 1.92 valence units (v.u.) in the [(H 4.5 PtMo 6 O 24 ) 2 ] 7À polyanion. The reasonable BVS values of very short or very long O-H bond distances can be obtained from the graphical correlation (Brown, 2001).
However, the title compound belongs to the B-series Anderson-type polyanions (Tsigdinos, 1978) viz. [X n+ (OH) 6 MoO 18 ] (12Àn)À (X = heteroatom), in which such dimerization is impossible because all six 3 -O atoms are fully protonated. The polyanion structure in the title compound is shown in Fig. 1.

Structural commentary
This study was carried out to clearly identify the position of the seventh or extra H atom in the [HCr III (OH) 6 Mo 6 O 18 ] 2À polyanion. After considering the electron density maps and BVS values of the protonated OB (O-bridged 2 -O atom) atoms in the previously reported structure (Lee, 2007), we came to the conclusion that the positional disorder model of the H atom was wrong. The electron density (Fig. 2) is not symmetric in the title compound, but we expect the H atoms to lie in the middle of the bond because of the short OÁ Á ÁO distance of 2.461 (3) Å , which corresponds to a pseudosymmetric hydrogen bond. The description of the rest of the structure and the composition of the atoms in the polyanion are the same as in the previous report of the compound, viz. K 2 [H 7 Cr III Mo 6 O 24 ]Á8H 2 O (Lee, 2007). The O atoms of the polyoxometalate are designated as OT (terminal Mo O atom), OB, and OC (centre of two Mo and one Cr atom, 3 -O atom), respectively.
As a result, we consider that the present model of the title compound is more reasonable, and the one extra H atom is located at the mid-point between the O5B and O16B atoms, and shared equally by two discrete polyanions. All H atoms and hydrogen bonds are well-defined in the title compound (Table 1).

Supramolecular features
Two discrete polyanions A and B are linked into chains along [011] by two normal, and one strong and pseudosymmetric hydrogen bonds (Table 1 and

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All the H atoms in the polyanions and all water H atoms were positioned using difference Fourier maps. All H atoms in the polyanion were refined freely, but the H5 atoms were refined with a distance restraint of O5B-H5 and O16B iii -H5 (Table 1) using the SADI command in SHELXL97 (Sheldrick, 2008); = 0.01, the distances between the first and second named bonds were restrained to be equal with an effective standard deviation sigma in order to locate the H5 atom on the pseudocentre between the O5B and O16B atoms. The H atoms of all water molecules (OW) were refined with a distance restraint of O-H = 0.95 (3) Å using the DFIX command, and were included in the refinement with U iso (H) = 1.5U eq (O). The highest peak in the difference map is 1.62 Å from O9T. Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), PLATON (Spek, 2009) and DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Dipotassium heptahydrogen hexamolybdochromate(III) octahydrate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.58 e Å −3 Δρ min = −0.67 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00391 (16) 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. 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 > 2sigma(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.