Ammonium diphosphitoindate(III)

The crystal structure of the title compound, NH4[In(HPO3)2], is built up from InIII cations (site symmetry 3m.) adopting an octahedral environment and two different phosphite anions (each with site symmetry 3m.) exhibiting a triangular–pyramidal geometry. Each InO6 octahedron shares its six apices with hydrogen phosphite groups. Reciprocally, each HPO3 group shares all its O atoms with three different metal cations, leading to [In(HPO3)2]− layers which propagate in the ab plane. The ammonium cation likewise has site symmetry 3m.. In the structure, the cations are located between the [In(HPO3)2]− layers of the host framework. The sheets are held together by hydrogen bonds formed between the NH4 + cations and the O atoms of the framework.

The crystal structure of the title compound, NH 4 [In(HPO 3 ) 2 ], is built up from In III cations (site symmetry 3m.) adopting an octahedral environment and two different phosphite anions (each with site symmetry 3m.) exhibiting a triangularpyramidal geometry. Each InO 6 octahedron shares its six apices with hydrogen phosphite groups. Reciprocally, each HPO 3 group shares all its O atoms with three different metal cations, leading to [In(HPO 3 ) 2 ] À layers which propagate in the ab plane. The ammonium cation likewise has site symmetry 3m.. In the structure, the cations are located between the [In(HPO 3 ) 2 ] À layers of the host framework. The sheets are held together by hydrogen bonds formed between the NH 4 + cations and the O atoms of the framework.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: RU2050).
Consequently the literature is currently dominated by reports of organically template phosphite frame-works (Natarajan & Mandal, 2008). Purely inorganic phosphite structures have also been evidenced with magnetic and non-magnetic cations (Marcos et al., 1993, Morris et al., 1994, Orive et al., 2011 (Hamchaoui et al., 2013). The structural model is also related to the yavapaiite aluns type (Graeber & Rosenzweig, 1971) and the mixed selenite-selenate [(RbFe(SeO 4 )(SeO 3 )] (Giester, 2000). As shown in Fig. 1, the asymmetric unit contains one crystallographic independant In III cation and two ones for both phosphorus and oxygen atoms. Six oxygen atoms define an octahedral geometry around the metallic center while three oxygen atoms and one hydrogen atom define the triangular pyramidal environment of the phosphorus atom. The quaternary ammonium ions are displayed between the [M(HPO 3 ) 2 ]layers of the host framework (Fig. 2). They exhibit N-H bond distances in the range usually found for this cation and the angles are similar to those expected for sp 3 hybridization. Thus for 1 the sheets are held together by hydrogen bonds formed between and the oxygen atoms of the framework. This H-bonding arrangement is illustrated in Fig. 3. It shows that the ammonium ion is firmly fixed in the structure by means of nine N-H···O hydrogen bonds, which prevent free ammonium-ion rotation at 298 K. The ammonium cations are located at the center of the six-ring windows of the upper layer.

Experimental
The title compound was prepared under mild hydrothermal conditions and autogenous pressure. The starting reagents were InCl 3 (Sigma-Aldrich, 98%), H 3 PO 3 (Aldrich, 99%), (NH 4 ) 2 CO 3 (Fluka, 30-33% of NH 3 ) and deionized water in a 2:15:4:280 molar ratio. The mixture was placed in a 23 ml Teflon-lined steel autoclave, heated at 453 K for 72 h and followed by slow cooling to room temperature. Well formed colorless crystals were recovered by vacuum filtration, washed with deionized water and dried in a desiccator.

Refinement
Part of the H atoms was localized from a difference Fourier map (HP2 and HN1) others were placed in calculated position according to geometrical constraints. Hydrogen atom positions of the ammonium cation were refined with their N-H and H-H distances restrained to one common refined value (0.87 Å and 1.33 Å respectively)

Ammonium diphosphitoindate(III)
Crystal data  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.003 Δρ max = 0.51 e Å −3 Δρ min = −1.39 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.092 (4) Absolute structure: Flack (1983), 459 Friedel pairs Flack parameter: −0.01 (2) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq