The anhydrous alkaline earth metal chlorites, Ca(ClO2)2, Sr(ClO2)2 and Ba(ClO2)2, were first investigated crystallographically by Riganti & Garrini (1960). Because these compounds were obtained as microcrystalline powders, only powder diffraction patterns were obtained, which were not indexed. These authors proposed that Ca(ClO2)2 and Sr(ClO2)2 are isomorphic and have pseudo-cubic cells with similar parameters. However, the complete structures of all anhydrous alkaline- arth metal chlorites are still unknown. They also reported that barium chlorite may form a hydrate with 3.5 H2O molecules (unlike Ca and Sr chlorites), but its crystal structure was not determined because of its very low stability.
The purpose of the present investigation was to obtain structural data for the title hydrate. As the crystal structures of only some chlorites have been determined to date, a new contribution to the crystal chemistry of these simple compounds seemed useful. The present paper continues our research on chlorous acid salts, which has included LiClO2 and KClO2 (Smolentsev & Naumov, 2005a) and the redetermined structure of NH4ClO2 (Smolentsev & Naumov, 2005b).
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In the structure of Ba(ClO2)2·3.5H2O, the coordination sphere of the Ba2+ cation includes ten O atoms (Fig. 1). Seven of these belong to the chlorite anions and the other three to the H2O molecules. The resultant coordination is a distorted bicapped square antiprism. By sharing edges, the antiprisms form layers parallel to the bc plane (Fig. 2). The layers are stacked in such way that adjacent layers are shifted along the b axis by 1/2 a unit translation.
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The structure contains both H2O molecules coordinated to the Ba2+ cations and solvate water molecules, which are involved only in hydrogen bonding of the layers. Non-coordinated water of crystallization has not been found in other hydrated chlorites studied so far, viz. NaClO2·3H2O (Tarimci & Schempp, 1975), Mg(ClO2)2·6H2O (Okuda et al., 1990; Marsh, 1991), Zn(ClO2)2·2H2O (Pakkanen, 1979) and La(ClO2)3·3H2O (Castellani Bisi, 1984), in which all H2O molecules are coordinated to the metal cations. In this regard, Mg(ClO2)2·6H2O is outstanding: the water molecules form the complex cation [Mg(H2O)6]2+, which is then hydrogen bonded to ClO2− anions. A layered structure is found in Zn(ClO2)2·2H2O, while the sodium salt contains chains, and both magnesium and lanthanum salts have three-dimensional frameworks. Unlike these other hydrated chlorites, the title compound has one more feature, i.e. two independent ClO2− groups with symmetry non-equivalent O atoms. Anions of the first type (O11—Cl1—O12) serve as tetradentate bridging ligands between the Ba2+ cations and are not included in the hydrogen-bond system. The O atoms of the second anion type (O21—Cl2—O22) have different environments. They are both coordinated to the same Ba2+ cation and involved in hydrogen bonding. However, atom O21 is also coordinated to the neighbouring cation and forms a hydrogen bond with only one coordinated H2O molecule within the layer, while atom O22 is hydrogen-bonded to two interlayer H2O molecules.
Due to the non-equivalence of the ClO2− anions, their geometric parameters are slightly different. Nevertheless, they are in good agreement with the values determined for other chlorites. Comparison can be made with the average Cl—O distance and O—Cl—O angle calculated from the corresponding values reported previously (Tarimci & Schempp, 1975; Tarimci et al., 1976; Pakkanen, 1979; Castellani Bisi, 1984; Okuda et al., 1990; Marsh, 1991; Smolentsev & Naumov, 2005a,b), which are 1.571 (14) Å and 110 (2)°, respectively.
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