CsGa(H_1.5AsO₄)₂(H₂AsO₄) and isotypic CsCr(H_1.5AsO₄)₂(H₂AsO₄): decorated kröhnkite-like chains in two unusual hydrogen arsenates

Hydro­thermally synthesized caesium gallium(III) hydrogen arsenate(V), CsGa(H_1.5AsO₄)₂(H₂AsO₄), (I), and isotypic caesium chromium(III) hydrogen arsenate(V), CsCr(H_1.5AsO₄)₂(H₂AsO₄), (II), represent a new structure type and stoichiometry among M^I-M^III hydrogen arsenates. The crystal structure, determined from single-crystal X-ray diffraction data, is based on an infinite octa­hedral-tetra­hedral chain and can be described as a decorated kröhnkite-like chain. The chains extend parallel to [100] and are separated by ten-coordinated Cs atoms. The hydrogen-bonding scheme comprises one very short symmetry-restricted hydrogen bond, with O...O distances of 2.519 (4) and 2.508 (4) Å in (I) and (II), respectively, and two further medium-strong hydrogen bonds, all of which reinforce the connections between adjacent chains. The average Ga-O and Cr-O bond lengths are 1.973 (15) and 1.980 (13) Å, respectively, and the average As-O bond lengths in the two protonated arsenate groups lie within a very narrow range [1.690 (18)-1.69 (3) Å]. The Cs atom is located on a centre of inversion, while the M^III and As2 atoms lie on twofold axes. Relationships to CaBa₂(HPO₄)₂(H₂PO₄)₂ and other compounds containing decorated kröhnkite-type or kröhnkite-like chains are discussed.

S1. Comment

The crystallography and topology of hydrothermally synthesized M^I—M^III hydrogen arsenates (M^I = Li, Na, K, Rb, Cs, Tl, Ag, NH₄ and H₃O; M^III = Al, Ga, In, Sc, Cr and Fe) are currently being studied in a systematic way by the authors. While our first investigations focused on alkali scandium arsenates (Schwendtner & Kolitsch, 2004a,b; Kolitsch, 2004b), more recent studies have included the remaining M^I and M^III cations. So far, these studies have yielded two novel triclinic diarsenate structure types represented by M^IInAs₂O₇ (M = Tl, Rb and NH₄) and AgScAs₂O₇, respectively, and a large number of new representatives for several of the six known structure types of the compounds with the general formula A^IM^III(HXO₄)₂ (X = P or As), as well as several new representatives for M^IM^IIIAs₂O₇-type diarsenates (Schwendtner & Kolitsch, 2005a,b,c).

The present work reports the novel chain-based crystal structures of two unusual isotypic caesium metal(III) hydrogen arsenates, CsGa(H_1.5AsO₄)₂(H₂AsO₄), (I), and CsCr(H_1.5AsO₄)₂(H₂AsO₄), (II). The asymmetric unit of the structure type contains one Cs, one M^III, two As, six O and three H atoms. The Cs atom is located on a centre of inversion, while the M^III and As2 atoms lie on twofold axes. One of the H atoms, involved in a very short hydrogen bond, is split (see discussion below).

The crystal structure contains octahedrally coordinated M^III, tetrahedrally coordinated As and ten-coordinated Cs atoms (Figs. 1–3). The average M^III—O bond lengths are 1.973 (15) Å in (I) and 1.980 (13) Å in (II). The latter value is somewhat smaller than the average Cr—O bond distance reported for six-coordinate Cr^III (1.999 Å; Baur, 1981).

The two non-equivalent AsO₄ tetrahedra are both protonated. The involved H atoms, H1, H2 and H3, all play important roles in the hydrogen-bonding scheme. Atom O4 of the As1O₄ group is bonded to atom H1 on a general position, and the O4/H1 group is involved in a medium-strong hydrogen bond (Table 2). In contrast, atom O3 is involved in a very short symmetry-restricted hydrogen bond, with an O3···O3' distance of 2.519 (4) and 2.508 (4) Å in (I) and (II), respectively. The split H atom bonded to O3 (H3) could be located experimentally only with difficulty, a situation typical for such strong hydrogen bonds (e.g. Clearfield et al., 1976; Giester, 1989; Büchner & Wickleder, 2004). The refinement agrees with observations that the H atom involved in similarly strong hydrogen bonds is invariably located on a half-occupied double-well position slightly off the centre of symmetry (e.g. Chevrier et al., 1990, 1993; Macíček et al., 1994; Thomas, 1995; Beran et al., 1997; Olovsson et al., 2001, 2002; Dörsam et al., 2003), and that there is a rapid proton transfer between the two split positions. The two symmetry-equivalent O6 atoms of the doubly protonated As2O₄ group are each bonded to an H atom (H2) on a general position, and the O6/H2 group is involved in a medium-strong hydrogen bond (Table 2), similar to that of the O4/H1 group. The site symmetry of the split H3 atom results in the unusual stoichiometry of the title compounds.

The average As—O bond distances in the two protonated arsenate groups [1.691 (20) and 1.690 (18) Å for As1, and 1.690 (24) and 1.691 (25) Å for As2, in (I) and (II), respectively] are both slightly longer than the mean length in arsenate compounds generally (1.682 Å; Baur, 1981). The slight increase can be attributed to the influence of the protonation and the distinct distortion of the tetrahedra (cf. Brown, 1981). As expected, the As—OH bonds are distinctly elongated in comparison with the As—O bonds (Table 1), as is typical of protonated AsO₄ tetrahedra (Ferraris, 1970; Ferraris & Ivaldi, 1984). The connectivity of the polyhedral units is rather complex but aesthetic. The M^IIIO₆ (M = Ga and Cr) octahedron shares each of its six vertices with protonated AsO₄ tetrahedra. The As1O₄ group acts as a bridge connecting two adjacent M^IIIO₆ (M = Ga and Cr) octahedra via the O1 and O2 ligands (which are in a cis configuration). Thus, an octahedral–tetrahedral kröhnkite-like chain along [100] is formed (see more detailed discussion below). This chain is decorated by the As2O₄ group, which shares two of its ligands (two O5 atoms) with the M^IIIO₆ (M = Ga and Cr) octahedron. The very strong hydrogen bond involving atom H3 (see above), as well as the other two, medium-strong, hydrogen bonds, all provide a direct connection between adjacent chains (Fig. 2).

The Cs atom is located on the origin and has ten O ligands within 3.4 Å [the 11t h and 12t h O neighbors are both at a distance of 3.9094 (19) Å in (I) and 3.921 (2) Å in (II)]. The CsO₁₀ polyehedron closely resembles a flattened pentagonal antiprism, and is characterized by average Cs—O bond lengths of 3.25 (10) and 3.25 (11) Å in (I) and (II), respectively.

Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O'Keeffe (1991). The values obtained are as follows [values for (I) given first]: 1.09/1.11 (Cs), 3.11/3.00 (Ga/Cr), 4.92/4.93 (As1), 4.93/4.93 (As2), 1.97/1.95 (O1), 1.76/1.74 (O2), 1.30/1.30 (O3 = O ligand involved in very short O3···O3' hydrogen bond donated by the H_1.5AsO₄ group), 1.26/1.27 (O4 = `normal' OH of H_1.5AsO₄ group), 1.91/1.89 (O5), 1.30/1.30 (O6 = OH of H₂AsO₄ group) valence units (v.u.). These bond-valence sums are all reasonably close to expected ideal valencies, and confirm that atoms O3, O4 and O6 represent hydroxy groups. The most underbonded O atom (O2) is the acceptor of one of the two medium-strong hydrogen bonds; the acceptor of the remaining hydrogen bond is O3 (Table 2).

The infinite octahedral–tetrahedral chain in CsM^III(H_1.5AsO₄)₂(H₂AsO₄) (M^III = Ga and Cr) may be considered a decorated variant of the widespread kröhnkite-type chain, which is built from MO₆ octahedra corner-linked to bridging XO₄ tetrahedra. This chain is named after kröhnkite, Na₂Cu^II(S^VIO₄)₂·2H₂O (Dahlman, 1952; Hawthorne & Ferguson, 1975), and is representative of a larger number of compounds containing infinite chains [M(XO₄)₂(H₂O)₂], where M is divalent (Mg, Mn, Fe, Co, Ni, Cu, Zn and Cd) or trivalent (Al, Fe, In and Tl), and where X is pentavalent (P and As) or hexavalent (S, Se, Cr, Mo and W) (Fleck et al., 2002; Fleck & Kolitsch, 2003; Kolitsch & Fleck, 2005a,b).

While kröhnkite-type chains are not uncommon, decorated versions of either these chains or closely related chains (kröhnkite-like chains according to the classification of Fleck et al., 2002) are rare. Among the few compounds containing such decorated chains are (NH₄)₄Cd(HSe^IVO₃)₂(Se^VIO₄)₂ (Kolitsch, 2004a), which features a kröhnkite-like infinite chain built of CdO₆ octahedra and pyramidal HSeO₃ groups, and decorated by Se^VIO₄ tetrahedra. In Ba₂Ca(HPO₄)₂(H₂PO₄)₂ (Toumi et al., 1997), CaO₆ octahedra and H₂PO₄ tetrahedra are corner-linked to form kröhnkite-like infinite chains, which are decorated by additional H₂PO₄ tetrahedra. Both Rb₅Er(MoO₄)₄ (Klevtsova & Glinskaya, 1976; note: atomic coordinates are not included in the ICSD) and isotypic Rb₅In(MoO₄)₄ (Tillmanns et al., 2005) contain very similar, but slightly twisted, decorated kröhnkite-like chains based on MO₆ (M = Er and In) octahedra and MoO₄ tetrahedra.

S2. Experimental

Large colorless glassy prisms of (I) were prepared hydrothermally (493 K, 7 d) in a Teflon-lined stainless steel autoclave from a mixture of Cs₂CO₃, Ga₂O₃ (approximate molar ratio Cs:Ga of 1:1), arsenic acid and distilled water. Enough arsenic acid was added to keep the pH between about 1.5 and 0.5. The Teflon cylinders were filled with distilled water up to approxomately 80% of their inner volume. The initial and final pH values were about 1.5 and 1, respectively. The prisms were accompanied by a small amount (about 20 vol%) of very small hexagonal colorless platelets of CsGa(HAsO₄)₂ (novel structure type, space group R32; Schwendtner & Kolitsch, 2005b, 2005c). Tiny green transparent prisms of (II) were prepared hydrothermally (493 K, 7 d) in a Teflon-lined stainless steel autoclave from a mixture of Cs₂CO₃, Cr₂O₃ (approximate molar ratio Cs:Cr of 1:1), arsenic acid and distilled water. Enough arsenic acid was added to keep the pH between about 1.5 and 0.5. The Teflon cylinders were filled with distilled water up to approxomately 80% of their inner volume. Initial and final pH values were 1 and 1/2, respectively. The prisms were accompanied by about 30 vol% of an uninvestigated green material (possibly unreacted Cr₂O₃).

S3. Refinement

The split hydrogen atom, H3, which is involved in the very short hydrogen bond (O3···O3' = about 2.5 Å), could be located experimentally in both (I) and (II) with the help of difference Fourier maps calculated by PLATON (Spek, 2003). It was refined with U_iso(H) fixed to 0.04 Ų and the O3—H3 bond length restrained to 0.90 (5) Å. The refined site coordinates of atom H3 in (I) and (II) are distinctly off the centre of symmetry at (3/4, 1/4, 0), which links atoms O3 and O3'. The H3—H3' distance is approximately 0.99 Å in (I) and 0.86 Å in (II). The remaining two H atoms in each arsenate were refined with O—H bond lengths restrained to 0.90 (5) Å. The highest electron-density peak in (I) is 1.05 Å from the As1 site.

Computing details

For both compounds, data collection: COLLECT (Nonius, 2004); cell refinement: HKL SCALEPACK (Otwinowski et al., 2003); data reduction: HKL DENZO (Otwinowski et al., 2003) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Diamond (Brandenburg, 2005); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The crystal structure of (I), in a view along [100], in the direction of the infinite decorated kröhnkite-like chains. CdO6 octahedra are corner-linked to two non-equivalent protonated AsO4 tetrahedra. The Cs atoms (shown as spheres) separate the chains. Note the very short symmetry-restricted hydrogen bond involving the split H3 atom.
	Fig. 2. Connectivity in (I), shown with displacement ellipsoids of the non-H atoms at the 70% probability level. [Symmetry codes: (i) −x, y, 1/2 − z; (ii) 1 − x, y, 1/2 − z; (viii) x − 1, y, z.]
	Fig. 3. A polyhedral view of the decorated kröhnkite-like chain in (I). The left-most As2O4 tetrahedron has been omitted to show the chain more clearly.

(I) caesium gallium(III) hydrogen arsenate(V)

Crystal data

CsGa(H1.5AsO4)2(H2AsO4)	F(000) = 1144
Mr = 624.43	Dx = 3.961 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2378 reflections
a = 4.714 (1) Å	θ = 2.0–35.0°
b = 14.674 (3) Å	µ = 15.52 mm^−1
c = 15.162 (3) Å	T = 293 K
β = 93.31 (3)°	Fragment, colorless
V = 1047.1 (4) Å^3	0.15 × 0.10 × 0.03 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	2309 independent reflections
Radiation source: fine-focus sealed tube	2074 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.019
ϕ and ω scans	θmax = 35.0°, θmin = 2.7°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski et al., 2003)	h = −7→7
Tmin = 0.204, Tmax = 0.653	k = −23→23
4542 measured reflections	l = −24→24

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.066	w = 1/[σ^2(Fo^2) + (0.039P)^2 + 2.03P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
2309 reflections	Δρmax = 1.29 e Å^−3
92 parameters	Δρmin = −0.98 e Å^−3
3 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00299 (19)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cs	0.0000	0.0000	0.0000	0.02798 (8)
Ga	0.0000	0.14299 (2)	0.2500	0.00809 (7)
As1	0.47349 (4)	0.216622 (14)	0.122816 (13)	0.00887 (7)
O1	0.2208 (3)	0.14131 (11)	0.14451 (10)	0.0109 (3)
O2	0.7462 (3)	0.23884 (11)	0.19596 (10)	0.0113 (3)
O3	0.5923 (4)	0.18632 (14)	0.02371 (11)	0.0183 (3)
H3	0.683 (17)	0.237 (4)	0.018 (6)	0.040*	0.50
O4	0.3264 (4)	0.32322 (12)	0.10974 (14)	0.0211 (3)
H1	0.181 (9)	0.320 (4)	0.078 (3)	0.060 (16)*
As2	0.5000	−0.00664 (2)	0.2500	0.00895 (7)
O5	0.2566 (3)	0.05000 (11)	0.30401 (10)	0.0118 (3)
O6	0.3393 (4)	−0.07491 (13)	0.17038 (12)	0.0192 (3)
H2	0.327 (12)	−0.129 (3)	0.191 (4)	0.068 (17)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.04361 (16)	0.01845 (12)	0.02089 (12)	−0.00286 (9)	−0.00660 (10)	−0.00133 (7)
Ga	0.00617 (12)	0.00871 (14)	0.00941 (13)	0.000	0.00056 (9)	0.000
As1	0.00691 (9)	0.00975 (10)	0.00987 (10)	−0.00154 (6)	−0.00036 (6)	0.00110 (6)
O1	0.0085 (5)	0.0124 (6)	0.0121 (6)	−0.0043 (5)	0.0024 (5)	−0.0009 (5)
O2	0.0081 (5)	0.0117 (6)	0.0134 (6)	−0.0003 (5)	−0.0043 (5)	0.0001 (5)
O3	0.0172 (7)	0.0272 (9)	0.0108 (7)	−0.0077 (7)	0.0046 (6)	−0.0027 (6)
O4	0.0168 (7)	0.0124 (7)	0.0332 (10)	0.0009 (6)	−0.0057 (7)	0.0047 (7)
As2	0.00677 (12)	0.00763 (12)	0.01239 (14)	0.000	0.00015 (9)	0.000
O5	0.0086 (6)	0.0131 (6)	0.0138 (6)	0.0038 (5)	0.0020 (5)	0.0019 (5)
O6	0.0200 (7)	0.0146 (8)	0.0224 (8)	−0.0047 (6)	−0.0024 (6)	−0.0062 (6)

Geometric parameters (Å, º)

As1—O1	1.6714 (15)	Ga—O5	1.9700 (16)
As1—O2	1.6804 (16)	Ga—O2^i	1.9920 (16)
As1—O3	1.6930 (17)	Ga—O2^iii	1.9920 (16)
As1—O4	1.7179 (19)	Cs—O1^iv	3.1496 (16)
O3—H3	0.87 (4)	Cs—O1	3.1496 (16)
O4—H1	0.82 (4)	Cs—O6	3.156 (2)
As2—O5	1.6693 (15)	Cs—O6^iv	3.156 (2)
As2—O5^i	1.6693 (15)	Cs—O4^v	3.213 (2)
As2—O6	1.7115 (18)	Cs—O4^vi	3.213 (2)
As2—O6^i	1.7115 (18)	Cs—O5^vii	3.3536 (17)
O6—H2	0.86 (4)	Cs—O5^ii	3.3536 (17)
Ga—O1	1.9582 (16)	Cs—O3^viii	3.373 (2)
Ga—O1^ii	1.9582 (16)	Cs—O3^iii	3.373 (2)
Ga—O5^ii	1.9700 (16)
O1^iv—Cs—O1	180.00 (6)	O6—Cs—O3^iii	117.38 (5)
O1^iv—Cs—O6	118.14 (5)	O6^iv—Cs—O3^iii	62.62 (5)
O1—Cs—O6	61.86 (5)	O4^v—Cs—O3^iii	115.81 (5)
O1^iv—Cs—O6^iv	61.86 (5)	O4^vi—Cs—O3^iii	64.19 (5)
O1—Cs—O6^iv	118.14 (5)	O5^vii—Cs—O3^iii	120.57 (4)
O6—Cs—O6^iv	180.00 (8)	O5^ii—Cs—O3^iii	59.43 (4)
O1^iv—Cs—O4^v	75.22 (5)	O3^viii—Cs—O3^iii	180.00 (8)
O1—Cs—O4^v	104.78 (5)	O1—Ga—O1^ii	178.55 (9)
O6—Cs—O4^v	55.06 (5)	O1—Ga—O5^ii	89.67 (6)
O6^iv—Cs—O4^v	124.94 (5)	O1^ii—Ga—O5^ii	89.33 (7)
O1^iv—Cs—O4^vi	104.78 (5)	O1—Ga—O5	89.33 (7)
O1—Cs—O4^vi	75.22 (5)	O1^ii—Ga—O5	89.67 (6)
O6—Cs—O4^vi	124.94 (5)	O5^ii—Ga—O5	92.32 (10)
O6^iv—Cs—O4^vi	55.06 (5)	O1—Ga—O2^i	90.57 (6)
O4^v—Cs—O4^vi	180.00 (8)	O1^ii—Ga—O2^i	90.45 (7)
O1^iv—Cs—O5^vii	50.30 (4)	O5^ii—Ga—O2^i	178.90 (7)
O1—Cs—O5^vii	129.70 (4)	O5—Ga—O2^i	88.75 (7)
O6—Cs—O5^vii	117.22 (5)	O1—Ga—O2^iii	90.45 (7)
O6^iv—Cs—O5^vii	62.78 (5)	O1^ii—Ga—O2^iii	90.57 (6)
O4^v—Cs—O5^vii	113.45 (5)	O5^ii—Ga—O2^iii	88.75 (7)
O4^vi—Cs—O5^vii	66.55 (5)	O5—Ga—O2^iii	178.90 (7)
O1^iv—Cs—O5^ii	129.70 (4)	O2^i—Ga—O2^iii	90.17 (9)
O1—Cs—O5^ii	50.30 (4)	O1—As1—O2	121.68 (8)
O6—Cs—O5^ii	62.78 (5)	O1—As1—O3	106.08 (8)
O6^iv—Cs—O5^ii	117.22 (5)	O2—As1—O3	110.92 (8)
O4^v—Cs—O5^ii	66.55 (5)	O1—As1—O4	109.70 (9)
O4^vi—Cs—O5^ii	113.45 (5)	O2—As1—O4	100.83 (9)
O5^vii—Cs—O5^ii	180.00 (5)	O3—As1—O4	106.78 (10)
O1^iv—Cs—O3^viii	63.88 (5)	As1—O3—H3	93 (6)
O1—Cs—O3^viii	116.12 (5)	As1—O4—H1	110 (4)
O6—Cs—O3^viii	62.62 (5)	O5—As2—O5^i	120.28 (11)
O6^iv—Cs—O3^viii	117.38 (5)	O5—As2—O6	110.40 (9)
O4^v—Cs—O3^viii	64.19 (5)	O5^i—As2—O6	103.55 (8)
O4^vi—Cs—O3^viii	115.81 (5)	O5—As2—O6^i	103.55 (8)
O5^vii—Cs—O3^viii	59.43 (4)	O5^i—As2—O6^i	110.40 (9)
O5^ii—Cs—O3^viii	120.57 (4)	O6—As2—O6^i	108.35 (14)
O1^iv—Cs—O3^iii	116.12 (5)	As2—O6—H2	109 (4)
O1—Cs—O3^iii	63.88 (5)

Symmetry codes: (i) −x+1, y, −z+1/2; (ii) −x, y, −z+1/2; (iii) x−1, y, z; (iv) −x, −y, −z; (v) x−1/2, y−1/2, z; (vi) −x+1/2, −y+1/2, −z; (vii) x, −y, z−1/2; (viii) −x+1, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O3^ix	0.87 (4)	1.69 (5)	2.519 (4)	158 (8)
O4—H1···O3^vi	0.82 (4)	1.95 (4)	2.747 (3)	164 (5)
O6—H2···O2^v	0.86 (4)	1.98 (4)	2.799 (2)	160 (6)

Symmetry codes: (v) x−1/2, y−1/2, z; (vi) −x+1/2, −y+1/2, −z; (ix) −x+3/2, −y+1/2, −z.

(II) caesium chromium(III) hydrogen arsenate(V)

Crystal data

CsCr(H1.5AsO4)2(H2AsO4)	F(000) = 1116
Mr = 606.71	Dx = 3.847 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1973 reflections
a = 4.744 (1) Å	θ = 2.0–32.6°
b = 14.625 (3) Å	µ = 13.98 mm^−1
c = 15.127 (3) Å	T = 293 K
β = 93.48 (3)°	Prism, pale green
V = 1047.6 (4) Å^3	0.08 × 0.02 × 0.02 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	1918 independent reflections
Radiation source: fine-focus sealed tube	1593 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
ϕ and ω scans	θmax = 32.6°, θmin = 2.7°
Absorption correction: multi-scan HKL SCALEPACK (Otwinowski et al., 2003)	h = −7→7
Tmin = 0.401, Tmax = 0.767	k = −21→22
3740 measured reflections	l = −22→22

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.053	w = 1/[σ^2(Fo^2) + (0.023P)^2 + 2.08P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
1918 reflections	Δρmax = 0.98 e Å^−3
89 parameters	Δρmin = −0.80 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00049 (9)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cs	0.0000	0.0000	0.0000	0.02773 (8)
Cr	0.0000	0.14373 (4)	0.2500	0.00847 (11)
As1	0.47360 (5)	0.216663 (18)	0.122801 (17)	0.01003 (7)
O1	0.2226 (4)	0.14100 (13)	0.14451 (12)	0.0124 (3)
O2	0.7443 (4)	0.23967 (12)	0.19633 (12)	0.0119 (3)
O3	0.5927 (4)	0.18680 (15)	0.02376 (14)	0.0189 (4)
H3	0.684 (18)	0.234 (5)	0.011 (7)	0.040*	0.50
O4	0.3259 (5)	0.32334 (13)	0.11011 (16)	0.0208 (4)
H1	0.192 (9)	0.324 (3)	0.073 (3)	0.056 (15)*
As2	0.5000	−0.00576 (2)	0.2500	0.01002 (8)
O5	0.2585 (4)	0.05068 (12)	0.30442 (12)	0.0128 (4)
O6	0.3402 (4)	−0.07461 (14)	0.17022 (15)	0.0200 (4)
H2	0.307 (11)	−0.122 (3)	0.189 (3)	0.072 (18)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.04241 (18)	0.01932 (14)	0.02050 (14)	−0.00307 (11)	−0.00607 (12)	−0.00064 (9)
Cr	0.0075 (2)	0.0088 (2)	0.0091 (2)	0.000	0.00074 (18)	0.000
As1	0.00869 (11)	0.01110 (13)	0.01023 (12)	−0.00145 (8)	−0.00009 (8)	0.00107 (9)
O1	0.0103 (8)	0.0143 (9)	0.0126 (8)	−0.0031 (6)	0.0022 (6)	−0.0010 (7)
O2	0.0095 (8)	0.0114 (8)	0.0144 (9)	−0.0004 (6)	−0.0026 (7)	0.0008 (7)
O3	0.0177 (10)	0.0273 (11)	0.0123 (9)	−0.0061 (8)	0.0055 (7)	−0.0036 (8)
O4	0.0184 (10)	0.0138 (9)	0.0292 (12)	0.0027 (7)	−0.0062 (9)	0.0044 (8)
As2	0.00859 (16)	0.00897 (16)	0.01245 (17)	0.000	0.00023 (12)	0.000
O5	0.0106 (8)	0.0133 (9)	0.0148 (9)	0.0030 (6)	0.0029 (7)	0.0010 (7)
O6	0.0222 (10)	0.0149 (10)	0.0222 (11)	−0.0054 (8)	−0.0037 (8)	−0.0033 (8)

Geometric parameters (Å, º)

As1—O1	1.6727 (18)	Cr—O5	1.9776 (18)
As1—O2	1.6811 (18)	Cr—O2^i	1.9950 (18)
As1—O3	1.690 (2)	Cr—O2^iii	1.9950 (18)
As1—O4	1.716 (2)	Cs—O1	3.1409 (19)
O3—H3	0.84 (4)	Cs—O1^iv	3.1409 (19)
O4—H1	0.82 (4)	Cs—O6^iv	3.150 (3)
As2—O5	1.6692 (18)	Cs—O6	3.150 (3)
As2—O5^i	1.6692 (18)	Cs—O4^v	3.209 (2)
As2—O6	1.713 (2)	Cs—O4^vi	3.209 (2)
As2—O6^i	1.713 (2)	Cs—O5^ii	3.354 (2)
O6—H2	0.76 (4)	Cs—O5^vii	3.354 (2)
Cr—O1	1.9667 (19)	Cs—O3^iii	3.378 (2)
Cr—O1^ii	1.9667 (19)	Cs—O3^viii	3.378 (2)
Cr—O5^ii	1.9776 (18)
O1—Cs—O1^iv	180.00 (7)	O6^iv—Cs—O3^viii	117.47 (5)
O1—Cs—O6^iv	118.39 (5)	O6—Cs—O3^viii	62.53 (5)
O1^iv—Cs—O6^iv	61.61 (5)	O4^v—Cs—O3^viii	115.41 (5)
O1—Cs—O6	61.61 (5)	O4^vi—Cs—O3^viii	64.59 (5)
O1^iv—Cs—O6	118.39 (5)	O5^ii—Cs—O3^viii	120.98 (5)
O6^iv—Cs—O6	180.00 (9)	O5^vii—Cs—O3^viii	59.02 (5)
O1—Cs—O4^v	75.31 (6)	O3^iii—Cs—O3^viii	180.00 (7)
O1^iv—Cs—O4^v	104.69 (6)	O1—Cr—O1^ii	177.67 (11)
O6^iv—Cs—O4^v	55.39 (5)	O1—Cr—O5^ii	89.89 (7)
O6—Cs—O4^v	124.61 (5)	O1^ii—Cr—O5^ii	88.50 (8)
O1—Cs—O4^vi	104.69 (6)	O1—Cr—O5	88.50 (8)
O1^iv—Cs—O4^vi	75.31 (6)	O1^ii—Cr—O5	89.89 (7)
O6^iv—Cs—O4^vi	124.61 (5)	O5^ii—Cr—O5	93.03 (11)
O6—Cs—O4^vi	55.39 (5)	O1—Cr—O2^i	90.08 (8)
O4^v—Cs—O4^vi	180.00 (4)	O1^ii—Cr—O2^i	91.56 (8)
O1—Cs—O5^ii	50.69 (4)	O5^ii—Cr—O2^i	178.79 (8)
O1^iv—Cs—O5^ii	129.31 (4)	O5—Cr—O2^i	88.18 (7)
O6^iv—Cs—O5^ii	116.61 (5)	O1—Cr—O2^iii	91.56 (8)
O6—Cs—O5^ii	63.39 (5)	O1^ii—Cr—O2^iii	90.08 (8)
O4^v—Cs—O5^ii	113.54 (5)	O5^ii—Cr—O2^iii	88.18 (7)
O4^vi—Cs—O5^ii	66.46 (5)	O5—Cr—O2^iii	178.79 (8)
O1—Cs—O5^vii	129.31 (4)	O2^i—Cr—O2^iii	90.61 (10)
O1^iv—Cs—O5^vii	50.69 (4)	O1—As1—O2	121.80 (9)
O6^iv—Cs—O5^vii	63.39 (5)	O1—As1—O3	106.46 (10)
O6—Cs—O5^vii	116.61 (5)	O2—As1—O3	110.80 (10)
O4^v—Cs—O5^vii	66.46 (5)	O1—As1—O4	109.44 (10)
O4^vi—Cs—O5^vii	113.54 (5)	O2—As1—O4	100.50 (10)
O5^ii—Cs—O5^vii	180.00 (5)	O3—As1—O4	106.97 (11)
O1—Cs—O3^iii	64.23 (5)	As1—O3—H3	101 (7)
O1^iv—Cs—O3^iii	115.77 (5)	As1—O4—H1	112 (4)
O6^iv—Cs—O3^iii	62.53 (5)	O5^i—As2—O5	120.73 (13)
O6—Cs—O3^iii	117.47 (5)	O5^i—As2—O6	103.35 (10)
O4^v—Cs—O3^iii	64.59 (5)	O5—As2—O6	110.52 (10)
O4^vi—Cs—O3^iii	115.41 (5)	O5^i—As2—O6^i	110.52 (10)
O5^ii—Cs—O3^iii	59.02 (5)	O5—As2—O6^i	103.35 (10)
O5^vii—Cs—O3^iii	120.98 (5)	O6—As2—O6^i	107.99 (16)
O1—Cs—O3^viii	115.77 (5)	As2—O6—H2	112 (4)
O1^iv—Cs—O3^viii	64.23 (5)

Symmetry codes: (i) −x+1, y, −z+1/2; (ii) −x, y, −z+1/2; (iii) x−1, y, z; (iv) −x, −y, −z; (v) −x+1/2, −y+1/2, −z; (vi) x−1/2, y−1/2, z; (vii) x, −y, z−1/2; (viii) −x+1, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O3^ix	0.84 (4)	1.68 (4)	2.508 (4)	169 (10)
O4—H1···O3^v	0.82 (4)	1.94 (4)	2.753 (3)	172 (5)
O6—H2···O2^vi	0.76 (4)	2.05 (4)	2.786 (3)	161 (5)

Symmetry codes: (v) −x+1/2, −y+1/2, −z; (vi) x−1/2, y−1/2, z; (ix) −x+3/2, −y+1/2, −z.