RbCr^III(C₂O₄)₂·2H₂O, Cs₂Mg(C₂O₄)₂·4H₂O and Rb₂Cu^II(C₂O₄)₂·2H₂O: three new complex oxalate hydrates

Rubidium chromium(III) dioxalate dihydrate [di­aqua­bis([mu]-oxalato)­chromium(III)­rubidium(I)], [RbCr(C₂O₄)₂(H₂O)₂], (I), and dicaesium magnesium dioxalate tetrahydrate [tetra­aqua­bis([mu]-oxalato)­magnesium(II)­dicaesium(I)], [Cs₂Mg(C₂­O₄)₂(H₂O)₄], (II), have layered structures which are new among double-metal oxalates. In (I), the Rb and Cr atoms lie on sites with imposed 2/m symmetry and the unique water molecule lies on a mirror plane; in (II), the Mg atom lies on a twofold axis. The two non-equivalent Cr and Mg atoms both show octahedral coordination, with a mean Cr-O distance of 1.966 Å and a mean Mg-O distance of 2.066 Å. Dirubid­ium copper(II) dioxalate dihydrate [di­aqua­bis([mu]-oxalato)­copper(II)­dirubidium(I)], [Rb₂Cu(C₂O₄)₂(H₂O)₂], (III), is also layered and is isotypic with the previously described K₂- and (NH₄)₂Cu^II(C₂O₄)₂·2H₂O compounds. The two non-equivalent Cu atoms lie on inversion centres and are both (4+2)-coordinated. Hydro­gen bonds are medium-strong to weak in the three compounds. The oxalate groups are slightly non-planar only in the Cs-Mg compound, (II), and are more distinctly non-planar in the K-Cu compound, (III).

S1. Comment

As part of recent work on the crystal chemistry and topology of, firstly, complex metal oxalates (Fleck & Kolitsch, 2004) and, secondly, bismuth(III) oxalates (Kolitsch, 2003), the three title compounds have been prepared from aqueous solutions at room temperature, and their crystal structures have been determined from single-crystal X-ray data collected at 293 K. Two of the three oxalates, (I) and (II), represent novel structure types. \sch

Compound (I) has a layered atomic arrangement with space-group symmetry C2/m. The asymmetric unit contains one unique Rb atom, one unique C, three O atoms and one H atom. Layers of Rb atoms parallel to the (001) plane are connected to parallel layers of CrO₄(H₂O)₂ octahedra via tetradentate bridging oxalate groups approximately along [102] (Figs. 1 and 2). The layered character of the structure explains the tabular (001) habit of the crystals. The Rb atoms are coordinated to eight O3 atoms in a very narrow distance range (Table 1), with a mean Rb—O distance of 3.017 Å. The RbO₈ polyhedron may be described as a rather distorted tetragonal prism. The CrO₄(H₂O)₂ octahedron has four ligands from tetradentate oxalate groups and two apical trans ligands which are water molecules (OW2). The CrO₄(H₂O)₂ octahedron is only slightly distorted (Table 1) and has a mean Cr—O distance of 1.966 Å; the bonds to the water molecules are slightly longer by comparison. In the only other Rb—Cr^III oxalate known, Rb₃Cr^III(C₂O₄)₃·3H₂O (Van Niekerk & Schoening, 1952a; Merrachi et al., 1987), which is isotypic with its K analogue (Van Niekerk & Schoening, 1952b; Taylor, 1978), the Cr atom also has an octahedral coordination.

Both mean Rb—O and mean Cr—O distances are close to expected values, although the Cr—O value is slightly smaller than the average value in oxidic Cr compounds (1.999 Å; Baur, 1981). This may be explained by the influence of the strong oxalate C—O bonds, which force the Cr atom to keep its four oxalate O ligands at a close distance. The oxalate group is planar and shows expected bond lengths (Table 1). The single hydrogen bond in (I) is medium strong, with OW2···O1 2.6514 (13) Å (Table 2, Fig. 1). It is directed approximately along [492] and [492] to the O1 ligands of adjacent CrO₄(H₂O)₂ octahedra, and thus provides a strong linkage within the `octahedral' layer.

Compound (I) is not isostructural with KCr^III(C₂O₄)₂·3H₂O, which has space group P2/c and one more water molecule per formula unit (Van Niekerk & Schoening, 1951), although CrO₄(H₂O)₂ octahedra exist in both compounds. The Cr—OW bonds in the K—Cr compound are longer than the Cr—O bonds, as in the Rb—Cr compound.

Compound (II) represents a second new structure type, which also has a layered character, although the layering is less distinct. The compound crystallizes in space group C2/c. The asymmetric unit contains one unique Cs atom, one Mg, two C, six O atoms and four H atoms. Only the Mg atom is located on a special position (0, y, 1/4). Slightly distorted MgO₄(H₂O)₂ octahedra are located in layers parallel to (101), which are separated by somewhat corrugated layers of Cs atoms (Figs. 3 and 4). The MgO₄(H₂O)₂ octahedron has two cis ligands which are water molecules (OW5), and four cis ligands from bidentate oxalate groups. The unusual cis arrangements of the different ligands are probably responsible for the slight distortion of the MgO₄(H₂O)₂ octahedron (Table 3, Fig. 4). The average Mg—O bond length is 2.066 Å, which is slightly smaller than the average value in oxidic Mg compounds (2.085 Å; Baur, 1981). The Cs atom has a geometrically irregular but otherwise clearly defined coordination sphere involving nine O atoms (mean Cs—O 3.303 Å), two of which belong to water molecules (Table 3). Atom OW6 is only bonded to Cs, while the less strongly bonded atom OW5 is shared with the Mg atom. The oxalate group is nearly planar and has expected bond geometries (Table 3). Hydrogen bonding in (I) is provided by water molecules OW5 and OW6 bonded to the Mg and Cs atoms. The four hydrogen bonds are medium-strong to weak, with O···O distances of between 2.684 (2) and 2.760 (2) Å (Table 4). The three strongest bonds are all directed along vectors approximately in the (101) plane, i.e. the plane of the layers in the structure.

Compound (III) (space group P1) is isotypic with both the K analogue, K₂Cu^II(C₂O₄)₂·2H₂O (Viswamitra, 1962a; Weichert & Löhn, 1974), and the NH₄ analogue, (NH₄)₂Cu^II(C₂O₄)₂·2H₂O (Viswamitra, 1962b). It should be mentioned that the presently determined structure of the Rb compound is based on a standard (reduced) unit cell, whereas the previous structure models of the two above-mentioned analogues were described using a non-standard unit cell. Compound (III) is metrically pseudo-monoclinic, with a C-centred cell and cell parameters a = 10.53, b = 14.51 and c = 7.00 Å, and α = 90.2, β = 103.4 and g\ = 90.2°.

The atomic arrangement in (III) (Figs. 5 and 6) is based on layers of CuO₆ polyhedra separated by layers of Rb atoms, similar to the situation in the other two title compounds. Oxalate groups provide the necessary linkage between these units. The two non-equivalent Rb atoms are each surrounded by eight O atoms (mean Rb—O 3.031 Å for atom Rb1 and 3.045 Å for atom Rb2; Table 5). The Rb—O polyhedra are highly irregular. Atom Cu1 has a (four + two) coordination in which all six bonds are to oxalate O atoms. In contrast, atom Cu2 has a (less pronounced) (four + two) coordination in which the two long bonds are to atoms OW9 of one of the two non-equivalent water molecules (Table 5, Figs. 5 and 6); atom OW10 of the other water molecule is bonded to Rb1 only. The two CuO₆ polyhedra can both be described as elongated octahedra with only weak angular distortions (Table 5). The two non-equivalent oxalate groups play different roles. The first tetradentate group links adjacent Cu1O₆ and Cu2O₆ polyhedra in the (001) plane via the single bridge O3—C1—O5 (three of the four O ligands are shared with the two Cu atoms, while the fourth O ligand is shared with the Rb2 atom). The second equally tetradentate group provides a connection along approximately [114] between the Cu1O₆ polyhedra and the two RbO₈ polyhedra. Both oxalate groups are slightly non-planar [maximum deviation for the group containing atoms C1 and C2 is 7.62 (19)°, and that for the group containing atoms C3 and C4 is 3.7 (2)°], whereas in the K analogue (Weichert & Löhn, 1974) only one of the two groups is non-planar (maximum deviation 9°). The reported structure data for the NH₄ analogue (Viswamitra, 1962b) are of a precision too low to allow any statement about the aplanarity of the oxalate groups. Hydrogen bonding in (III) is weak, with all O···O distances greater than 2.793 (3) Å (Table 6).

S2. Experimental

All three title compounds crystallized from slowly evaporated aqueous solutions at room temperature. Compound (I) formed dark-pinkish indistinctly tabular (001) crystals, arranged to form clusters of radiating individuals. The crystals grew from a solution containing dissolved Rb₂CO₃, Cr(NO₃)₃·9H₂O and oxalic acid dihydrate (pH = 4). No other compound crystallized from the solution. Compound (II) grew from a solution containing dissolved CsCO₃, magnesium hydroxide carbonate, minor CsF and oxalic acid dihydrate (pH = 4). The colourless thick tabular crystal used for the present structural study was distinctly monoclinic, but the large majority of the other crystals showed a (pseudo?) hexagonal tabular habit, with flat bipyramidal faces, and with re-entrant angles between adjacent crystal faces bordering the plane of flattening. The apparance of these crystals is similar to that of snow crystals and they may be twins [the MgO₄(H₂O)₂ octahedra in Fig. 2 show a pseudohexagonal arrangement] or possibly a dimorph (?). No other compound crystallized from the solution. Compound (III) crystallized as blocky to isometric blue pleochroitic individuals from a solution containing dissolved Rb₂CO₃, copper hydroxide carbonate, CuCl₂, nitric acid and oxalic acid dihydrate (pH = 8). The blue crystals formed thick crusts which were covered by smaller amounts of blue-green indistinct opaque platelets of an uninvestigated compound.

S3. Refinement

The H atoms in all three compounds were freely refined.

Computing details

For all compounds, data collection: COLLECT (Nonius, 2002); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1999) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The layered structure of (I) projected along [110]. Oxalate groups provide a linkage between (001) layers of CrO4(H2O)2 octahedra and parallel layers of eight-coordinated Rb atoms (large spheres). The single medium-strong hydrogen bond strengthening the octahedral layer is shown as a dashed line in the top left-hand part of the figure. The unit cell is outlined.
	Fig. 2. The connectivity in RbCr^III(C2O4)2·2H2O, shown with displacement ellipsoids at the 50% probability level [symmetry codes: (i) 1 − x, y, −z; (ii) 1 − x, 1 − y, −z Please check; (iii) x, 1 − y, z].
	Fig. 3. The layered structure of (II) projected along [010]. Layers of MgO4(H2O)2 octahedra parallel to (101) are separated by corrugated layers of nine-coordinate Cs atoms (large spheres). Oxalate groups link these units together in different directions. The unit cell is outlined.
	Fig. 4. The connectivity in (II), shown with displacement ellipsoids at the 50% probability level [symmetry code: (i) −x, y, 1/2 − z].
	Fig. 5. The layered structure of (III) projected along [100]. Layers of CuO6 polyhedra parallel to (001) are separated by layers of eight-coordinated Rb atoms (large spheres). Oxalate groups link these units together in different directions (see text). Only the stronger Rb—O bonds (those within 3.1 Å) are shown. The unit cell is outlined.
	Fig. 6. The connectivity in (III), shown with displacement ellipsoids at the 50% probability level [symmetry codes: (i) −x, −y − z; (ii) 1 − x, 1 − y, −z].

(I) Diaquabis(µ-oxalato)chromium(III)rubidium(I)

Crystal data

[RbCr(C2O4)2(H2O)2]	F(000) = 338
Mr = 349.54	Dx = 2.375 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 752 reflections
a = 6.639 (1) Å	θ = 2.0–30.0°
b = 7.313 (1) Å	µ = 6.16 mm^−1
c = 10.078 (2) Å	T = 293 K
β = 92.46 (3)°	Plate, pink
V = 488.85 (14) Å^3	0.20 × 0.18 × 0.03 mm
Z = 2

Data collection

Nonius KappaCCD area-detector diffractometer	759 independent reflections
Radiation source: fine-focus sealed tube	708 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.012
ψ and ω scans	θmax = 30.1°, θmin = 4.1°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −9→9
Tmin = 0.372, Tmax = 0.837	k = −10→10
1410 measured reflections	l = −14→14

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)] = 0.022	All H-atom parameters refined
wR(F^2) = 0.060	w = 1/[σ^2(Fo^2) + (0.036P)^2 + 0.23P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max < 0.001
759 reflections	Δρmax = 0.43 e Å^−3
47 parameters	Δρmin = −0.96 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.022 (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 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
Rb	0.0000	0.5000	0.5000	0.02849 (15)
Cr	0.5000	0.5000	0.0000	0.01441 (14)
O1	0.43538 (15)	0.32232 (13)	0.13719 (10)	0.0204 (2)
OW2	0.2128 (2)	0.5000	−0.06230 (18)	0.0254 (3)
O3	0.31678 (19)	0.30771 (17)	0.34099 (12)	0.0340 (3)
C	0.3731 (2)	0.3934 (2)	0.24689 (14)	0.0208 (3)
H	0.165 (4)	0.415 (3)	−0.102 (2)	0.055 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb	0.0371 (2)	0.02514 (19)	0.0239 (2)	0.000	0.00899 (12)	0.000
Cr	0.0168 (2)	0.0104 (2)	0.0163 (2)	0.000	0.00376 (15)	0.000
O1	0.0274 (5)	0.0133 (4)	0.0208 (5)	−0.0010 (4)	0.0058 (4)	0.0014 (3)
OW2	0.0215 (7)	0.0145 (6)	0.0397 (9)	0.000	−0.0045 (6)	0.000
O3	0.0466 (7)	0.0302 (6)	0.0263 (6)	0.0014 (5)	0.0133 (5)	0.0084 (5)
C	0.0234 (6)	0.0194 (6)	0.0199 (6)	−0.0002 (5)	0.0030 (5)	0.0018 (5)

Geometric parameters (Å, º)

Rb—O3^i	2.9906 (13)	Cr—O1	1.9583 (10)
Rb—O3^ii	2.9906 (13)	Cr—O1^viii	1.9583 (10)
Rb—O3^iii	2.9906 (13)	Cr—O1^ix	1.9583 (10)
Rb—O3^iv	2.9906 (13)	Cr—OW2	1.9816 (15)
Rb—O3^v	3.0426 (14)	Cr—OW2^ix	1.9816 (15)
Rb—O3^vi	3.0426 (14)	O1—C	1.3047 (17)
Rb—O3	3.0426 (14)	O3—C	1.2093 (17)
Rb—O3^vii	3.0426 (14)	C—C^vii	1.559 (3)
Cr—O1^vii	1.9583 (10)	OW2—H	0.80 (2)
O1^vii—Cr—O1	83.14 (6)	O1^vii—Cr—OW2^ix	90.76 (5)
O1^vii—Cr—O1^viii	180.00 (4)	O1—Cr—OW2^ix	90.76 (5)
O1—Cr—O1^viii	96.86 (6)	O1^viii—Cr—OW2^ix	89.24 (5)
O1^vii—Cr—O1^ix	96.86 (6)	O1^ix—Cr—OW2^ix	89.24 (5)
O1—Cr—O1^ix	180.00 (6)	OW2—Cr—OW2^ix	180.0
O1^viii—Cr—O1^ix	83.14 (6)	O3—C—O1	125.31 (14)
O1^vii—Cr—OW2	89.24 (5)	O3—C—C^vii	121.21 (9)
O1—Cr—OW2	89.24 (5)	O1—C—C^vii	113.47 (8)
O1^viii—Cr—OW2	90.76 (5)	Rb^x—OW2—H	54.4 (17)
O1^ix—Cr—OW2	90.76 (5)	H—OW2—H^vii	102 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW2—H···O1^xi	0.80 (2)	1.88 (2)	2.6514 (13)	160 (2)

Symmetry code: (xi) −x+1/2, −y+1/2, −z.

(II) Tetraaquabis(µ-oxalato)magnesium(II)dicaesium(I)

Crystal data

[Cs2Mg(C2O4)2(H2O)4]	F(000) = 1000
Mr = 538.23	Dx = 2.681 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2059 reflections
a = 17.045 (3) Å	θ = 2.0–30.0°
b = 7.368 (1) Å	µ = 5.57 mm^−1
c = 13.588 (3) Å	T = 293 K
β = 128.61 (3)°	Fragment, colourless
V = 1333.5 (7) Å^3	0.14 × 0.13 × 0.10 mm
Z = 4

Data collection

Nonius KappaCCD area-detector diffractometer	1935 independent reflections
Radiation source: fine-focus sealed tube	1773 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.009
ψ and ω scans	θmax = 30.0°, θmin = 3.2°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −23→23
Tmin = 0.482, Tmax = 0.573	k = −10→10
3719 measured reflections	l = −19→19

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)] = 0.018	All H-atom parameters refined
wR(F^2) = 0.040	w = 1/[σ^2(Fo^2) + (0.009P)^2 + 2.37P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max = 0.001
1935 reflections	Δρmax = 0.68 e Å^−3
104 parameters	Δρmin = −0.73 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00120 (11)

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
Cs	0.152825 (10)	0.316641 (18)	0.107435 (12)	0.03296 (6)
Mg	0.0000	0.33317 (11)	0.2500	0.02091 (18)
C1	0.12842 (14)	0.1839 (3)	0.49764 (18)	0.0241 (4)
C2	0.15869 (13)	0.0878 (2)	0.42361 (18)	0.0233 (4)
O1	0.06078 (11)	0.30331 (18)	0.43747 (13)	0.0269 (3)
O2	0.17076 (12)	0.1346 (3)	0.60756 (14)	0.0400 (4)
O3	0.22516 (12)	−0.0283 (2)	0.47684 (15)	0.0377 (4)
O4	0.10857 (11)	0.13669 (19)	0.30969 (13)	0.0273 (3)
OW5	0.09999 (13)	0.5245 (2)	0.28078 (17)	0.0355 (4)
OW6	−0.03594 (13)	0.1615 (2)	−0.15258 (16)	0.0343 (3)
H1	0.083 (3)	0.628 (5)	0.252 (3)	0.056 (9)*
H2	0.153 (3)	0.529 (5)	0.347 (3)	0.069 (11)*
H3	−0.077 (2)	0.161 (4)	−0.137 (3)	0.051 (9)*
H4	−0.053 (2)	0.083 (4)	−0.196 (3)	0.042 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.03167 (9)	0.03670 (9)	0.02957 (9)	−0.00206 (5)	0.01865 (7)	−0.00134 (5)
Mg	0.0214 (4)	0.0205 (4)	0.0192 (4)	0.000	0.0119 (4)	0.000
C1	0.0212 (9)	0.0290 (9)	0.0204 (9)	−0.0024 (7)	0.0122 (8)	−0.0027 (7)
C2	0.0205 (8)	0.0238 (8)	0.0231 (9)	−0.0010 (7)	0.0125 (7)	−0.0013 (7)
O1	0.0286 (7)	0.0295 (7)	0.0231 (7)	0.0049 (5)	0.0164 (6)	−0.0004 (5)
O2	0.0354 (8)	0.0587 (10)	0.0221 (7)	0.0106 (8)	0.0160 (7)	0.0084 (7)
O3	0.0315 (8)	0.0420 (9)	0.0309 (8)	0.0158 (7)	0.0152 (7)	0.0046 (7)
O4	0.0309 (7)	0.0277 (7)	0.0230 (7)	0.0052 (6)	0.0167 (6)	0.0011 (5)
OW5	0.0258 (8)	0.0288 (8)	0.0343 (9)	−0.0035 (6)	0.0101 (7)	0.0068 (7)
OW6	0.0405 (9)	0.0304 (8)	0.0340 (9)	−0.0048 (7)	0.0242 (8)	−0.0055 (7)

Geometric parameters (Å, º)

Cs—O2^i	3.104 (2)	Mg—O4	2.0748 (16)
Cs—O3^ii	3.1486 (16)	Mg—O1^iii	2.0778 (16)
Cs—OW6	3.155 (2)	Mg—O1	2.0778 (16)
Cs—O1^iii	3.2962 (16)	C1—O2	1.240 (2)
Cs—O1^iv	3.3338 (15)	C1—O1	1.263 (2)
Cs—O2^v	3.339 (2)	C1—C2	1.558 (3)
Cs—OW5	3.3694 (19)	C2—O3	1.231 (2)
Cs—O3^v	3.4514 (18)	C2—O4	1.267 (2)
Cs—O4	3.5309 (15)	OW5—H1	0.82 (4)
Mg—OW5^iii	2.0439 (17)	OW5—H2	0.78 (4)
Mg—OW5	2.0439 (17)	OW6—H3	0.84 (3)
Mg—O4^iii	2.0748 (16)	OW6—H4	0.74 (3)
OW5—Mg—OW5^iii	92.77 (11)	O2—C1—O1	126.41 (19)
OW5—Mg—O4^iii	171.00 (6)	O2—C1—C2	117.79 (17)
OW5^iii—Mg—O4^iii	88.56 (7)	O1—C1—C2	115.78 (16)
OW5—Mg—O4	88.56 (7)	O3—C2—O4	125.94 (18)
OW5^iii—Mg—O4	171.00 (6)	O3—C2—C1	119.39 (17)
O4^iii—Mg—O4	91.51 (9)	O4—C2—C1	114.65 (16)
OW5—Mg—O1^iii	92.18 (7)	Mg—OW5—H1	123 (2)
OW5^iii—Mg—O1^iii	96.20 (7)	Cs—OW5—H1	105 (2)
O4^iii—Mg—O1^iii	78.82 (6)	Mg—OW5—H2	119 (3)
O4—Mg—O1^iii	92.64 (7)	Cs—OW5—H2	101 (2)
OW5—Mg—O1	96.20 (7)	H1—OW5—H2	108 (3)
OW5^iii—Mg—O1	92.18 (7)	Cs—OW6—H3	98 (2)
O4^iii—Mg—O1	92.64 (7)	Cs—OW6—H4	142 (2)
O4—Mg—O1	78.82 (6)	H3—OW6—H4	103 (3)
O1^iii—Mg—O1	167.84 (9)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW5—H1···OW6^vi	0.82 (4)	1.88 (4)	2.684 (2)	166 (3)
OW5—H2···O3^i	0.78 (4)	1.96 (4)	2.735 (3)	170 (3)
OW6—H3···O2^iii	0.84 (3)	1.89 (3)	2.729 (2)	174 (3)
OW6—H4···O4^vii	0.74 (3)	2.02 (3)	2.760 (2)	174 (3)

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

(III) Diaquabis(µ-oxalato)copper(II)dirubidium(I)

Crystal data

[Rb2Cu(C2O4)2(H2O)2]	Z = 2
Mr = 446.55	F(000) = 422
Triclinic, P1	Dx = 2.850 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.000 (1) Å	Cell parameters from 2903 reflections
b = 8.949 (2) Å	θ = 2.0–30.0°
c = 8.982 (2) Å	µ = 11.44 mm^−1
α = 108.05 (3)°	T = 293 K
β = 97.69 (3)°	Fragment, blue
γ = 97.99 (3)°	0.12 × 0.07 × 0.07 mm
V = 520.3 (2) Å^3

Data collection

Nonius KappaCCD area-detector diffractometer	3011 independent reflections
Radiation source: fine-focus sealed tube	2641 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.016
ψ and ω scans	θmax = 30.0°, θmin = 2.4°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −9→9
Tmin = 0.305, Tmax = 0.449	k = −12→12
5809 measured reflections	l = −12→12

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	All H-atom parameters refined
wR(F^2) = 0.059	w = 1/[σ^2(Fo^2) + (0.026P)^2 + 0.24P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max < 0.001
3011 reflections	Δρmax = 0.71 e Å^−3
174 parameters	Δρmin = −0.80 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0221 (10)

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
Rb1	0.36833 (4)	0.11883 (3)	0.66573 (2)	0.03173 (8)
Rb2	0.25523 (3)	0.55140 (3)	0.63007 (3)	0.03259 (8)
Cu1	0.0000	0.0000	0.0000	0.02151 (9)
Cu2	0.5000	0.5000	0.0000	0.02131 (10)
C1	0.1311 (3)	0.3660 (2)	0.0004 (2)	0.0194 (4)
C2	0.2251 (3)	0.4665 (2)	0.1773 (2)	0.0199 (4)
C3	0.2628 (3)	0.1559 (3)	0.2745 (2)	0.0238 (4)
C4	0.0562 (3)	0.1520 (3)	0.3179 (2)	0.0246 (4)
O1	−0.0331 (2)	0.2808 (2)	−0.03125 (19)	0.0312 (4)
O2	0.2361 (2)	0.37932 (19)	−0.10198 (17)	0.0239 (3)
O3	0.1293 (2)	0.4699 (2)	0.28201 (19)	0.0313 (4)
O4	0.4047 (2)	0.53626 (19)	0.19850 (17)	0.0238 (3)
O5	0.4100 (3)	0.2198 (2)	0.3775 (2)	0.0392 (4)
O6	0.2631 (2)	0.0892 (2)	0.12515 (17)	0.0260 (3)
O7	0.0398 (3)	0.2196 (2)	0.45588 (19)	0.0373 (4)
O8	−0.0872 (2)	0.0760 (2)	0.20220 (18)	0.0268 (3)
OW9	0.5727 (3)	0.2387 (3)	0.0175 (2)	0.0358 (4)
OW10	0.7878 (3)	0.1353 (2)	0.6452 (2)	0.0351 (4)
H1	0.848 (6)	0.171 (4)	0.595 (4)	0.054 (11)*
H2	0.842 (5)	0.180 (4)	0.739 (4)	0.053 (10)*
H3	0.502 (7)	0.175 (5)	0.039 (5)	0.075 (14)*
H4	0.689 (6)	0.219 (5)	0.016 (4)	0.065 (11)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb1	0.03486 (14)	0.03152 (13)	0.02394 (13)	0.00059 (9)	0.00844 (9)	0.00374 (9)
Rb2	0.02269 (12)	0.03945 (14)	0.03162 (13)	−0.00294 (9)	−0.00095 (9)	0.01260 (10)
Cu1	0.01680 (17)	0.0273 (2)	0.01724 (18)	−0.00022 (14)	0.00411 (12)	0.00445 (13)
Cu2	0.01554 (17)	0.0286 (2)	0.01555 (17)	−0.00190 (14)	0.00433 (12)	0.00349 (13)
C1	0.0171 (9)	0.0206 (9)	0.0209 (9)	0.0036 (7)	0.0042 (7)	0.0074 (7)
C2	0.0208 (9)	0.0204 (9)	0.0202 (9)	0.0045 (8)	0.0059 (7)	0.0083 (7)
C3	0.0240 (10)	0.0244 (10)	0.0220 (10)	−0.0027 (8)	0.0033 (8)	0.0098 (8)
C4	0.0292 (11)	0.0215 (10)	0.0218 (10)	0.0000 (8)	0.0067 (8)	0.0068 (8)
O1	0.0199 (8)	0.0371 (9)	0.0325 (9)	−0.0047 (7)	0.0047 (6)	0.0103 (7)
O2	0.0186 (7)	0.0298 (8)	0.0194 (7)	0.0001 (6)	0.0039 (5)	0.0047 (6)
O3	0.0298 (9)	0.0395 (10)	0.0257 (8)	0.0027 (7)	0.0148 (7)	0.0101 (7)
O4	0.0207 (7)	0.0294 (8)	0.0186 (7)	0.0012 (6)	0.0056 (5)	0.0051 (6)
O5	0.0310 (9)	0.0504 (11)	0.0270 (9)	−0.0154 (8)	−0.0041 (7)	0.0137 (8)
O6	0.0177 (7)	0.0344 (8)	0.0223 (8)	−0.0008 (6)	0.0042 (6)	0.0064 (6)
O7	0.0473 (11)	0.0377 (10)	0.0224 (8)	0.0000 (8)	0.0151 (7)	0.0037 (7)
O8	0.0229 (8)	0.0326 (8)	0.0226 (7)	0.0004 (6)	0.0075 (6)	0.0070 (6)
OW9	0.0298 (10)	0.0414 (11)	0.0443 (11)	0.0130 (8)	0.0152 (8)	0.0195 (8)
OW10	0.0381 (10)	0.0332 (9)	0.0281 (10)	−0.0047 (8)	0.0057 (8)	0.0070 (7)

Geometric parameters (Å, º)

Rb1—OW10	2.953 (2)	Cu1—O1^vi	2.6512 (19)
Rb1—OW10^i	2.954 (2)	Cu1—O1	2.6512 (19)
Rb1—O2^ii	2.9575 (18)	Cu2—O4	1.9363 (15)
Rb1—O5	3.0304 (19)	Cu2—O4^vii	1.9363 (15)
Rb1—O8^iii	3.0450 (18)	Cu2—O2	1.9487 (16)
Rb1—O4^iv	3.054 (2)	Cu2—O2^vii	1.9487 (16)
Rb1—OW9^ii	3.073 (2)	Cu2—OW9	2.508 (2)
Rb1—O7	3.181 (2)	Cu2—OW9^vii	2.508 (2)
Rb1—O5^i	3.533 (2)	C1—O1	1.232 (3)
Rb2—O3^v	2.9024 (17)	C1—O2	1.274 (2)
Rb2—O5^iv	2.912 (2)	C1—C2	1.561 (3)
Rb2—O3	2.9586 (19)	C2—O3	1.221 (3)
Rb2—O7	2.964 (2)	C2—O4	1.285 (3)
Rb2—O4^iv	2.9901 (17)	C3—O5	1.224 (3)
Rb2—OW9^iv	3.109 (2)	C3—O6	1.289 (3)
Rb2—O2^ii	3.2414 (17)	C3—C4	1.546 (3)
Rb2—O7^v	3.285 (2)	C4—O7	1.228 (3)
Rb2—O5	3.536 (2)	C4—O8	1.282 (3)
Cu1—O6^vi	1.9429 (17)	OW9—H3	0.79 (5)
Cu1—O6	1.9429 (17)	OW9—H4	0.86 (4)
Cu1—O8	1.9444 (16)	OW10—H1	0.76 (4)
Cu1—O8^vi	1.9444 (16)	OW10—H2	0.82 (4)
O6^vi—Cu1—O6	180.00 (9)	O4^vii—Cu2—OW9	91.31 (7)
O6^vi—Cu1—O8	94.73 (7)	O2—Cu2—OW9	85.22 (7)
O6—Cu1—O8	85.27 (7)	O2^vii—Cu2—OW9	94.78 (7)
O6^vi—Cu1—O8^vi	85.27 (7)	O4—Cu2—OW9^vii	91.31 (7)
O6—Cu1—O8^vi	94.73 (7)	O4^vii—Cu2—OW9^vii	88.69 (7)
O8—Cu1—O8^vi	180.00 (9)	O2—Cu2—OW9^vii	94.78 (7)
O6^vi—Cu1—O1^vi	90.62 (7)	O2^vii—Cu2—OW9^vii	85.22 (7)
O6—Cu1—O1^vi	89.38 (7)	OW9—Cu2—OW9^vii	180.0
O8—Cu1—O1^vi	92.83 (7)	O1—C1—O2	124.87 (19)
O8^vi—Cu1—O1^vi	87.17 (7)	O1—C1—C2	119.80 (18)
O6^vi—Cu1—O1	89.38 (7)	O2—C1—C2	115.33 (17)
O6—Cu1—O1	90.62 (7)	O3—C2—O4	125.8 (2)
O8—Cu1—O1	87.17 (7)	O3—C2—C1	119.68 (19)
O8^vi—Cu1—O1	92.83 (7)	O4—C2—C1	114.50 (17)
O1^vi—Cu1—O1	180.00 (7)	O5—C3—O6	124.8 (2)
O4—Cu2—O4^vii	180.00 (10)	O5—C3—C4	120.60 (19)
O4—Cu2—O2	85.75 (7)	O6—C3—C4	114.56 (18)
O4^vii—Cu2—O2	94.25 (7)	O7—C4—O8	125.1 (2)
O4—Cu2—O2^vii	94.25 (7)	O7—C4—C3	119.5 (2)
O4^vii—Cu2—O2^vii	85.75 (7)	O8—C4—C3	115.43 (18)
O2—Cu2—O2^vii	180.0	H3—OW9—H4	113 (4)
O4—Cu2—OW9	88.69 (7)	H1—OW10—H2	107 (4)
O1—C1—C2—O3	−5.7 (3)	O5—C3—C4—O7	3.1 (3)
O2—C1—C2—O3	174.8 (2)	O6—C3—C4—O7	−176.3 (2)
O1—C1—C2—O4	172.38 (19)	O5—C3—C4—O8	−177.1 (2)
O2—C1—C2—O4	−7.1 (3)	O6—C3—C4—O8	3.5 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW9—H3···O6	0.79 (5)	2.08 (5)	2.824 (3)	156 (4)
OW9—H4···O1^viii	0.86 (4)	2.08 (4)	2.843 (3)	147 (4)
OW10—H1···O7^viii	0.76 (4)	2.05 (4)	2.793 (3)	166 (4)
OW10—H2···O1^ix	0.82 (4)	1.99 (4)	2.817 (3)	177 (4)

Symmetry codes: (viii) x+1, y, z; (ix) x+1, y, z+1.