Dicerium(IV) tetrachromate(VI) dihydrate, Ce
(CrO
4)
4·2H
2O, has been prepared from an acidic aqueous solution at room temperature. Its novel crystal structure, which was solved from single-crystal X-ray diffraction data, is built from isolated CrO
4 tetrahedra and isolated Ce(O,H
2O)
n (
n = 8 and 9) polyhedra. All atoms are in general positions. The mean Ce-O and Cr-O bond lengths are 2.358 and 1.651 Å, respectively. Comparisons are drawn with the structure of Ce
IV(CrO
4)
2·2H
2O.
Supporting information
Clusters of small dark-red bipyramidal crystals of the title compound formed at room temperature on slow evaporation of an acidic aqueous solution of CeIIICl3·7H2O and CrVIO3 (pH = 1–2). The reaction must have involved the oxidation of CeIII to CeIV. The compound is stable in air.
H atoms were freely refined; the O—H distances are listed in Table 2.
Data collection: COLLECT (Nonius, 2003); 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: Diamond (Pennington, 1999); ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.
dicerium(IV) tetrachromate(VI) dihydrate
top
Crystal data top
Ce2(CrO4)4·2H2O | F(000) = 2880 |
Mr = 780.27 | Dx = 3.751 Mg m−3 |
Orthorhombic, Pbca | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ac 2ab | Cell parameters from 5599 reflections |
a = 10.938 (2) Å | θ = 2.0–32.6° |
b = 11.464 (2) Å | µ = 9.59 mm−1 |
c = 22.038 (4) Å | T = 293 K |
V = 2763.4 (9) Å3 | Fragment, dark red |
Z = 8 | 0.15 × 0.07 × 0.07 mm |
Data collection top
Nonius KappaCCD diffractometer | 5028 independent reflections |
Radiation source: fine-focus sealed tube | 4611 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.011 |
ψ and ω scans | θmax = 32.6°, θmin = 2.7° |
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997) | h = −16→16 |
Tmin = 0.327, Tmax = 0.554 | k = −17→17 |
9513 measured reflections | l = −33→33 |
Refinement top
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.018 | All H-atom parameters refined |
wR(F2) = 0.044 | w = 1/[σ2(Fo2) + (0.019P)2 + 4.3P] where P = (Fo2 + 2Fc2)/3 |
S = 1.15 | (Δ/σ)max = 0.001 |
5028 reflections | Δρmax = 0.78 e Å−3 |
234 parameters | Δρmin = −1.01 e Å−3 |
0 restraints | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.00037 (3) |
Crystal data top
Ce2(CrO4)4·2H2O | V = 2763.4 (9) Å3 |
Mr = 780.27 | Z = 8 |
Orthorhombic, Pbca | Mo Kα radiation |
a = 10.938 (2) Å | µ = 9.59 mm−1 |
b = 11.464 (2) Å | T = 293 K |
c = 22.038 (4) Å | 0.15 × 0.07 × 0.07 mm |
Data collection top
Nonius KappaCCD diffractometer | 5028 independent reflections |
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997) | 4611 reflections with I > 2σ(I) |
Tmin = 0.327, Tmax = 0.554 | Rint = 0.011 |
9513 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.018 | 0 restraints |
wR(F2) = 0.044 | All H-atom parameters refined |
S = 1.15 | Δρmax = 0.78 e Å−3 |
5028 reflections | Δρmin = −1.01 e Å−3 |
234 parameters | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
Ce1 | 0.137846 (11) | 0.214546 (11) | 0.052712 (5) | 0.00979 (3) | |
Ce2 | 0.196727 (11) | 0.412728 (10) | 0.295996 (5) | 0.00962 (3) | |
Cr1 | 0.03009 (3) | 0.45626 (3) | 0.142602 (16) | 0.01192 (7) | |
Cr2 | 0.08110 (3) | 0.14059 (3) | 0.220914 (15) | 0.00953 (6) | |
Cr3 | 0.28545 (3) | 0.15941 (3) | 0.408329 (15) | 0.00922 (6) | |
Cr4 | 0.43643 (3) | 0.38158 (3) | 0.081713 (16) | 0.00918 (6) | |
O1 | 0.0026 (2) | 0.58981 (17) | 0.12766 (10) | 0.0261 (4) | |
O2 | −0.09370 (16) | 0.39269 (17) | 0.16873 (8) | 0.0181 (3) | |
O3 | 0.14049 (17) | 0.44736 (18) | 0.19415 (9) | 0.0212 (4) | |
O4 | 0.07468 (17) | 0.38859 (17) | 0.07588 (8) | 0.0196 (4) | |
O5 | −0.05350 (17) | 0.08156 (17) | 0.21529 (9) | 0.0207 (4) | |
O6 | 0.17722 (17) | 0.05569 (17) | 0.25662 (9) | 0.0217 (4) | |
O7 | 0.06593 (15) | 0.26400 (15) | 0.25978 (8) | 0.0157 (3) | |
O8 | 0.13853 (17) | 0.17100 (17) | 0.15253 (8) | 0.0192 (4) | |
O9 | 0.42946 (16) | 0.19179 (18) | 0.41777 (9) | 0.0213 (4) | |
O10 | 0.22201 (16) | 0.25493 (16) | 0.36197 (8) | 0.0183 (3) | |
O11 | 0.21871 (17) | 0.16269 (17) | 0.47566 (8) | 0.0194 (4) | |
O12 | 0.27599 (17) | 0.02515 (15) | 0.38022 (8) | 0.0179 (3) | |
O13 | 0.51012 (16) | 0.39957 (16) | 0.14532 (8) | 0.0184 (3) | |
O14 | 0.39287 (19) | 0.50889 (17) | 0.05480 (9) | 0.0237 (4) | |
O15 | 0.5258 (2) | 0.3151 (2) | 0.03219 (9) | 0.0278 (4) | |
O16 | 0.31572 (17) | 0.29867 (17) | 0.09545 (9) | 0.0199 (4) | |
OW17 | 0.31761 (18) | 0.29515 (18) | 0.22843 (9) | 0.0174 (3) | |
OW18 | 0.3159 (2) | 0.1044 (2) | 0.01138 (14) | 0.0350 (6) | |
H1 | 0.296 (4) | 0.292 (4) | 0.197 (2) | 0.051 (15)* | |
H2 | 0.375 (4) | 0.283 (4) | 0.235 (2) | 0.041 (13)* | |
H3 | 0.328 (7) | 0.048 (7) | 0.016 (3) | 0.11 (3)* | |
H4 | 0.376 (5) | 0.130 (5) | 0.006 (3) | 0.071 (18)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Ce1 | 0.00984 (5) | 0.01135 (6) | 0.00818 (6) | −0.00186 (4) | 0.00019 (4) | −0.00012 (4) |
Ce2 | 0.00975 (6) | 0.00930 (6) | 0.00980 (6) | −0.00095 (4) | 0.00088 (4) | −0.00103 (4) |
Cr1 | 0.01219 (15) | 0.01144 (15) | 0.01213 (15) | 0.00084 (12) | 0.00118 (12) | −0.00060 (12) |
Cr2 | 0.01054 (14) | 0.01003 (15) | 0.00802 (14) | −0.00025 (11) | −0.00004 (11) | 0.00067 (11) |
Cr3 | 0.00907 (14) | 0.01027 (15) | 0.00833 (14) | 0.00059 (11) | 0.00010 (11) | −0.00090 (11) |
Cr4 | 0.00858 (14) | 0.00949 (14) | 0.00948 (14) | 0.00000 (11) | 0.00003 (11) | 0.00033 (12) |
O1 | 0.0304 (10) | 0.0148 (8) | 0.0333 (11) | 0.0051 (7) | 0.0038 (9) | 0.0027 (8) |
O2 | 0.0146 (8) | 0.0214 (9) | 0.0183 (8) | −0.0011 (7) | 0.0030 (6) | −0.0012 (7) |
O3 | 0.0181 (8) | 0.0258 (9) | 0.0198 (9) | −0.0004 (7) | −0.0040 (7) | −0.0010 (7) |
O4 | 0.0233 (9) | 0.0199 (8) | 0.0157 (8) | 0.0006 (7) | 0.0076 (7) | −0.0013 (7) |
O5 | 0.0190 (8) | 0.0219 (9) | 0.0211 (8) | −0.0077 (7) | 0.0021 (7) | −0.0062 (7) |
O6 | 0.0219 (9) | 0.0226 (9) | 0.0207 (9) | 0.0094 (7) | 0.0021 (7) | 0.0069 (7) |
O7 | 0.0145 (7) | 0.0156 (8) | 0.0170 (8) | −0.0007 (6) | −0.0001 (6) | −0.0044 (6) |
O8 | 0.0220 (9) | 0.0257 (9) | 0.0099 (7) | −0.0051 (7) | 0.0010 (6) | 0.0021 (7) |
O9 | 0.0123 (8) | 0.0263 (9) | 0.0253 (9) | −0.0036 (7) | −0.0036 (7) | 0.0017 (8) |
O10 | 0.0176 (8) | 0.0170 (8) | 0.0204 (8) | −0.0001 (7) | −0.0028 (7) | 0.0049 (7) |
O11 | 0.0232 (9) | 0.0228 (9) | 0.0123 (7) | 0.0047 (7) | 0.0047 (6) | −0.0026 (7) |
O12 | 0.0197 (8) | 0.0162 (8) | 0.0179 (8) | −0.0009 (6) | 0.0034 (6) | −0.0051 (6) |
O13 | 0.0166 (8) | 0.0203 (8) | 0.0182 (8) | −0.0030 (7) | −0.0061 (6) | −0.0003 (7) |
O14 | 0.0275 (10) | 0.0148 (8) | 0.0289 (10) | 0.0016 (7) | −0.0064 (8) | 0.0031 (7) |
O15 | 0.0302 (10) | 0.0360 (11) | 0.0172 (9) | 0.0148 (9) | 0.0088 (8) | 0.0027 (8) |
O16 | 0.0154 (8) | 0.0233 (9) | 0.0211 (9) | −0.0068 (7) | −0.0048 (7) | 0.0020 (7) |
OW17 | 0.0115 (8) | 0.0267 (10) | 0.0141 (8) | 0.0040 (7) | −0.0012 (6) | −0.0034 (7) |
OW18 | 0.0232 (11) | 0.0197 (10) | 0.0620 (17) | 0.0033 (9) | 0.0139 (11) | 0.0012 (11) |
Geometric parameters (Å, º) top
Ce1—O4 | 2.1723 (19) | Cr1—O3 | 1.6611 (19) |
Ce1—O8 | 2.2557 (18) | Cr1—O4 | 1.7326 (18) |
Ce1—O15i | 2.2625 (19) | Cr2—O5 | 1.6251 (19) |
Ce1—O16 | 2.3670 (18) | Cr2—O6 | 1.6346 (18) |
Ce1—O11ii | 2.3762 (18) | Cr2—O7 | 1.6621 (18) |
Ce1—O14iii | 2.382 (2) | Cr2—O8 | 1.6696 (18) |
Ce1—O9iv | 2.3847 (18) | Cr3—O9 | 1.6316 (18) |
Ce1—OW18 | 2.493 (2) | Cr3—O10 | 1.6505 (18) |
Ce2—O12v | 2.2793 (18) | Cr3—O11 | 1.6541 (18) |
Ce2—O6v | 2.3108 (18) | Cr3—O12 | 1.6624 (18) |
Ce2—O10 | 2.3373 (18) | Cr4—O13 | 1.6301 (18) |
Ce2—O3 | 2.3608 (19) | Cr4—O14 | 1.6458 (19) |
Ce2—O7 | 2.3645 (17) | Cr4—O15 | 1.6514 (19) |
Ce2—OW17 | 2.4048 (19) | Cr4—O16 | 1.6548 (18) |
Ce2—O13iv | 2.4210 (18) | OW17—H1 | 0.72 (5) |
Ce2—O2vi | 2.4314 (18) | OW17—H2 | 0.66 (4) |
Ce2—O5vii | 2.5024 (18) | OW18—H3 | 0.67 (8) |
Cr1—O1 | 1.5947 (19) | OW18—H4 | 0.73 (6) |
Cr1—O2 | 1.6420 (18) | | |
| | | |
O4—Ce1—O8 | 88.57 (7) | O7—Ce2—O13iv | 67.99 (6) |
O4—Ce1—O15i | 99.20 (8) | OW17—Ce2—O13iv | 139.41 (7) |
O8—Ce1—O15i | 140.81 (7) | O12v—Ce2—O2vi | 70.71 (6) |
O4—Ce1—O16 | 78.09 (7) | O6v—Ce2—O2vi | 67.95 (7) |
O8—Ce1—O16 | 72.51 (7) | O10—Ce2—O2vi | 67.44 (6) |
O15i—Ce1—O16 | 146.68 (7) | O3—Ce2—O2vi | 124.44 (6) |
O4—Ce1—O11ii | 75.07 (7) | O7—Ce2—O2vi | 127.61 (6) |
O8—Ce1—O11ii | 145.76 (7) | OW17—Ce2—O2vi | 68.08 (7) |
O15i—Ce1—O11ii | 72.52 (7) | O13iv—Ce2—O2vi | 128.18 (6) |
O16—Ce1—O11ii | 74.76 (7) | O12v—Ce2—O5vii | 74.07 (7) |
O4—Ce1—O14iii | 149.28 (7) | O6v—Ce2—O5vii | 77.74 (7) |
O8—Ce1—O14iii | 76.26 (7) | O10—Ce2—O5vii | 137.27 (6) |
O15i—Ce1—O14iii | 77.93 (8) | O3—Ce2—O5vii | 67.19 (7) |
O16—Ce1—O14iii | 120.77 (7) | O7—Ce2—O5vii | 98.36 (6) |
O11ii—Ce1—O14iii | 130.73 (7) | OW17—Ce2—O5vii | 135.74 (7) |
O4—Ce1—O9iv | 74.48 (7) | O13iv—Ce2—O5vii | 64.75 (6) |
O8—Ce1—O9iv | 73.32 (7) | O2vi—Ce2—O5vii | 134.03 (7) |
O15i—Ce1—O9iv | 72.02 (7) | O1—Cr1—O2 | 110.06 (10) |
O16—Ce1—O9iv | 136.20 (7) | O1—Cr1—O3 | 109.72 (11) |
O11ii—Ce1—O9iv | 127.99 (7) | O2—Cr1—O3 | 109.41 (10) |
O14iii—Ce1—O9iv | 75.62 (7) | O1—Cr1—O4 | 107.92 (10) |
O4—Ce1—OW18 | 143.07 (8) | O2—Cr1—O4 | 109.33 (9) |
O8—Ce1—OW18 | 103.98 (9) | O3—Cr1—O4 | 110.39 (10) |
O15i—Ce1—OW18 | 92.59 (10) | O5—Cr2—O6 | 111.81 (10) |
O16—Ce1—OW18 | 73.12 (9) | O5—Cr2—O7 | 107.66 (9) |
O11ii—Ce1—OW18 | 75.41 (8) | O6—Cr2—O7 | 108.84 (10) |
O14iii—Ce1—OW18 | 67.42 (8) | O5—Cr2—O8 | 111.04 (10) |
O9iv—Ce1—OW18 | 142.27 (8) | O6—Cr2—O8 | 108.48 (10) |
O12v—Ce2—O6v | 80.02 (7) | O7—Cr2—O8 | 108.96 (9) |
O12v—Ce2—O10 | 85.16 (7) | O9—Cr3—O10 | 109.50 (10) |
O6v—Ce2—O10 | 135.39 (7) | O9—Cr3—O11 | 107.85 (10) |
O12v—Ce2—O3 | 135.49 (7) | O10—Cr3—O11 | 110.77 (9) |
O6v—Ce2—O3 | 71.29 (7) | O9—Cr3—O12 | 108.56 (10) |
O10—Ce2—O3 | 138.80 (7) | O10—Cr3—O12 | 110.94 (10) |
O12v—Ce2—O7 | 139.63 (6) | O11—Cr3—O12 | 109.14 (10) |
O6v—Ce2—O7 | 138.22 (7) | O13—Cr4—O14 | 109.93 (10) |
O10—Ce2—O7 | 73.95 (6) | O13—Cr4—O15 | 109.52 (10) |
O3—Ce2—O7 | 69.07 (7) | O14—Cr4—O15 | 110.03 (11) |
O12v—Ce2—OW17 | 138.52 (7) | O13—Cr4—O16 | 108.05 (9) |
O6v—Ce2—OW17 | 80.62 (7) | O14—Cr4—O16 | 110.14 (10) |
O10—Ce2—OW17 | 83.47 (7) | O15—Cr4—O16 | 109.14 (11) |
O3—Ce2—OW17 | 69.44 (7) | Ce2—OW17—H1 | 115 (4) |
O7—Ce2—OW17 | 73.71 (6) | Ce2—OW17—H2 | 120 (4) |
O12v—Ce2—O13iv | 73.18 (6) | H1—OW17—H2 | 121 (5) |
O6v—Ce2—O13iv | 138.40 (7) | Ce1—OW18—H3 | 126 (6) |
O10—Ce2—O13iv | 73.70 (6) | Ce1—OW18—H4 | 124 (5) |
O3—Ce2—O13iv | 107.38 (7) | H3—OW18—H4 | 103 (7) |
Symmetry codes: (i) x−1/2, −y+1/2, −z; (ii) x, −y+1/2, z−1/2; (iii) −x+1/2, y−1/2, z; (iv) x−1/2, y, −z+1/2; (v) −x+1/2, y+1/2, z; (vi) x+1/2, y, −z+1/2; (vii) −x, y+1/2, −z+1/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
OW17—H1···O16 | 0.72 (5) | 2.26 (5) | 2.931 (3) | 155 (5) |
OW17—H1···O8 | 0.72 (5) | 2.42 (5) | 2.943 (3) | 130 (5) |
OW17—H2···O7vi | 0.66 (4) | 2.10 (4) | 2.752 (3) | 168 (5) |
OW18—H3···O4iii | 0.67 (8) | 2.49 (8) | 3.095 (3) | 151 (8) |
OW18—H3···O11viii | 0.67 (8) | 2.62 (7) | 3.185 (3) | 143 (8) |
Symmetry codes: (iii) −x+1/2, y−1/2, z; (vi) x+1/2, y, −z+1/2; (viii) −x+1/2, −y, z−1/2. |
Experimental details
Crystal data |
Chemical formula | Ce2(CrO4)4·2H2O |
Mr | 780.27 |
Crystal system, space group | Orthorhombic, Pbca |
Temperature (K) | 293 |
a, b, c (Å) | 10.938 (2), 11.464 (2), 22.038 (4) |
V (Å3) | 2763.4 (9) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 9.59 |
Crystal size (mm) | 0.15 × 0.07 × 0.07 |
|
Data collection |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | Multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997) |
Tmin, Tmax | 0.327, 0.554 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 9513, 5028, 4611 |
Rint | 0.011 |
(sin θ/λ)max (Å−1) | 0.758 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.018, 0.044, 1.15 |
No. of reflections | 5028 |
No. of parameters | 234 |
H-atom treatment | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.78, −1.01 |
Selected bond lengths (Å) topCe1—O4 | 2.1723 (19) | Cr1—O1 | 1.5947 (19) |
Ce1—O8 | 2.2557 (18) | Cr1—O2 | 1.6420 (18) |
Ce1—O15i | 2.2625 (19) | Cr1—O3 | 1.6611 (19) |
Ce1—O16 | 2.3670 (18) | Cr1—O4 | 1.7326 (18) |
Ce1—O11ii | 2.3762 (18) | Cr2—O5 | 1.6251 (19) |
Ce1—O14iii | 2.382 (2) | Cr2—O6 | 1.6346 (18) |
Ce1—O9iv | 2.3847 (18) | Cr2—O7 | 1.6621 (18) |
Ce1—OW18 | 2.493 (2) | Cr2—O8 | 1.6696 (18) |
Ce2—O12v | 2.2793 (18) | Cr3—O9 | 1.6316 (18) |
Ce2—O6v | 2.3108 (18) | Cr3—O10 | 1.6505 (18) |
Ce2—O10 | 2.3373 (18) | Cr3—O11 | 1.6541 (18) |
Ce2—O3 | 2.3608 (19) | Cr3—O12 | 1.6624 (18) |
Ce2—O7 | 2.3645 (17) | Cr4—O13 | 1.6301 (18) |
Ce2—OW17 | 2.4048 (19) | Cr4—O14 | 1.6458 (19) |
Ce2—O13iv | 2.4210 (18) | Cr4—O15 | 1.6514 (19) |
Ce2—O2vi | 2.4314 (18) | Cr4—O16 | 1.6548 (18) |
Ce2—O5vii | 2.5024 (18) | | |
Symmetry codes: (i) x−1/2, −y+1/2, −z; (ii) x, −y+1/2, z−1/2; (iii) −x+1/2, y−1/2, z; (iv) x−1/2, y, −z+1/2; (v) −x+1/2, y+1/2, z; (vi) x+1/2, y, −z+1/2; (vii) −x, y+1/2, −z+1/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
OW17—H1···O16 | 0.72 (5) | 2.26 (5) | 2.931 (3) | 155 (5) |
OW17—H1···O8 | 0.72 (5) | 2.42 (5) | 2.943 (3) | 130 (5) |
OW17—H2···O7vi | 0.66 (4) | 2.10 (4) | 2.752 (3) | 168 (5) |
OW18—H3···O4iii | 0.67 (8) | 2.49 (8) | 3.095 (3) | 151 (8) |
OW18—H3···O11viii | 0.67 (8) | 2.62 (7) | 3.185 (3) | 143 (8) |
Symmetry codes: (iii) −x+1/2, y−1/2, z; (vi) x+1/2, y, −z+1/2; (viii) −x+1/2, −y, z−1/2. |
CeIV2(CrO4)4·2H2O, dicerium(IV) tetrachromate(VI) dihydrate, (I), represents a new structure type among chromates (and sulfates or molybdates) of comparable MIV cations (M = Ce, Th, U, Zr and Ti). The atomic arrangement has space-group symmetry Pbca, and the asymmetric unit contains two Ce, four Cr, 18 O and four H atoms, all of which occupy general positions. The structure is built from isolated CrO4 tetrahedra, Ce atoms and two water molecules, which belong to the coordination environments of the Ce atoms (Figs. 1 and 2). The mean Cr—O bond lengths in the four CrO4 tetrahedra are similar (1.658, 1.648, 1.650 and 1.646 Å for atoms Cr1, Cr2, Cr3 and Cr4, respectively). The Cr1O4 tetrahedron shows the largest bond-length distortion (Table 1) and, accordingly, the largest mean Cr—O bond length, an observation in strict accordance with the distortion theorem (Brown & Shannon, 1973; Brown, 1981). The observed distortion of the Cr1O4 tetrahedron (with an unusually short Cr1—O1 distance and a long Cr1—O4 bond) is due to the fact that atom O1 is bonded only to one metal atom (Cr1), whereas atoms O2, O3 and O4 are each bonded to atom Cr1 and one of the two Ce atoms; atom O4 has a very short bond to Ce1 and therefore needs a relatively long bond to Cr1 in order to satisfy its bond-valence requirements. The four mean Cr—O bond lengths are all larger than the corresponding average (1.642 Å) in the single CrO4 tetrahedron present in the more highly hydrated cerium(IV) chromate CeIV(CrO4)2·2H2O (monoclinic, P21/m; Lindgren, 1977). The geometry of the CrO4 tetrahedra in (I) is regular; the maximum deviations from ideal tetrahedral O—Cr—O angles are 1.81 °.
The two Ce atoms are, as seen in a view along [001] (Fig. 1a), located in undulating layers roughly parallel to (100). Interestingly, in monoclinic CeIV(CrO4)2·2H2O (Lindgren, 1977), the Ce atoms are also located in (slightly less) undulating layers parallel to (100). Atom Ce1 is coordinated to eight oxygen ligands (one of which is a water molecule, OW18), while atom Ce2 has a coordination sphere consisting of nine O atoms (again, one of these is a water molecule, OW17). The two Ce-(O,H2O) coordination polyhedra do not share any faces, edges or corners. The polyhedra can be described as a distorted bicapped trigonal prism, where two faces of the prism are capped (Ce1), and a distorted monocapped square antiprism, where one of the basal planes is capped (Ce2). The single Ce atom in CeIV(CrO4)2·2H2O (Lindgren, 1977) is eight-coordinate and the isolated Ce(O,H2O)8 coordination polyhedron is similar to that of atom Ce1 in the title compound. The mean Ce1—O and Ce2—O bond lengths in (I) are 2.337 and 2.379 Å, respectively. In CeIV(CrO4)2·2H2O (Lindgren, 1977), the corresponding value is very similar (2.342 Å).
The O—H vectors of the two water molecules in (I) point towards small voids in the structure. The hydrogen bonds are all more or less weak (Table 2), and the bonds donated by atoms H1 and H3 appear to be bifurcated (note that atom H4 has no acceptor). The H atoms in CeIV(CrO4)2·2H2O could not be located by Lindgren (1977), but the distances between the two non-equivalent OW atoms and probable O acceptor atoms suggest that the hydrogen bonds are also weak in CeIV(CrO4)2·2H2O (O···O > 2.75 Å).
Bond-valence sums for the metal atoms were calculated using the bond-valence parameters of Brese & O'Keeffe (1991) for Cr—O bonds and the parameters from Roulhac & Palenik (2003) for CeIV—O bonds. These parameters are 4.00 (Ce1), 3.94 (Ce2), 5.84 (Cr1), 5.95 (Cr2), 5.91 (Cr3) and 5.98 (Cr4) valence units (v.u.), and thus are all reasonably close to ideal valences. It is noted that the use of bond-valence parameters from Brese & O'Keeffe (1991) for CeIV—O bonds would result in considerably underestimated sums for the Ce atoms (3.59 v.u. for Ce1 and 3.54 v.u. for Ce2). Even the improved parameters of Brown (1996; updated values, R0 = 2.09, b = 0.37; www.CCP14.ac.uk/CCP/web-mirrors/i_d_brown) would give unsatisfactory (overestimated) values, viz. 4.24 v.u. (Ce1) and 4.18 v.u. (Ce2).