Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807035581/bg3045sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536807035581/bg3045Isup2.hkl |
CCDC reference: 657633
The compound was the unexpected product from the reaction of zinc sulfate monohydrate (0.09 g, 5 mmol), 3-(3-carboxyphenoxy)propionic acid (0.10 g, 5 mmol), sodium hydroxide (0.04 g, 10 mmol) and 4,4'-bipyridine (0.08 g, 5 mmol) in methanol. Colorless prismatic crystals were obtained from the filtered solution after several days.
The structure is twinned; the use of the TwinRotMat routine of the PLATON suite (Spek, 2003) gave the TWIN command as (1 1 0 0 - 1 0 0 0 - 1). The twin component refined to 0.54 (1).
For the 4,4'-bipyridine molecule, the C–N bond distances were restrained to 0.005 Å of each other as were the C–C bond distances of the rings. There was some disorder in the carbon atoms as some temperature factors were too small whereas others were too large. As the molecule has approximate twofold rotational symmetry along the N···N vector, the temperature factors of pairs of carbon atoms (i.e., C1/C5, C2/C4, C6/C10 and C7/C9) were restrained to be the same.
The carbon-bound hydrogen atoms were placed at calculated positions (C–H 0.93 Å) and were inlcuded in the refinement in the riding model approximation, with U(H) set to 1.2Ueq(C). Although the hydronge atoms of the water molecules could be placed in chemically sensible positions on the basis of hydrogen bonds, more than one such scheme may be envisaged. As such, hydrogen atoms were not included in the refinement.
The final difference Fourier map had a large peak in the neighbourhood of O1W, but was otherwise diffuse.
The crystal structure of (4,4'-bipyridine)triaquasulfatozinc dihydrate, which is described in the P65 space group, shows a bipyridine-bridged, helical chain structure having unidentate sulfate groups (Kondo et al., 1999; Ma et al., 2000). The compound with the formulation 2[C10H10N2]2+ [(C10H8N2)3(H2O)6(SO4)4Zn3]4-.10H2O, isolated at a different pH, has the sulfate groups in µ2-bridging modes (Tong & Chen, 2000). Under solvothermal conditions, a monoaqua monohydrate is also known but the sulfate group engages in a different bonding mode (Huang et al., 1998).
In the present study, the crystal structure of (4,4'-bipyridine)triaquasulfatozinc dihydrate when refined in the P61 space group gave a Flack parameter (Flack, 1983) of nearly zero. Interestingly, two previous studies have described the compound in the P65 space group (Kondo et al., 1999; Ma et al., 2000), so that the present and previous studies hint at the possibility that helices of opposite handedness could be present in a synthesis. Such has not been mentioned in the literature. Unfortunately, the Flack parameter is not given in the earlier reports, and one (Ma et al., 2000) used a large weighting scheme.
The crystal structure of the analogous (4,4'-bipyridine)triaquasulfatocopper dihydrate is known in the P65 form its Flack parameter refined to nearly zero (Hagrman et al., 1998; Lin & Liu, 2003). A P61 polymorph has also been reported; however, as its Flack parameter refined to 0.93 (Yuan et al., 2003), the form may be the P65 form only.
The existence of pairs of enantiomorphic structures is rarely discussed in the literature as finding such compounds is regarded as being unlikely (Brock & Dunitz, 1991). Amonium dioxalatotitanate dihydrate was refined by SHELX-76 in P6222; the authors attempted a refinement in P6422 but in view of statistical tests available then, concluded that the space group was P62222 as it led to significantly better agreement (English & Eve, 1993). A later study on the charge density (Sheu et al., 1996) did not comment on the alternative choice of the P6422 setting. The title zinc compound (in P61) and the isostructural copper compound (in P65) can be regarded as exemplifying such a pair of achiral metal-organic compounds. In this case, the space groups can be distinguished on the basis of anomalous dispersion (Ha & Allewell, 1997).
For adducts of zinc sulfate and 4,4'-bipyridine in molar stoichiometries, see Huang et al. (1998) and Tong & Chen (2000). For the title compound described in the P65 space group, see Kondo et al. (1999) and Ma et al. (2000). For the analogous copper compound that is described in the P61 space group, see Lin & Liu (2003), and for the same compound described in the P65 space group, see Hagrman et al. (1998) and Yuan et al. (2003).
For literature on enantiomorphous space groups, see Ha & Allewell (1997) and for a comment on the rarity of compounds crystallizing in both enantiomorphous space groups, see Brock & Dunitz (1991). For the possible existence of two enantiomorphic forms of ammonium dioxalatotitanate dihydrate, see English & Eve (1993) and Sheu et al. (1996).
For related literature, see: Flack (1983); Spek (2003).
Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO; data reduction: CrystalStructure (Molecular Structure Corporation & Rigaku, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2007).
[Zn(SO4)(C10H8N2)(H2O)3]·2H2O | Dx = 1.716 Mg m−3 |
Mr = 407.69 | Mo Kα radiation, λ = 0.71073 Å |
Hexagonal, P61 | Cell parameters from 21095 reflections |
Hall symbol: P 61 | θ = 3.6–27.4° |
a = 11.4275 (3) Å | µ = 1.74 mm−1 |
c = 20.9322 (6) Å | T = 295 K |
V = 2367.27 (9) Å3 | Prism, colorless |
Z = 6 | 0.35 × 0.29 × 0.18 mm |
F(000) = 1260 |
Rigaku RAXIS-RAPID IP diffractometer | 3514 independent reflections |
Radiation source: fine-focus sealed tube | 3309 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.034 |
ω scan | θmax = 27.4°, θmin = 3.6° |
Absorption correction: multi-scan ABSCOR (Higashi, 1995) | h = −14→14 |
Tmin = 0.45, Tmax = 0.73 | k = −14→14 |
22988 measured reflections | l = −25→27 |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.045 | H-atom parameters constrained |
wR(F2) = 0.125 | w = 1/[σ2(Fo2) + (0.0882P)2 + 0.7556P] where P = (Fo2 + 2Fc2)/3 |
S = 1.11 | (Δ/σ)max = 0.001 |
3514 reflections | Δρmax = 1.46 e Å−3 |
185 parameters | Δρmin = −0.43 e Å−3 |
41 restraints | Absolute structure: Flack parameter for 1473 Friedel pairs |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: −0.05 (2) |
[Zn(SO4)(C10H8N2)(H2O)3]·2H2O | Z = 6 |
Mr = 407.69 | Mo Kα radiation |
Hexagonal, P61 | µ = 1.74 mm−1 |
a = 11.4275 (3) Å | T = 295 K |
c = 20.9322 (6) Å | 0.35 × 0.29 × 0.18 mm |
V = 2367.27 (9) Å3 |
Rigaku RAXIS-RAPID IP diffractometer | 3514 independent reflections |
Absorption correction: multi-scan ABSCOR (Higashi, 1995) | 3309 reflections with I > 2σ(I) |
Tmin = 0.45, Tmax = 0.73 | Rint = 0.034 |
22988 measured reflections |
R[F2 > 2σ(F2)] = 0.045 | H-atom parameters constrained |
wR(F2) = 0.125 | Δρmax = 1.46 e Å−3 |
S = 1.11 | Δρmin = −0.43 e Å−3 |
3514 reflections | Absolute structure: Flack parameter for 1473 Friedel pairs |
185 parameters | Absolute structure parameter: −0.05 (2) |
41 restraints |
x | y | z | Uiso*/Ueq | ||
Zn1 | 0.57094 (6) | 1.00177 (10) | 0.49998 (4) | 0.02328 (15) | |
S1 | 0.38825 (14) | 0.64977 (15) | 0.47291 (7) | 0.0273 (3) | |
O1 | 0.4543 (5) | 0.7708 (5) | 0.51774 (19) | 0.0286 (10) | |
O2 | 0.2571 (5) | 0.5504 (5) | 0.4983 (3) | 0.0487 (13) | |
O3 | 0.3648 (5) | 0.6913 (5) | 0.4104 (2) | 0.0472 (14) | |
O4 | 0.4787 (7) | 0.5936 (6) | 0.4647 (3) | 0.0566 (16) | |
O1w | 0.6807 (5) | 1.2160 (5) | 0.4796 (2) | 0.0282 (10) | |
O2w | 0.5570 (5) | 0.9639 (6) | 0.4045 (3) | 0.0389 (13) | |
O3w | 0.5760 (6) | 1.0252 (6) | 0.6000 (3) | 0.0412 (15) | |
O4w | 0.1575 (7) | 0.6249 (9) | 0.6013 (3) | 0.074 (2) | |
O5w | 0.5635 (6) | 0.6791 (6) | 0.3194 (3) | 0.0489 (12) | |
N1 | 0.7614 (4) | 1.0049 (5) | 0.5031 (3) | 0.0261 (9) | |
N2 | 1.3815 (4) | 1.0025 (6) | 0.4974 (4) | 0.0316 (10) | |
C1 | 0.7649 (6) | 0.8902 (6) | 0.5018 (5) | 0.0347 (9) | |
H1 | 0.6836 | 0.8088 | 0.5005 | 0.042* | |
C2 | 0.8846 (5) | 0.8873 (6) | 0.5022 (4) | 0.0318 (8) | |
H2 | 0.8827 | 0.8050 | 0.5023 | 0.038* | |
C3 | 1.0071 (4) | 1.0071 (6) | 0.5026 (4) | 0.0246 (8) | |
C4 | 1.0036 (6) | 1.1263 (6) | 0.5042 (4) | 0.0318 (8) | |
H4 | 1.0831 | 1.2093 | 0.5054 | 0.038* | |
C5 | 0.8795 (5) | 1.1195 (6) | 0.5038 (5) | 0.0347 (9) | |
H5 | 0.8784 | 1.2004 | 0.5041 | 0.042* | |
C6 | 1.2611 (5) | 0.8924 (7) | 0.4970 (7) | 0.0646 (18) | |
H6 | 1.2595 | 0.8105 | 0.4935 | 0.078* | |
C7 | 1.1377 (7) | 0.8860 (7) | 0.5012 (8) | 0.076 (2) | |
H7 | 1.0578 | 0.8038 | 0.5050 | 0.092* | |
C8 | 1.1372 (5) | 1.0062 (6) | 0.4997 (4) | 0.0263 (9) | |
C9 | 1.2614 (6) | 1.1228 (7) | 0.4997 (8) | 0.076 (2) | |
H9 | 1.2661 | 1.2062 | 0.5029 | 0.092* | |
C10 | 1.3794 (6) | 1.1172 (7) | 0.4948 (6) | 0.0646 (18) | |
H10 | 1.4607 | 1.1975 | 0.4896 | 0.078* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Zn1 | 0.0185 (3) | 0.0317 (3) | 0.0245 (2) | 0.0161 (3) | −0.0012 (3) | −0.0016 (2) |
S1 | 0.0231 (6) | 0.0313 (7) | 0.0282 (6) | 0.0142 (6) | −0.0029 (5) | −0.0036 (6) |
O1 | 0.028 (2) | 0.038 (3) | 0.026 (2) | 0.022 (2) | 0.0011 (17) | −0.0011 (18) |
O2 | 0.046 (3) | 0.044 (3) | 0.036 (2) | 0.007 (2) | 0.008 (2) | 0.003 (2) |
O3 | 0.058 (3) | 0.040 (3) | 0.039 (3) | 0.020 (3) | −0.025 (3) | −0.009 (2) |
O4 | 0.062 (3) | 0.062 (4) | 0.068 (4) | 0.048 (3) | −0.021 (3) | −0.026 (3) |
O1w | 0.023 (2) | 0.023 (2) | 0.036 (2) | 0.0106 (18) | 0.0037 (19) | 0.0042 (17) |
O2w | 0.030 (3) | 0.044 (3) | 0.029 (3) | 0.008 (2) | −0.011 (2) | 0.000 (2) |
O3w | 0.069 (4) | 0.049 (3) | 0.017 (2) | 0.038 (3) | −0.002 (2) | −0.002 (2) |
O4w | 0.047 (3) | 0.086 (5) | 0.070 (4) | 0.018 (3) | 0.013 (3) | −0.011 (4) |
O5w | 0.049 (3) | 0.052 (3) | 0.054 (3) | 0.031 (3) | 0.002 (2) | −0.013 (2) |
N1 | 0.019 (2) | 0.032 (2) | 0.029 (2) | 0.013 (2) | 0.000 (2) | 0.0033 (19) |
N2 | 0.016 (2) | 0.035 (2) | 0.044 (2) | 0.012 (2) | 0.006 (3) | −0.0010 (19) |
C1 | 0.017 (2) | 0.032 (2) | 0.051 (2) | 0.009 (2) | 0.000 (2) | 0.0027 (18) |
C2 | 0.024 (2) | 0.0298 (19) | 0.050 (2) | 0.020 (2) | −0.007 (2) | −0.0013 (17) |
C3 | 0.018 (3) | 0.034 (3) | 0.0283 (19) | 0.018 (3) | 0.005 (3) | −0.0024 (19) |
C4 | 0.024 (2) | 0.0298 (19) | 0.050 (2) | 0.020 (2) | −0.007 (2) | −0.0013 (17) |
C5 | 0.017 (2) | 0.032 (2) | 0.051 (2) | 0.009 (2) | 0.000 (2) | 0.0027 (18) |
C6 | 0.020 (2) | 0.032 (2) | 0.147 (6) | 0.017 (2) | −0.003 (4) | −0.013 (3) |
C7 | 0.022 (2) | 0.031 (2) | 0.183 (7) | 0.019 (2) | 0.016 (4) | 0.000 (3) |
C8 | 0.010 (2) | 0.032 (3) | 0.035 (2) | 0.009 (2) | 0.002 (3) | 0.001 (2) |
C9 | 0.022 (2) | 0.031 (2) | 0.183 (7) | 0.019 (2) | 0.016 (4) | 0.000 (3) |
C10 | 0.020 (2) | 0.032 (2) | 0.147 (6) | 0.017 (2) | −0.003 (4) | −0.013 (3) |
Zn1—O1 | 2.316 (5) | C1—H1 | 0.9300 |
Zn1—O1W | 2.163 (5) | C2—C3 | 1.384 (4) |
Zn1—O2W | 2.034 (6) | C2—H2 | 0.9300 |
Zn1—O3W | 2.108 (5) | C3—C4 | 1.383 (9) |
Zn1—N1 | 2.160 (4) | C3—C8 | 1.492 (5) |
Zn1—N2i | 2.170 (4) | C4—C5 | 1.381 (10) |
S1—O2 | 1.455 (5) | C4—H4 | 0.9300 |
S1—O3 | 1.461 (5) | C5—H5 | 0.9300 |
S1—O4 | 1.474 (5) | C6—C7 | 1.378 (11) |
S1—O1 | 1.523 (5) | C6—H6 | 0.9300 |
N1—C1 | 1.331 (4) | C7—C8 | 1.377 (10) |
N1—C5 | 1.330 (4) | C7—H7 | 0.9300 |
N2—C6 | 1.321 (9) | C8—C9 | 1.378 (10) |
N2—C10 | 1.324 (9) | C9—C10 | 1.385 (11) |
N2—Zn1ii | 2.170 (4) | C9—H9 | 0.9300 |
C1—C2 | 1.385 (10) | C10—H10 | 0.9300 |
O2w—Zn1—O3w | 175.7 (3) | N1—C1—H1 | 118.7 |
O2w—Zn1—N1 | 90.3 (2) | C2—C1—H1 | 118.7 |
O3w—Zn1—N1 | 90.3 (3) | C3—C2—C1 | 119.9 (5) |
O2w—Zn1—O1w | 89.2 (2) | C3—C2—H2 | 120.0 |
O3w—Zn1—O1w | 95.1 (2) | C1—C2—H2 | 120.0 |
N1—Zn1—O1w | 88.8 (2) | C2—C3—C4 | 117.4 (4) |
O2w—Zn1—N2i | 90.2 (3) | C2—C3—C8 | 120.8 (5) |
O3w—Zn1—N2i | 89.3 (3) | C4—C3—C8 | 121.8 (5) |
N1—Zn1—N2i | 178.9 (2) | C5—C4—C3 | 118.7 (5) |
O1w—Zn1—N2i | 90.3 (2) | C5—C4—H4 | 120.7 |
O2w—Zn1—O1 | 88.6 (2) | C3—C4—H4 | 120.7 |
O3w—Zn1—O1 | 87.09 (19) | N1—C5—C4 | 124.2 (5) |
N1—Zn1—O1 | 90.86 (19) | N1—C5—H5 | 117.9 |
O1w—Zn1—O1 | 177.80 (19) | C4—C5—H5 | 117.9 |
N2i—Zn1—O1 | 90.1 (2) | N2—C6—C7 | 126.9 (6) |
O2—S1—O3 | 107.2 (3) | N2—C6—H6 | 116.5 |
O2—S1—O4 | 112.1 (4) | C7—C6—H6 | 116.5 |
O3—S1—O4 | 108.6 (4) | C8—C7—C6 | 117.4 (6) |
O2—S1—O1 | 109.4 (3) | C8—C7—H7 | 121.3 |
O3—S1—O1 | 110.2 (3) | C6—C7—H7 | 121.3 |
O4—S1—O1 | 109.3 (3) | C7—C8—C9 | 116.6 (4) |
S1—O1—Zn1 | 132.7 (2) | C7—C8—C3 | 120.4 (5) |
C1—N1—C5 | 117.1 (4) | C9—C8—C3 | 122.8 (5) |
C1—N1—Zn1 | 120.6 (4) | C8—C9—C10 | 120.7 (6) |
C5—N1—Zn1 | 122.3 (4) | C8—C9—H9 | 119.6 |
C6—N2—C10 | 114.6 (5) | C10—C9—H9 | 119.6 |
C6—N2—Zn1ii | 124.2 (4) | N2—C10—C9 | 122.9 (6) |
C10—N2—Zn1ii | 121.1 (4) | N2—C10—H10 | 118.6 |
N1—C1—C2 | 122.6 (5) | C9—C10—H10 | 118.6 |
O2—S1—O1—Zn1 | −136.6 (4) | C2—C3—C4—C5 | −1.4 (16) |
O3—S1—O1—Zn1 | −18.9 (4) | C8—C3—C4—C5 | 177.0 (8) |
O4—S1—O1—Zn1 | 100.3 (4) | C1—N1—C5—C4 | −1.0 (18) |
O2w—Zn1—O1—S1 | −4.5 (4) | Zn1—N1—C5—C4 | −178.3 (7) |
O3w—Zn1—O1—S1 | 175.0 (4) | C3—C4—C5—N1 | 1.1 (17) |
N1—Zn1—O1—S1 | −94.8 (4) | C10—N2—C6—C7 | −8 (2) |
N2i—Zn1—O1—S1 | 85.7 (4) | Zn1ii—N2—C6—C7 | 173.6 (13) |
O2w—Zn1—N1—C1 | −76.9 (8) | N2—C6—C7—C8 | 7 (3) |
O3w—Zn1—N1—C1 | 98.8 (8) | C6—C7—C8—C9 | −5 (2) |
O1w—Zn1—N1—C1 | −166.1 (8) | C6—C7—C8—C3 | 178.6 (11) |
O1—Zn1—N1—C1 | 11.7 (8) | C2—C3—C8—C7 | −4.7 (16) |
O2w—Zn1—N1—C5 | 100.3 (8) | C4—C3—C8—C7 | 177.0 (11) |
O3w—Zn1—N1—C5 | −84.0 (8) | C2—C3—C8—C9 | 179.7 (11) |
O1w—Zn1—N1—C5 | 11.1 (8) | C4—C3—C8—C9 | 1.3 (16) |
O1—Zn1—N1—C5 | −171.1 (8) | C7—C8—C9—C10 | 6 (2) |
C5—N1—C1—C2 | 1.2 (17) | C3—C8—C9—C10 | −177.9 (12) |
Zn1—N1—C1—C2 | 178.5 (7) | C6—N2—C10—C9 | 8 (2) |
N1—C1—C2—C3 | −1.5 (17) | Zn1ii—N2—C10—C9 | −173.2 (12) |
C1—C2—C3—C4 | 1.6 (16) | C8—C9—C10—N2 | −8 (2) |
C1—C2—C3—C8 | −176.8 (8) |
Symmetry codes: (i) x−1, y, z; (ii) x+1, y, z. |
Experimental details
Crystal data | |
Chemical formula | [Zn(SO4)(C10H8N2)(H2O)3]·2H2O |
Mr | 407.69 |
Crystal system, space group | Hexagonal, P61 |
Temperature (K) | 295 |
a, c (Å) | 11.4275 (3), 20.9322 (6) |
V (Å3) | 2367.27 (9) |
Z | 6 |
Radiation type | Mo Kα |
µ (mm−1) | 1.74 |
Crystal size (mm) | 0.35 × 0.29 × 0.18 |
Data collection | |
Diffractometer | Rigaku RAXIS-RAPID IP diffractometer |
Absorption correction | Multi-scan ABSCOR (Higashi, 1995) |
Tmin, Tmax | 0.45, 0.73 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 22988, 3514, 3309 |
Rint | 0.034 |
(sin θ/λ)max (Å−1) | 0.647 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.045, 0.125, 1.11 |
No. of reflections | 3514 |
No. of parameters | 185 |
No. of restraints | 41 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 1.46, −0.43 |
Absolute structure | Flack parameter for 1473 Friedel pairs |
Absolute structure parameter | −0.05 (2) |
Computer programs: RAPID-AUTO (Rigaku, 1998), RAPID-AUTO, CrystalStructure (Molecular Structure Corporation & Rigaku, 2002), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), X-SEED (Barbour, 2001), publCIF (Westrip, 2007).
Zn1—O1 | 2.316 (5) | Zn1—O3W | 2.108 (5) |
Zn1—O1W | 2.163 (5) | Zn1—N1 | 2.160 (4) |
Zn1—O2W | 2.034 (6) | Zn1—N2i | 2.170 (4) |
Symmetry code: (i) x−1, y, z. |
The crystal structure of (4,4'-bipyridine)triaquasulfatozinc dihydrate, which is described in the P65 space group, shows a bipyridine-bridged, helical chain structure having unidentate sulfate groups (Kondo et al., 1999; Ma et al., 2000). The compound with the formulation 2[C10H10N2]2+ [(C10H8N2)3(H2O)6(SO4)4Zn3]4-.10H2O, isolated at a different pH, has the sulfate groups in µ2-bridging modes (Tong & Chen, 2000). Under solvothermal conditions, a monoaqua monohydrate is also known but the sulfate group engages in a different bonding mode (Huang et al., 1998).
In the present study, the crystal structure of (4,4'-bipyridine)triaquasulfatozinc dihydrate when refined in the P61 space group gave a Flack parameter (Flack, 1983) of nearly zero. Interestingly, two previous studies have described the compound in the P65 space group (Kondo et al., 1999; Ma et al., 2000), so that the present and previous studies hint at the possibility that helices of opposite handedness could be present in a synthesis. Such has not been mentioned in the literature. Unfortunately, the Flack parameter is not given in the earlier reports, and one (Ma et al., 2000) used a large weighting scheme.
The crystal structure of the analogous (4,4'-bipyridine)triaquasulfatocopper dihydrate is known in the P65 form its Flack parameter refined to nearly zero (Hagrman et al., 1998; Lin & Liu, 2003). A P61 polymorph has also been reported; however, as its Flack parameter refined to 0.93 (Yuan et al., 2003), the form may be the P65 form only.
The existence of pairs of enantiomorphic structures is rarely discussed in the literature as finding such compounds is regarded as being unlikely (Brock & Dunitz, 1991). Amonium dioxalatotitanate dihydrate was refined by SHELX-76 in P6222; the authors attempted a refinement in P6422 but in view of statistical tests available then, concluded that the space group was P62222 as it led to significantly better agreement (English & Eve, 1993). A later study on the charge density (Sheu et al., 1996) did not comment on the alternative choice of the P6422 setting. The title zinc compound (in P61) and the isostructural copper compound (in P65) can be regarded as exemplifying such a pair of achiral metal-organic compounds. In this case, the space groups can be distinguished on the basis of anomalous dispersion (Ha & Allewell, 1997).