Redetermination of [Gd(NO3)3(H2O)4]·2H2O

The crystal structure of the title compound, tetraaquatris(nitrato-κ2 O,O′)gadolinium(III) dihydrate, was redetermined from single-crystal X-ray data. In comparison with the first determination [Ma et al. (1991 ▶). Wuji Huaxue Xuebao, 7, 351–353], all H atoms could be located, accompanied with higher accuracy and precision. The GdIII atom shows a ten-coordination with three nitrate ligands behaving in a bidentate manner and the other positions being occupied by four water molecules, forming a distorted bicapped square antiprism. Two nitrate ions coordinate to the metal atom with similar bond lengths while the third shows a more asymmetric bonding behaviour. An intricate network of O—H⋯O hydrogen bonds, including the lattice water molecules, stabilizes the crystal packing.

The crystal structure of the title compound, tetraaquatris-(nitrato-2 O,O 0 )gadolinium(III) dihydrate, was redetermined from single-crystal X-ray data. In comparison with the first determination [Ma et al. (1991). Wuji Huaxue Xuebao,7,[351][352][353], all H atoms could be located, accompanied with higher accuracy and precision. The Gd III atom shows a tencoordination with three nitrate ligands behaving in a bidentate manner and the other positions being occupied by four water molecules, forming a distorted bicapped square antiprism. Two nitrate ions coordinate to the metal atom with similar bond lengths while the third shows a more asymmetric bonding behaviour. An intricate network of O-HÁ Á ÁO hydrogen bonds, including the lattice water molecules, stabilizes the crystal packing.
As shown in Figure 1, the Gd III atom is ten-coordinated, being bound to six nitrate-oxygen atoms, O1, O2, O4, O5, O7, and O8, and four water-oxygen atoms O10, O11, O12, and O13. The nitrate ions are coordinated to the central gadolinium atom in a bidentate mode and show clearly the changes in bond lengths and angles noted previously for the isotypic Eu(III) structure (Stumpf & Bolte, 2001). The distances between the Gd III ion and the nitrate-O atoms (Gd-O) are in the range 2.494 (3)-2.754 (2) Å, with a mean value of 2.571 Å. The differences between the Gd III atom and the two oxygen atoms of the same nitrate group are 0.034 Å and 0.060 Å for nitrate N1 and nitrate N2, respectively. Nitrate N1 and nitrate N2 groups appear to be more symmetrically bonded to the Gd III atom because the third nitrate group N3 exhibits one Gd-O distance that is 0.202 Å longer than the other. This asymmetric bonding seem to be associated with a steric effect of the coordinating water molecules. There is simply not space enough for all the oxygen atoms around the Gd III atom at the same distance. The Gd-O water distances are in the range of 2.364 (3)-2.398 (2) Å with an average distance of 2.386 Å. Hence the water molecules are closer to the Gd III atom than the nitrate groups by ca. 0.185 Å.
These results are comparable to that reported for other lanthanide complexes with bidentately coordinating nitrate groups. The coordination polyhedron around the Gd III ion can be best described as a distorted bi-capped square antiprism ( Fig. 1), as previously reported for other hydrated lanthanide(III) nitrate complexes (Ribár et al., 1986;Moret et al., 1990;Ma et al., 1991, Stockhause & Meyer, 1997Kawashima et al., 2000;Stumpf & Bolte, 2001;Junk et al., 1999;Gao et al., 1990;Eriksson et al., 1980;Rogers et al., 1983). The structure contains additional two lattice water molecules, which are associated with the complex by a network of hydrogen bonds. All hydrogen atoms and except for O1 and O2 all oxygen atoms are involved in a complicated network of O-H···O hydrogen bonds, stabilizing the crystal packing (Table 1, Fig. 2). H5, H6 and H10 act as bifurcated bridging atoms. In contrast to the original work of Ma et al. (1991), we report much more precise results. All hydrogen atoms could be located in the difference Fourier maps and were allowed to refine freely. The final R1 value changed from 0.076 to 0.0158 and the overall e.s.d.'s for the positional parameters dropped from e.g. Gd1 x = 0.8016 (1) to 0.80352 (2). The same is valid for distances e.g. Gd1-O1 dropped from 2.558 (56) Å to 2.528 (2) Å and for the angels e.g. O1-Gd1-O2 dropped from 51.1 (22) to 50.56 (7).

Refinement
Hydrogen atoms could be located in difference Fourier maps and were allowed to refine freely. The residual electron density of Δρmax = 1.04 e Å -3 is located 0.89 Å next to Gd1, whereas Δρmin = 1.10 e Å -3 is located 0.96 Å next to Gd1.

Tetraaquatris(nitrato-κ 2 O,O′)gadolinium(III) dihydrate
Extinction coefficient: 0.0129 (7) Special details Experimental. Diffractometer operator E. Herdtweck scanspeed 10 s per frame dx 45 4932 frames measured in 9 data sets phi-scan with delta_phi = 0.50 omega-scans with delta_omega = 0.50 Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except for 0 with very negative F 2 or flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The observed criterion of F 2 > σ(F 2 ) is used only for calculating R_factor_obs etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.