Crystal structure of Cs2[Th(NO3)6]

The crystal structure of Cs2[Th(NO3)6] can be derived from a dense packing of idealized CsO12 and ThO12 units. The CsO12 units form a distorted hcp arrangement with half of the octahedral sites occupied by ThO12 units.

Dicaesium hexanitratothorate(IV), Cs 2 [Th(NO 3 ) 6 ], was synthesized in the form of colourless crystals by reaction of thorium nitrate and caesium nitrate in aqueous solution. The Th atom is located on an inversion centre and is coordinated by six chelating nitrate anions. The resulting ThO 12 coordination polyhedron is best described as a slightly distorted icosahedron. The Cs atom also has a coordination number of 12, but its coordination polyhedron is considerably more distorted. The crystal packing can be derived from an hexagonal dense packing (hcp) of idealized spherical CsO 12 and ThO 12 units. The CsO 12 units form a distorted hcp arrangement and half of the octahedral sites are occupied by the ThO 12 units.

Chemical context
Nitrato complexes of the actinoids (Ryan, 1961;Strnad & Kohler, 1989) play an important role in the production of nuclear fuel as well as in its reprocessing. Moreover, multinary thorium nitrate compounds are of potential interest as anhydrous starting materials for further chemical conversion.
The caesium cation is surrounded by eleven NO 3 À -anions, one of which is chelating, leading to an overall coordination number of 12. The Cs-O distances of the chelating O-atoms range from 3.150 (2) to 3.436 (3) Å , whereas the other ten Cs-O distances are between 3.090 (2) and 3.552 (2) Å .
The crystal structure of Cs 2 [Th(NO 3 ) 6 ] can be derived from a dense packing if the CsO 12 and ThO 12 units are idealized as spheres. The CsO 12 units form a distorted hexagonal closepacked arrangement with the ThO 12 units situated in half of the octahedral sites. The unit cell of Cs 2 [Th(NO 3 ) 6 ] is shown in Fig. 2, pointing out the pseudo-hexagonal arrangement.
The structure of the title compound is assumed to be isotypic with that of Rb 2 [Th(NO 3 ) 6 ] (Walker et al., 1956), although atom positions have not been reported for the Rb compound so far. However, the unit cells are similar and the space group types are identical.

Synthesis and crystallization
0.1 g (0.18 mmol, 1 eq) Th(NO 3 ) 4 Á5H 2 O and 70 mg (0.36 mmol, 2 eq) CsNO 3 were placed in a reaction flask and 100 ml water were added. The turbid solution was stirred and 1 ml of HNO 3 conc. was additionally added, which led to a clear solution. The mixture was heated to 333 K and evaporated at 22 mbar in a rotary evaporator leading to a colourless powder. After dissolving the colourless solid in as little water as possible, the solution was allowed to evaporate at room temperature for one month. Single crystals of the title compound were obtained in an almost quantitative yield.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The highest remaining electron density was found in Wyckoff position 2a. Inclusion of this density in the refinement led to unreasonable models. In the final model, this density was therefore not further considered.  program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and SHELXLE (Hübschle et al., 2011); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).

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 F 2 against all reflections. The weighted R-factor wR and goodness 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 threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) 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.