Uranyl interactions: The title compound is the first example of an actinide compound that contains three different actinyl‐ion coordination geometries, namely, octahedral, pentagonal bipyramidal, and hexagonal bipyramidal. Strong cation–cation interactions between the uranyl ions lead to pentamers with a rather unusual configuration (see picture; U dark gray, O light gray).

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The crystal structure of Li4[(UO2)2(W2O10)] (1) has been solved by direct methods and refined to R 1 0.036. The compound is orthorhombic, Cmc21, a = 7.3823(3), b = 14.0498(5), c = 10.1887(3) Å, V = 1056.77(7) Å3. The structure is based upon uranyl tungstate sheets with the composition [(UO2)2(W2O10)]4–. The sheets are strongly corrugated and are parallel to the (001) plane. The sheet can be described as consisting of two types of chains. UO7 pentagonal bipyramids share edges to form UO5 chains, whereas WO6 octahedra share corners to form WO5 chains. Both chains extend along the a axis. The Li+ cations reside in between the sheets providing their linkage into a three-dimensional structure. The structure can be considered as a derivative of complex oxides observed in the U—W—O and U—Mo—W—O systems. However, the topology of polyhedral linkage is different as a result of the presence of Li+ cations in the structure. The crystal chemistry of Li uranyl tungstates is discussed.

The chemistry of actinides is an important and currently active field of inorganic chemistry. Actinides and their compounds play a prime role in nuclear waste management and safety assessment. [1][2][3] In particular, investigations of complexation of uranyl ions 4-6 with oxygen-based complexes consisting of highvalent elements such as S, Cr, Se, Mo or W have high relevance for nuclear waste disposal, owing to their use in nuclear engineering (e.g., Mo 7 and W 8 are used in refractory alloys) or due to their formation in burn-up processes of nuclear fuel (Mo, Se). 9 Actinide sulfates, chromates and selenates are known to form diverse and complex structures with such interesting structural features as cation-cation interactions, microporous frameworks and nanotubules. [10][11][12][13][14][15][16] Actinide molybdates and tungstates may adopt structure types more resembling dense complex oxides than oxosalts. 4 In comparison to molybdates, uranyl tungstates are less studied with relatively few reports available in the literature. 17 The mixture was heated to 820°C for 4 h and cooled down to room temperature with the rate of 7°C h −1 . The obtained product consisted of yellowish crystals of 1 and 2 immersed within an amorphous yellow solid. Powder samples consisting of both 1 and 2 phases were prepared by heating a mixture of CsNO 3 (0.273 g, 1.40 mmol), UO 3 (0.400 g, 1.40 mmol) and WO 3 (0.324 g, 1.40 mmol) in a 1 : 1 : 1 molar ratio at 750°C for 72 h in a porcelain crucible. X-ray powder diffraction indicated that the sample is a mixture of α-and β-Cs 2 [(UO 2 ) 2 (W 2 O 9 )] in a ratio 1 : 4 (Fig. S1 †). Our attempts to obtain pure powders of either α-or β-Cs 2 [(UO 2 ) 2 (W 2 O 9 )] were not successful. Single crystals of 3 were obtained by high-temperature solid-state reaction of an initial mixture of RbNO 3 (0.103 g, 0.70 mmol), UO 3 (0.200 g, 0.70 mmol) and WO 3 (0.162 g, 0.70 mmol) with a Rb : U : W ratio equal to 1 : 1 : 1. The mixture was slowly heated to 870°C in a porcelain crucible and then cooled down to 400°C at a rate of 4°C h −1 . The product consisted of yellowish crystals of 3 in a glassy amorphous mass. Single crystals of the obtained phases were investigated by means of single-crystal X-ray diffraction analysis. It is noteworthy that the crystals of 1 were observed to be of considerably lower quality in comparison to the crystals of 2, which may indicate its lower stability.The structures of all three compounds are based upon 2D uranyl tungstate layers (Fig. 1). The layers in these structures are based on UO 2 2+ linear groups linked with oxo-tungstates polyhedra. In all three structures, U 6+ cations form linear uranyl groups, [OvUvO] 2+ , equatorially coordinated by five O atoms. In the structures of 1 and 2, UO 7 pentagonal bipyramids share edges to † Electronic supplementary information (ESI) available: Includes powder diffractograms and IR spectra. CCDC 881326-881328. For ESI and crystallographic data in CIF or other electronic format see

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Uranylwechselwirkungen: Die Titelverbindung ist die erste Actinoidverbindung mit drei verschiedenen Koordinationsumgebungen der Actinylionen: oktaedrisch, pentagonal bipyramidal und hexagonal bipyramidal. Starke Kation‐Kation‐Wechselwirkungen zwischen den Uranylionen bewirken die Bildung eines Pentamers mit einer recht ungewöhnlichen Konfiguration (siehe Bild; dunkelgrau: U dunkelgrau, hellgrau: O).