New series of niobium oxychlorides, M2RENb6Cl15O3 (M = monovalent cation, RE = rare earth) and M2UNb6Cl15O3. The crystal structure of Cs2UNb6Cl15O3

The new M2RENb6Cl15O3 (M = monovalent cation; RE = rare earth) and M2UNb6Cl15O3 oxychlorides, isotypic with the corresponding tantalum oxybromides (Cs2LaTa6Br15O3 structure-type: trigonal, P31c), have been isolated. The crystal structure of Cs2UNb6Cl15O3 (a = 9.2080(7) Å, c = 17.095(3) Å) has been refined from single crystal X-ray diffraction data. It is based on [(Nb6Cl9iO3i)Cl6a]5− units centered on the threefold axis at z = 14 and 34. Uranium is 9-coordinated by three Oi and six Cla belonging to three adjacent units, and cesium is 12-coordinated by six Cli and six Cla belonging to six adjacent units. The structural results indicate that the uranium should be in the trivalent state, leading to 14 valence electrons per Nb6 cluster. Magnetic measurements do not indicate any interaction between the uranium atoms in this compound, but great crystal field effects.