NEW SERIES OF NIOBIUM OXYCHLORIDES, M(2)RENB(6)CL(15)O(3) (M EQUALS MONOVALENT CATION, RE EQUALS RARE-EARTH) AND M(2)UNB(6)CL(15)O(3) - THECRYSTAL-STRUCTURE OF CS2UNB6CL15O3

The new M(2)RENb(6)Cl(15)O(3) (M = monovalent cation; RE = rare earth) and M(2)UNb(6)Cl(15)O(3) oxychlorides, isotypic with the correspondin g tantalum oxybromides (Cs2LaTa6Br15O3 structure-type: trigonal, P (3) over bar 1c), have been isolated. The crystal structure of Cs2UNb6Cl1 5O3 (a = 9.2080(7) Angstrom, c = 17.095(3) Angstrom) has been refined from single crystal X-ray diffraction data. It is based on [((Nb6Cl9O3 i)-O-i)Cl-6(a)](5-) units centered on the threefold axis at z = 1/4 an d 3/4. Uranium is 9-coordinated by three O-i and six Cl-i belonging to three adjacent units, and cesium is 12-coordinated by six Cl-i and si x Cl-a belonging to six adjacent units. The structural results indicat e that the uranium should be in the trivalent state, leading to 14 val ence electrons per Nb-6 cluster. Magnetic measurements do not indicate any interaction between the uranium atoms in this compound, but great crystal field effects.