TRIGONAL-PRISMATIC NB-6-UNITS IN COMPOUND S OF THE TYPE A(3)[NB6SBR17] (A=TL, K, CS)

The new compounds A(3)[Nb6SBr17] (A = Tl, K, Cs) were synthesized and their lattice parameters were calculated from indexed X-ray powder pat terns. An intensity measurement based on a single-crystal of Tl-3[Nb6S Br17] was performed with an automatic four-circle diffractometer (a = 1704.9(1) pm, b = 1862.8(3) pm, c = 932.51(7) pm; beta = 90.747(7)degr ees, V-m = 445.9(3) cm(3)/mol; C2/c, Z = 4). The structure refinement confirmed consistency with the Rb-3[Nb6SBr17] structure, found previou sly. The comparison of both structures yielded no significant differen ces concerning the distances within the sulfur-centered Nb6S cluster c ore. Extended Huckel calculations revealed a maximum of the niobium-ni obium overlap populations for (the actual present number of) 14 electr ons occupying metal-metal bonding states.