Preparation and Crystal Structure of Cs4Nb2S11

The new polychalcogenide Cs4Nb2S11 was prepared from the melt. Cs4Nb2S11 is orthorhombic, oP68, s.g. Pca21 (No.29), Z=4 with a=13.775(9)Ä, b=8.043(9)Ä, c=18.306(5)Ä. The crystal structure was determined from diffractometer data and refined to a conventional R of 0.052 (1104 Fo's, 154 variables). It is characterized by asymmetric discrete binuclear moieties [Nb2S11](4-) which are separated by the alkali cations. Each Nb atom is side-on coordinated by two S2(2-) groups, one bridging and one terminal sulfide ligand. Nb-S bond lenghts are 2.15(1)Ä to 2.22(1)Ä (terminal) and 2.44(1)Ä to 2.51(1)Ä (others). A further longer Nb-S bond (2.86(1)Ä and 2.90(1)Ä, resp.) expands the coordination of the two crystallographically independent transition metal atoms to distorted pentagonal bipyramidal configurations. The Nb-Nb-distance is 3.517(3)Ä. The anionic groups are arranged in hexagonal close packed slabs running parallel to (001). The atomic arrangement corresponds to that of K4Ta2S11.