More reduced Nb6I8 cluster. Synthesis and structure of CsNb6I11 and its hydride CsNb6I11H

The title compound is formed by direct reaction of Nb3I8 or Nb6I11 with Nb and CsI in a sealed Nb tube at 9100C. The structure and composition were established by X-ray crystallography: space group P6322, Z = 2, a = 11.007 (2) A, c = 11.894 (2) A, R = 0.047, and R/sub w/ = 0.060 for 777 independent reflections after correction for absorption and secondary extinction. The structure of Cs+[(Nb6I/sup i/8)I/sup a//sub 6/2/]- consists of distorted metal octahedra face-capped by I/sup i/ to give Nb6I8 clusters (D/sub 3d/ symmetry) which are interconnected to other octahedra in a hexagonal close-packed arrangement through bridging I/sup a/. The Nb-Nb distances range from 2.771 (2) to 2.940 (2) A, the average of 2.825 A being 0.025 (4) A less than that in the known and closely related Nb6I11(Nb6I8I/sub 6/2/). The observed distortion of the octahedra in both phases can be understood in terms of packing and strain at the bridging iodine atoms. The title phase is converted to the isostructural CsNb6I11H by H2 at 4000C, whereas Mo6CL12, Na4M6Cl18 (M = Nb, Ta), and Ta6Cl15 do not react with hydrogen at 300 to 400 0C. Hydrogen absorption by halides of transition groups 5 and 6 and zirconium appears to require an oxidation state below 2.0. The upper limits of composition (oxidation state) found for group 3 to 6 halohydrides correspond closely to the upper limits found with the binary hydrides of the same metals