No abstract

When tetraacetatodimolybdenum is dissolved in aqueous hydrobromic acid and a solution of CsBr is added, a precipitate of microcrystalline CssMozBr8 is formed. By a special technique, crystallographically useful single crystals were obtained and the substance has been structurally characterized. The structure consists of Cs + cations and MozBr 3-anions, the latter having the shape of an MzX9 confacial bioctahedron but with crystallographically disordered vacancies in the bridging positions. The dimensions of the Mo2Br~-ion indicate that there is bonding interaction of medium strength between the molybdenum atoms. The crystals belong to the hexagonal system, with space groups P~2c, P63/mmc and P63mc admissible on the basis of systematic absences. P6smc was eliminated convincingly on the basis of unsatisfactory refinement, but no clear choice between the other two space groups was possible using diffraction data. The unit-cellj~parameters are a= 7.757 (3), c= 17.804 (7)A, Z= 2; dcale =4"40; doh~=4"33 g cm -3. The Mo-Mo distance is 2.439 (7); Mo-Br(terminal)=2.554 (3); Mo-Br(bridge)= 2.672 (5) A.