3 NEW POLYMORPHIC FORMS OF MOLYBDENUM PENTACHLORIDE

Three new polymorphic modifications of molybdenum pentachloride could be obtained by solvothermal syntheses in CCl4 and SbCl5 as solvents. T he structures have been solved by single-crystal X-ray diffraction. Th e already known structure of monoclinic alpha-MoCl5 (C2/m) is not isom orphous with alpha-NbCl5 and is better derived from the closest packin g of Cl atoms of the Sm type with molybdenum occupying 1/5 of the octa hedral holes. The triclinic structure of beta-MoCl5 (P (1) over bar) c an be derived from hexagonal closest packing. The orthorhombic structu re of gamma-MoCl5 (Pnma) and the monoclinic structure of delta-MoCl5 ( P2(1)/c) can both be derived from double-hexagonal closest packing. Al l four forms of MoCl5 have in common the discrete Mo2Cl10 moieties bui lt from edge-sharing double octahedra with the metal atoms displaced f rom the octahedron centres away from each other. The differences betwe en the modifications lie in the different stacking sequences of the cl ose-packed Cl-atom layers and the different occupation of the octahedr al interstices. This is reflected in the group-subgroup relationships of the space groups of the closest packings and the molybdenum pentach lorides. X-ray powder diffraction shows that sublimed MoCl5 is a mixtu re of all four modifications in variable amounts and probably a furthe r unknown form.