Three 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. The structures have been solved by single-crystal X-ray diffraction. The already known structure of monoclinic alpha-MoCl5 (C2/m) is not isomorphous with alpha-NbCl5 and is better derived from the closest packing of Cl atoms of the Sm type with molybdenum occupying 1/5 of the octahedral holes. The triclinic structure of beta-MoCl5 (P (1) over bar) can be derived from hexagonal closest packing. The orthorhombic structure of gamma-MoCl5 (Pnma) and the monoclinic structure of delta-MoCl5 (P2(1)/c) can both be derived from double-hexagonal closest packing. All four forms of MoCl5 have in common the discrete Mo2Cl10 moieties built from edge-sharing double octahedra with the metal atoms displaced from the octahedron centres away from each other. The differences between the modifications lie in the different stacking sequences of the close-packed Cl-atom layers and the different occupation of the octahedral interstices. This is reflected in the group-subgroup relationships of the space groups of the closest packings and the molybdenum pentachlorides. X-ray powder diffraction shows that sublimed MoCl5 is a mixture of all four modifications in variable amounts and probably a further unknown form.