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 -MoCl₅ (C2/m) is not isomorphous with -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 -MoCl₅ (P) can be derived from hexagonal closest packing. The orthorhombic structure of -MoCl₅ (Pnma) and the monoclinic structure of -MoCl₅ (P2₁/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.

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