Three New Polymorphic Forms of Molybdenum Pentachloride

Abstract

Three new polymorphic modifications of molybdenum pentachloride could be obtained by solvothermal syntheses in CCl 4 and SbCl 5 as solvents. The structures have been solved by single-crystal X-ray diffraction. The already known structure of monoclinic \alpha-MoCl 5 ( C 2/ m ) is not isomorphous with \alpha-NbCl 5 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-MoCl 5 ( P \overline 1) can be derived from hexagonal closest packing. The orthorhombic structure of \gamma-MoCl 5 ( Pnma ) and the monoclinic structure of \delta-MoCl 5 ( P 2 1 / c ) can both be derived from double-hexagonal closest packing. All four forms of MoCl 5 have in common the discrete Mo 2 Cl 10 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 MoCl 5 is a mixture of all four modifications in variable amounts and probably a further unknown form.