Tungsten hexafluoride, structure

Another class of octahedral structures whose stereochemistry was established by NMR is tungsten hexafluoride complexes with donor molecules, e.g., WF6-(C6H6)3P (87a). For these, the F19 spectra consist of a low-field doublet, a medium-field quintet and a high-field singlet of relative intensities 4 1 1. The doublet and quintet resonances establish the geometry... 

Flu. 6. The structure of tungsten hexafluoride showing the approximate double-hexagonal close packing of fluorine atoms. Hatched circles show atoms at x = 0, double circles those at x = j, single circles those at x - 5, and crossed circles those at x = corresponding to the sequence ABAC. Metal atoms are shown as small, dashed circles. 

Tungsten hexafluoride, as well as other transition-metal hexafluorides, exists as a symmetrical octahedral structure with the symmetry point group Oh near room temperature and in a phase of lower symmetry below the transition point [59,60]. The Raman and infrared spectra [61], infrared spectra [62] and NMR [63] were measured. 

All of these hexafluorides are dimorphic, with a high-temperature, cubic form and an orthorhombic form, stable below the transition temperature (92). The cubic form corresponds to a body-centered arrangement of the spherical units, with very high thermal disorder of the molecules in the lattice, leading to a better approximation to a sphere. Recently, the structures of the cubic forms of molybdenum (93) and tungsten (94) hexafluorides have been studied using neutron powder data, with the profile-refinement method and Kubic Harmonic analysis. In both compounds the fluorine density is nonuniformly distributed in a spherical shell of radius equal to the M—F distance. Thus, rotation is not completely free, and there is some preferential orientation of fluorine atoms along the axial directions. The M—F distances are the same as in the gas phase and in the orthorhombic form. 

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