The gas phase molecular structures of phosphorus pentafluoride and pentachloride

4 The gas phase molecular structures of phosphorus pentafluoride and pentachloride 

Phosphorus forms a pentafluoride, PF5, which is gaseous at room temperature, and solid compounds of composition PCI5, PBrs and PI5. Evaporation of the pentachloride yields a gas consisting of PCI5, PCI3 and CI2 in a temperature and pressure dependent equilibrium  

The mole fraction of PCI5 in the saturated vapor over a solid sample of PCI5 at 90 °C is about 75%. Evaporation of the pentabromide yields a gas consisting largely of PBr3 and Br2 the mole fraction of PBrs in saturated vapor at 65 is less than 2%. The pentaiodide has never been observed in the gas phase. 

The gas phase molecular structures of PP5 and PCI5 have been determined by gas electron diffraction. See Fig. 15.5. 

In both molecules the coordination polyhedron of the P atom is a trigonal bipyramid or tbp. The phosphorus and the two axial halogen atoms define a vertical threefold symmetry axis, the three equatorial halogen atoms a horizontal symmetry plane. In addition to the threefold symmetry axis and the horizontal symmetry plane, each molecule has a twofold symmetry axes through each of the three equatorial P-X bonds. The symmetry of a molecules with one n-fold symmetry axis and n twofold symmetry axes perpendicular to it are denoted by D . The presence of a horizontal symmetry plane is indicated by the additional subscript h. The symmetry of PF5 or PCI5 then, is 1)3.  

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