Phosphorus pentafluoride

Phosphorus pentafluoride, PF5, is a phosphorus halide. The phosphous atom is in the highest oxidation state possible, +5. PF5 is a highly reactive compound, existing as a colourless gas at room temperature and pressure.

Structure
Single-crystal X-ray studies indicate PF5 molecule has two distinct P−F bonds (axial and equatorial): P−Fax = 158.0 pm and P−Feq = 152.2 pm. Gas-phase electron diffraction analysis gives similar values: P−Fax = 158 pm and P−Feq = 153 pm.

19F NMR spectroscopy, at temperatures as low as −100 °C fails to distinguish the axial from the equatorial fluorine environments. The apparent equivalency arises from the low barrier for pseudorotation via the Berry mechanism, by which the axial and equatorial fluorine atoms rapidly exchange positions. The apparent equivalency of the F centers in PF5 was first noted by Gutowsky. . The explanation was first described by R. Stephen Berry, after whom the Berry mechanism is named. Berry pseudorotation influences the 19F NMR spectrum of PF5 since NMR spectroscopy operates on a millisecond timescale. Electron diffraction and X-ray crystallography do not detect this effect as their timescales are significantly shorter than for NMR spectroscopy.