Phosphorus pentafluoride

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Phosphorus pentafluoride
Structure of the phosphorus pentafluoride molecule
Space-filling model of the phosphorus pentafluoride molecule
Identifiers
CAS number 7647-19-0 YesY
PubChem 24295
RTECS number TH4070000
Properties
Molecular formula PF5
Molar mass 125.966 g/mol
Appearance colourless gas
Density 5.527 g/cm3
Melting point −93.78 °C (−136.80 °F; 179.37 K)
Boiling point −84.6 °C (−120.3 °F; 188.6 K)
Solubility in water hydrolysis
Structure
Molecular shape trigonal bipyramidal
Dipole moment 0 D
Hazards
EU Index Not listed
Flash point Non-flammable
Related compounds
Other anions Phosphorus pentachloride
Phosphorus pentabromide
Phosphorus pentaiodide
Other cations Arsenic pentafluoride
Antimony pentafluoride
Bismuth pentafluoride
Related compounds Phosphorus trifluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Phosphorus pentafluoride, PF5, is a phosphorus halide. It is a colourless gas at room temperature and pressure.

Structure[edit]

Single-crystal X-ray studies indicate that the PF5 molecule has two distinct types of P−F bonds (axial and equatorial): the length of an axial P−F bond is 158.0 pm and the length of an equatorial P−F bond is 152.2 pm. Gas-phase electron diffraction analysis gives similar values: the axial P−F bonds are 158 pm long and the equatorial P−F bonds are 153 pm long.

Fluorine-19 NMR spectroscopy, even 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.[1] 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 the solid state structures are, relative to a molecule in solution, static and can not undergo the necessary changes in atomic position.

References[edit]

  1. ^ Gutowsky, H. S.; McCall, D. W.; Slichter, C. P. (1953). "Nuclear Magnetic Resonance Multiplets in Liquids". J. Chem. Phys. 21 (2): 279. doi:10.1063/1.1698874.