Phosphorus pentafluoride, P F₅, is a

phosphorus halide In chemistry, there are three series of binary phosphorus halides, containing phosphorus in the oxidation states +5, +3 and +2. All compounds have been described, in varying degrees of detail, although serious doubts have been cast on the existenc ...

. It is a colourless, toxic gas that fumes in air.

Preparation

Phosphorus pentafluoride was first prepared in 1876 by the fluorination of phosphorus pentachloride using

arsenic trifluoride Arsenic trifluoride is a chemical compound of arsenic and fluorine with the chemical formula AsF3. It is a colorless liquid which reacts readily with water. Preparation and properties It can be prepared by reacting hydrogen fluoride, HF, with ar ...

, which remains a favored method: :3 PCl₅ + 5 AsF₃ → 3 PF₅ + 5 AsCl₃

Structure

Single-crystal X-ray studies indicate that the PF₅ has

trigonal bipyramidal geometry In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. This is one geometry for which the bond angles surrounding the central atom are not identi ...

. Thus it has two distinct types of P−F bonds (axial and equatorial): the length of an axial P−F bond is distinct from the equatorial P−F bond in the solid phase, but not the liquid or gas phases due to Berry pseudo rotation. 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. The explanation was first described by

R. Stephen Berry Richard Stephen Berry (April 9, 1931 – July 26, 2020) was an American professor of physical chemistry. He was the James Franck Distinguished Service Professor Emeritus at The University of Chicago. He was also Special Advisor for National Sec ...

, after whom the Berry mechanism is named. Berry pseudorotation influences the ¹⁹F NMR spectrum of PF₅ since NMR spectroscopy operates on a

millisecond A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be called ...

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.

Lewis acidity

Phosphorus pentafluoride is a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

. This property is relevant to its ready hydrolysis. A well studied adduct is PF₅ with pyridine. With primary and secondary amines, the adducts convert readily to dimeric amido-bridged derivatives with the formula F₄(NR₂)sub>2. A variety of complexes are known with bidentate ligands.

Hexafluorophosphoric acid Hexafluorophosphoric acid is the inorganic compound with the chemical formula (also written ). This strong Brønsted acid features a non-coordinating anion, hexafluorophosphate (). It is formed from the reaction of hydrogen fluoride Hydroge ...

(HPF₆) is derived from phosphorus pentafluoride and

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

. Its conjugate base, hexafluorophosphate (PF₆^–), is a useful non-coordinating anion.

References

{{fluorine compounds Phosphorus(V) compounds Phosphorus fluorides