Phosphorus trifluoride explained

Phosphorus trifluoride (formula PF₃), is a colorless and odorless gas. It is highly toxic and reacts slowly with water. Its main use is as a ligand in metal complexes. As a ligand, it parallels carbon monoxide in metal carbonyls,^[1] and indeed its toxicity is due to its binding with the iron in blood hemoglobin in a similar way to carbon monoxide.

Physical properties

Phosphorus trifluoride has an F−P−F bond angle of approximately 96.3°. Gaseous PF₃ has a standard enthalpy of formation of −945 kJ/mol (−226 kcal/mol). The phosphorus atom has a nuclear magnetic resonance chemical shift of 97 ppm (downfield of H₃PO₄).

Properties

Phosphorus trifluoride hydrolyzes especially at high pH, but it is less hydrolytically sensitive than phosphorus trichloride. It does not attack glass except at high temperatures, and anhydrous potassium hydroxide may be used to dry it with little loss. With hot metals, phosphides and fluorides are formed. With Lewis bases such as ammonia addition products (adducts) are formed, and PF₃ is oxidized by oxidizing agents such as bromine or potassium permanganate.

As a ligand for transition metals, PF₃ is a strong π-acceptor.It forms a variety of metal complexes with metals in low oxidation states. PF₃ forms several complexes for which the corresponding CO derivatives (see metal carbonyl) are unstable or nonexistent. Thus, Pd(PF₃)₄ is known, but Pd(CO)₄ is not.^{[2] [3] [4]} Such complexes are usually prepared directly from the related metal carbonyl compound, with loss of CO. However, nickel metal reacts directly with PF₃ at 100 °C under 35 MPa pressure to form Ni(PF₃)₄, which is analogous to Ni(CO)₄. Cr(PF₃)₆, the analogue of Cr(CO)₆, may be prepared from dibenzenechromium:

Cr(C₆H₆)₂ + 6 PF₃ → Cr(PF₃)₆ + 2 C₆H₆

Preparation

Phosphorus trifluoride is usually prepared from phosphorus trichloride via halogen exchange using various fluorides such as hydrogen fluoride, calcium fluoride, arsenic trifluoride, antimony trifluoride, or zinc fluoride:^{[5] [6] [7]}

2 PCl₃ + 3 ZnF₂ → 2 PF₃ + 3 ZnCl₂

Biological activity

Phosphorus trifluoride is similar to carbon monoxide in that it is a gas which strongly binds to iron in hemoglobin, preventing the blood from absorbing oxygen.

Precautions

PF₃ is highly toxic, comparable to phosgene.

Further reading

• Book: Toy, A. D. F. . The Chemistry of Phosphorus . Pergamon Press . Oxford, UK . 1973 .
  • Book: Lide, D. R. . Handbook of Chemistry and Physics . registration . 71st . CRC Press . Ann Arbor, MI . 1990 . 978-0-8493-0471-2 .
• Book: March, J. . Advanced Organic Chemistry . 4th . 723 . Wiley . New York . 1992 . 978-0-471-60180-7 .
• Book: Stecher, P. G. . The Merck Index . 7th . Merck & Co . Rahway, NJ, USA . 1960 .
• Holmes, R. R. . An Examination of the Basic Nature of the Trihalides of Phosphorus, Arsenic and Antimony . . 1960 . 12 . 3–4 . 266–275 . 10.1016/0022-1902(60)80372-7 .

External links

• National Pollutant Inventory - Fluoride and compounds fact sheet
• WebBook page for PF3

Notes and References

1. Chatt, J. . The Co-Ordinate Link in Chemistry . Nature . 1950 . 165 . 4199 . 637–638 . 10.1038/165637a0 . 15416738 . 1950Natur.165..637C .
2. Book: Nicholls, D. . Complexes and First-Row Transition Elements . Macmillan Press . London . 1973 .
3. Kruck, T. . Trifluorphosphin-Komplexe von Übergangsmetallen . Angewandte Chemie . 1967 . 79 . 1 . 27–43 . 10.1002/ange.19670790104 . 1967AngCh..79...27K .
4. Clark, R. J. . Busch, M. A. . Stereochemical Studies of Metal Carbonyl-Phosphorus Trifluoride Complexes . Accounts of Chemical Research . 1973 . 6 . 7 . 246–252 . 10.1021/ar50067a005 .
5. Book: Williams, A. A. . Parry, R. W. . Dess, H. . Inorganic Syntheses . Phosphorus(III) Fluoride . . 1957 . 5 . 95–97 . 10.1002/9780470132364.ch26 . 978-0-470-13164-0 .
6. Book: Dubrisay, R. . Pascal, P. . Nouveau Traité de Chimie Minérale . Azote-Phosphore . 1956 . 10 . Masson . Paris, France . 978-2-225-57123-7 .
7. Book: Clark, R. J. . Belefant, H. . Williamson, S. M. . Phosphorus Trifluoride . . 1990 . 28 . 310–315 . 10.1002/9780470132593.ch77 . 978-0-470-13259-3 .