Phosphorus trifluoride

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

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Physical properties

Phosphorus trifluoride has a bond angle of 96.3°. Gaseous PF₃ has a standard enthalpy of formation of -945 kJ/mol (-226 kcal/ mol). The phosphorus atom has an NMR chemical shift of 97 ppm (downfield of H₃PO₄).

Properties

Phosphorus trifluoride hydrolyses 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 oxidised by oxidising agents such as bromine or potassium permanganate.

As a ligand for transition metals, PF₃ is a strong π-acceptor.^[2] 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.^[3][4][5] 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 + 6PF₃ → Cr(PF₃)₆ + 2C₆H₆

Preparation

Phosphorus trifluoride is usually prepared from phosphorus trichloride via halogen exchange using various fluorides, e.g. hydrogen fluoride, calcium fluoride, arsenic trifluoride, antimony trifluoride, or zinc fluoride:^[6][7][8]

2PCl₃ + 3ZnF₂ → 2PF₃ + 3ZnCl₂

Biological activity

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

Precautions

PF₃ is highly toxic, comparable to phosgene.^[9]

References

1. ^ J. Chatt, Nature 165, 637-8 (1950).
2. ^ N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997, p 494
3. ^ D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
4. ^ Kruck, T.“Trifluorphosphin-Komplexe von Übergangsmetallen” Angewandte Chemie 1967, volume 79, p 27-43. DOI: 10.1002/ange.19670790104
5. ^ Clark, R. J.; Busch, M. A. “Stereochemical studies of metal carbonylphosphorus trifluoride complexes” Accounts of Chemical Research, 1973, volume 6, pages 246-52.DOI: 10.1021/ar50067a005.
6. ^ A. A. Williams, in Inorganic Syntheses, Vol. V, 95-7 (1946).
7. ^ Nouveau traité de chimie minérale : Tome X, Masson, Paris, France, 1956.
8. ^ Ronald J. Clark, Helen Belefant, Stanley M. Williamson (1990). "Phosphorus Trifluoride". Inorganic Syntheses 28: 310-315. doi:10.1002/9780470132593.ch77.
9. ^ Greenwood, 1997

• Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
• J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
• The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
• A. D. F. Toy, The Chemistry of Phosphorus, Pergamon Press, Oxford, UK, 1973.