Phosphine

Phosphine is the common name for phosphorus hydride (PH₃), also known by the IUPAC name phosphane and, occasionally, phosphamine. It is a colorless, flammable gas with a boiling point of −88 °C at standard pressure. Pure phosphine is odorless, but technical grade phosphine has a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine (P₂H₄). Phosphines are also a group of substituted phosphines, with the structure R₃P, where other functional groups replace hydrogens. They are important in catalysts where they complex to various metal ions; a chiral metal phosphine complex can catalyze a reaction to give chiral products.

Phosphine is highly toxic; it can easily kill in relatively low concentrations. Because of this, the gas is used for pest control by fumigation. For farm use, it is often sold in the form of aluminium phosphide, calcium phosphide, or zinc phosphide pellets, which yield phosphine on contact with atmospheric water or rodents' stomach acid. These pellets also contain other chemicals which evolve ammonia which helps to reduce the potential for spontaneous ignition or explosion of the phosphine gas. They may also contain other agents, such as methanethiol, to give the gas a detectable garlic smell to help warn against its presence in the atmosphere.

Phosphine is also used as a dopant in the semiconductor industry, and a precursor for the deposition of compound semiconductors. Recently high purity tertiary butyl phosphine (TBP) has been developed as a less hazardous liquid alternative to highly toxic phosphine gas, for application in Metalorganic Vapor Phase Epitaxy (MOVPE) of III-V compound semiconductors.

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History

Perhaps because of its strong association with elemental phosphorus, phosphine was once regarded as a gaseous form of the element but Lavoisier (1789) recognised it as a combination of phosphorus with hydrogen by describing it as “hydruyet of phosphorus, or phosphuret of hydrogen”.

Ernst von Meyer (1891) described the early history of phosphine research thus: "The discovery of phosphuretted hydrogen (PH₃) by Gengembre in 1783, and the examination of it by Pelletier (who was the first to prepare it pure), only became fruitful after Humphry Davy’s investigations; and the last-named elucidated the composition of this gas, and pointed out its analogy to ammonia, this being emphasised still more sharply by H. Rose later on."

Thénard (1845) used a cold trap to separate diphosphine from phosphine that had been generated from calcium phosphide, thereby demonstrating that P₂H₄ is responsible for spontaneous flammability associated with PH₃, and also for the characteristic orange/brown colour that can form on surfaces, which is a polymerisation product. He considered diphosphine’s formula to be PH₂, and thus an intermediate between elemental phosphorus, the higher polymers, and phosphine. Calcium phosphide (nominally Ca₃P₂) produces more P₂H₄ than other phosphides because of the preponderance of P-P bonds in the starting material.

Structure and properties

PH₃ is a trigonal pyramidal molecule with C_3v molecular symmetry. The length of the P-H bond 1.42 Å, the H-P-H bond angles are 93.5°. The dipole moment is 0.58 D, which increases with substitution of methyl groups in the series: CH₃PH₂, 1.10 D; (CH₃)₂PH, 1.23 D; (CH₃)₃P, 1.19 D. In contrast, the dipole moments of amines decrease with substitution, starting with ammonia, which has a dipole moment of 1.47 D. The low dipole moment and almost orthogonal bond angles lead to the conclusion that in PH₃ the P-H bonds are almost entirely pσ(P) – sσ(H) and the lone pair contributes only a little to the molecular orbitals. The high positive chemical shift of the P atom in³¹P NMR spectrum accords with the conclusion that the lone pair electrons occupy the 3s orbital and so are close to the P atom (Fluck, 1973). This electronic structure leads to a lack of nucleophilicity and an inability to form hydrogen bonds.

The aqueous solubility of PH₃ is slight; 0.22 mL of gas dissolve in 1 mL of water. Phosphine dissolves more readily in non-polar solvents than in water because of the non-polar P-H bonds. It acts as neither an acid nor a base in water. Proton exchange proceeds via a phosphonium (PH₄⁺) ion in acidic solutions and via PH₂^- at high pH, with equilibrium constants K_b = 4 x 10^-28 and K_z = 41.6 x 10^-29.

Chemistry

Phosphine may be prepared in a variety of ways^[2]. Industrially it can be made by the reaction of white phosphorus with sodium hydroxide, producing sodium hypophosphite and sodium phosphite as a by-product. Alternatively the acid-catalyzed disproportioning of white phosphorus may be used, which yields phosphoric acid and phosphine. Both routes have industrial significance, with the acid route as the preferred method if further reaction of the phosphine to substituted phosphines is needed. This latter step requires purification and pressurizing. It can also be made (as described above) by the hydrolysis of a metal phosphide such as aluminium phosphide or calcium phosphide. Pure samples of phosphine, free from P₂H₄, may be prepared using the action of potassium hydroxide on phosphonium iodide (PH₄I).

Phosphines

Related to PH₃ is the class of compounds commonly called phosphines. These are alkyl or aryl derivatives of phosphine, just as amines can be regarded as derivatives of ammonia. Common examples include triphenylphosphine ((C₆H₅)₃P) and BINAP, both used as phosphine ligands in metal complexes such as Wilkinson's catalyst. Metal phosphine complexes are catalysts for reactions such as the Sonogashira coupling. Most of these phosphines, with the exception of triphenyl phosphine, are made from pressurized, purified phosphine gas as described above.

A large industrial application of phosphine is found in the production of tetrakis(hydroxymethyl) phosphonium salts, made by passing phosphine gas through a solution of formaldehyde and a mineral acid such as hydrochloric acid. These find application as flame retardants for textile ("Proban(r) - registered trademark of Rhodia UK Limited") and as biocides.

Phosphine is often confused with phosgene, (COCl₂) which has a similar-sounding name but contains no phosphorus.

Use as a fumigant

Phosphine is highly toxic to organisms undergoing oxidative respiration, but is non toxic to organisms kept under low oxygen (<1%) or that can anaerobically respire (i.e. ferment). Because of these characteristics, phosphine is widely used as a fumigant of metabolically dormant stored products such as grain. The toxicity of phosphine kills insect pests that might infest the grain, but does not affect the viability of the dormant grain.

Because continued use of the previously widely used fumigant methyl bromide has been banned under the Montreal Protocol, phosphine is the only widely used, cost effective, rapidly acting fumigant that does not leave residues on the stored product. Given the heavy reliance on phosphine as a means of protecting grain from insect infestation, it is disturbing to note that pests developing high levels of resistance toward phosphine have become commonplace in many countries of Asia and in Australia as well. Active research in Australia into the mode of action of phosphine and the mechanisms whereby insects acquire resistance is being carried out by the CSIRO in Canberra, QDPI&F in Queensland and the University of Queensland.

See also

• Phosphine oxide - OPR₃
• Phosphinite - P(OR)R₂
• Phosphonite - P(OR)₂R
• Phosphite - P(OR)₃
• Phosphinate - OP(OR)R₂
• Phosphonate - OP(OR)₂R
• Phosphate - OP(OR)₃

References

1. E. Fluck, The chemistry of phosphine, Topics in Current Chemistry Vol. 35, 64 pp, 1973.
2. A.D.F. Toy, The Chemistry of Phosphorus, Pergamon Press, Oxford, UK, 1973.
3. WHO (World Health Organisation), Phosphine and selected metal phosphides, Environmental Health Criteria. Published under the joint sponsorship of UNEP, ILO and WHO, Geneva, Vol. 73, 100 pp, 1988.