Phosphine

Structure and reactions

PH₃ is a trigonal pyramidal molecule with C_3v molecular symmetry. The length of the P−H bond is 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 phosphorus 3s orbital contributes little to the P-H bonding. For this reason, the lone pair on phosphorus is predominantly formed by the 3s orbital of phosphorus. The upfield chemical shift of its ³¹P NMR signal accords with the conclusion that the lone pair electrons occupy the 3s orbital (Fluck, 1973). This electronic structure leads to a lack of nucleophilicity in general and lack of basicity in particular (pK_aH = −14), as well as an ability to form only weak hydrogen bonds.

The aqueous solubility of PH₃ is slight: 0.22 cm³ of gas dissolves in 1 cm³ of water. Phosphine dissolves more readily in non-polar solvents than in water because of the non-polar P−H bonds. It is technically amphoteric in water, but acid and base activity is poor. Proton exchange proceeds via a phosphonium (PH+4) ion in acidic solutions and via phosphanide (PH−2) at high pH, with equilibrium constants K_b = 4×10⁻²⁸ and K_a = 41.6×10⁻²⁹. Phosphine reacts with water only at high pressure and temperature, producing phosphoric acid and hydrogen:

PH₃ + 4 H₂O H₃PO₄ + 4 H₂

Burning phosphine in the air produces phosphoric acid:

PH₃ + 2 O₂ H₃PO₄.

Preparation and occurrence

Phosphine may be prepared in a variety of ways. Industrially it can be made by the reaction of white phosphorus with sodium or potassium hydroxide, producing potassium or sodium hypophosphite as a by-product.

3 KOH + P₄ + 3 H₂O → 3 KH₂PO₂ + PH₃ 3 NaOH + P₄ + 3 H₂O → 3 NaH₂PO₂ + PH₃

Alternatively, the acid-catalyzed disproportionation of white phosphorus yields phosphoric acid and phosphine. Both routes have industrial significance; the acid route is the preferred method if further reaction of the phosphine to substituted phosphines is needed. The acid route requires purification and pressurizing.

Laboratory routes

It is prepared in the laboratory by disproportionation of phosphorous acid:

4 H₃PO₃ → PH₃ + 3 H₃PO₄
Phosphine evolution occurs at around 200 °C.

Alternative methods include the hydrolysis of zinc phosphide:

Zn₃P₂ + 6 H₂O → 3 Zn(OH)₂ + 2 PH₃

Some other metal phosphides could also be used, including aluminium phosphide or calcium phosphide. Pure samples of phosphine, free from P₂H₄, may be prepared using the action of potassium hydroxide on phosphonium iodide:

[PH₄]I + KOH → PH₃ + KI + H₂O

Occurrence

Phosphine is a worldwide constituent of the Earth's atmosphere at very low and highly variable concentrations. It may contribute significantly to the global phosphorus biochemical cycle. The most likely source is reduction of phosphate in decaying organic matter, possibly via partial reductions and disproportionations, since environmental systems do not have known reducing agents of sufficient strength to directly convert phosphate to phosphine.

It is also found in Jupiter's atmosphere.

Possible extraterrestrial biosignature

In 2020 a spectroscopic analysis was reported to show signs of phosphine in the atmosphere of Venus in quantities that could not be explained by known abiotic processes. Later re-analysis of this work showed interpolation errors had been made, and re-analysis of data with the fixed algorithm do not result in the detection of phosphine. The authors of the original study then claimed to detect it with a much lower concentration of 1 ppb.

Applications

Organophosphorus chemistry

Phosphine is a precursor to many organophosphorus compounds. It reacts with formaldehyde in the presence of hydrogen chloride to give tetrakis(hydroxymethyl)phosphonium chloride, which is used in textiles. The hydrophosphination of alkenes is versatile route to a variety of phosphines. For example, in the presence of basic catalysts PH₃ adds of Michael acceptors. Thus with acrylonitrile, it reacts to give tris(cyanoethyl)phosphine:

PH₃ + 3 CH₂=CHZ → P(CH₂CH₂Z)₃ (Z is NO₂, CN, or C(O)NH₂)

Acid catalysis is applicable to hydrophosphination with isobutylene and related analogues:

PH₃ + R₂C=CH₂ → R₂(CH₃)CPH₂

where R is CH₃, alkyl, etc.

Microelectronics

Phosphine is used as a dopant in the semiconductor industry, and a precursor for the deposition of compound semiconductors. Commercially significant products include gallium phosphide and indium phosphide.

Fumigant (pest control)

Phosphine is an attractive fumigant because it is lethal to insects and rodents, but degrades to phosphoric acid, which is non-toxic. As sources of phosphine, for farm use, pellets of aluminium phosphide (AlP), calcium phosphide (Ca₃P₂), or zinc phosphide (Zn₃P₂) are used. These phosphides release phosphine upon contact with atmospheric water or rodents' stomach acid. These pellets also contain reagents to reduce the potential for ignition or explosion of the released phosphine.

An alternative is the use of phosphine gas itself which requires dilution with either CO₂ or N₂ or even air to bring it below the flammability point. Use of the gas avoids the issues related with the solid residues left by metal phosphide and results in faster, more efficient control of the target pests.

One problem with phosphine fumigants is the increased resistance by insects.

Toxicity and safety

Deaths have resulted from accidental exposure to fumigation materials containing aluminium phosphide or phosphine. It can be absorbed either by inhalation or transdermally. As a respiratory poison, it affects the transport of oxygen or interferes with the utilization of oxygen by various cells in the body. Exposure results in pulmonary edema (the lungs fill with fluid). Phosphine gas is heavier than air so it stays near the floor.

Phosphine appears to be mainly a redox toxin, causing cell damage by inducing oxidative stress and mitochondrial dysfunction. Resistance in insects is caused by a mutation in a mitochondrial metabolic gene.

Phosphine can be absorbed into the body by inhalation. The main target organ of phosphine gas is the respiratory tract. According to the 2009 U.S. National Institute for Occupational Safety and Health (NIOSH) pocket guide, and U.S. Occupational Safety and Health Administration (OSHA) regulation, the 8 hour average respiratory exposure should not exceed 0.3 ppm. NIOSH recommends that the short term respiratory exposure to phosphine gas should not exceed 1 ppm. The Immediately Dangerous to Life or Health level is 50 ppm. Overexposure to phosphine gas causes nausea, vomiting, abdominal pain, diarrhea, thirst, chest tightness, dyspnea (breathing difficulty), muscle pain, chills, stupor or syncope, and pulmonary edema. Phosphine has been reported to have the odor of decaying fish or garlic at concentrations below 0.3 ppm. The smell is normally restricted to laboratory areas or phosphine processing since the smell comes from the way the phosphine is extracted from the environment. However, it may occur elsewhere, such as in industrial waste landfills. Exposure to higher concentrations may cause olfactory fatigue.

Fumigation hazards

Phosphine is used for pest control, but its usage is strictly regulated due to high toxicity. Gas from phosphine has high mortality rate and has caused deaths in Sweden and other countries.

Because the previously popular fumigant methyl bromide has been phased out in some countries under the Montreal Protocol, phosphine is the only widely used, cost-effective, rapidly acting fumigant that does not leave residues on the stored product. Pests with high levels of resistance toward phosphine have become common in Asia, Australia and Brazil. High level resistance is also likely to occur in other regions, but has not been as closely monitored. Genetic variants that contribute to high level resistance to phosphine have been identified in the dihydrolipoamide dehydrogenase gene. Identification of this gene now allows rapid molecular identification of resistant insects.

Explosiveness

Phosphine gas is denser than air and hence may collect in low-lying areas. It can form explosive mixtures with air, and may also self-ignite.

In fiction

Anne McCaffrey's Dragonriders of Pern series features genetically engineered dragons that breathe fire by producing phosphine by extracting it from minerals of their native planet.

In the 2008 pilot of the crime drama television series Breaking Bad, Walter White poisons two rival gangsters by adding red phosphorus to boiling water to produce phosphine gas. However, this reaction in reality would require white phosphorus instead, and for the water to contain sodium hydroxide.