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Phosphine and Arsine

Module by: Andrew R. Barron. E-mail the author

Because of their use in metal organic chemical vapor deposition (MOCVD) of 13-15 (III-V) semiconductor compounds phosphine (PH3) and arsine (AsH3) are prepared on an industrial scale.

Synthesis

Phosphine (PH3) is prepared by the reaction of elemental phosphorus (P4) with water, Equation 1. Ultra pure phosphine that is used by the electronics industry is prepared by the thermal disproportionation of phosphorous acid, Equation 2.

graphics1.jpg
(1)
graphics2.jpg
(2)

Arsine can be prepared by the reduction of the chloride, Equation 3 or Equation 4. The corresponding syntheses can also be used for stibine and bismuthine.

graphics3.jpg
(3)
AsH3.jpg
(4)

The hydrolysis of calcium phosphide or arsenide can also generate the trihydrides.

Structure

The phosphorus in phosphine adopts sp3 hybridization, and thus phosphine has an umbrella structure (Figure 1a) due to the stereochemically active lone pair. The barrier to inversion of the umbrella (Ea = 155 kJ/mol) is much higher than that in ammonia (Ea = 24 kJ/mol). Putting this difference in context, ammonia’s inversion rate is 1011 while that of phosphine is 103. As a consequence it is possible to isolate chiral organophosphines (PRR'R"). Arsine adopts the analogous structure (Figure 1b).

Figure 1: The structures of (a) phosphine and (b) arsine.
Figure 1 (graphics5.jpg)

Reactions

Phosphine is only slightly soluble in water (31.2 mg/100 mL) but it is readily soluble in non-polar solvents. Phosphine acts as neither an acid nor a base in water; however, proton exchange proceeds via the phosphonium ion (PH4+) in acidic solutions and via PH2- at high pH, with equilibrium constants Kb = 4 x 10-28 and Ka = 41.6 x 10-29, respectively.

Arsine has similar solubility in water to that of phosphine (i.e., 70 mg/100 mL), and AsH3 is generally considered non-basic, but it can be protonated by superacids to give isolable salts of AsH4+. Arsine is readily oxidized in air, Equation 5.

graphics6.jpg
(5)

Arsine will react violently in presence of strong oxidizing agents, such as potassium permanganate, sodium hypochlorite or nitric acid. Arsine decomposes to its constituent elements upon heating to 250 - 300 °C.

Gutzeit test

The Gutzeit test is the characteristic test for arsenic and involves the reaction of arsine with Ag+. Arsine is generated by reduction of aqueous arsenic compounds, typically arsenites, with Zn in the presence of H2SO4. The evolved gaseous AsH3 is then exposed to silver nitrate either as powder or as a solution. With solid AgNO3, AsH3 reacts to produce yellow Ag4AsNO3, while with a solution of AgNO3 black Ag3As is formed.

Hazards

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). The presence of P2H4 also causes spontaneous combustion in air. Phosphine is highly toxic; symptoms include pain in the chest, a sensation of coldness, vertigo, shortness of breath, and at higher concentrations lung damage, convulsions and death. The recommended limit (RL) is 0.3 ppm.

Arsine is a colorless odorless gas that is highly toxic by inhalation. Owing to oxidation by air it is possible to smell a slight, garlic-like scent when arsine is present at about 0.5 ppm. Arsine attacks hemoglobin in the red blood cells, causing them to be destroyed by the body. Further damage is caused to the kidney and liver. Exposure to arsine concentrations of 250 ppm is rapidly fatal: concentrations of 25 – 30 ppm are fatal for 30 min exposure, and concentrations of 10 ppm can be fatal at longer exposure times. Symptoms of poisoning appear after exposure to concentrations of 0.5 ppm and the recommended limit (RL) is as low as 0.05 ppm.

Bibliography

  • R. Minkwitz, A. Kornath, W. Sawodny, and H. Härtner, Z. Anorg. Allg. Chem., 1994, 620, 753.

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