Hydrogen selenide

Hydrogen selenide is H₂Se, the simplest hydride of selenium. H₂Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit: 0.3 ppm over an 8 hour period.

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Structure

H₂Se adopts a "bent" structure with a H-Se-H bond angle of 91°. Consistent with this structure, three IR-active vibrational bands are observed: 2358, 2345, and 1034 cm^-1.

Properties

The properties of H₂S and H₂Se are similar, although the selenide is more acidic with pK_a = 3.89, and the second pK_a = 11.0 at 25°C. Reflecting its acidity, H₂Se is soluble in water.

Preparation

H₂Se can be purchased in cylinders. A number of routes to H₂Se have been reported, which are suitable for both large and small scale preparations.

• H₂Se is usually prepared by the action of water on Al₂Se₃, concomitant with formation of hydrated alumina. A related reaction involves the acid hydrolysis of FeSe.

Al₂Se₃ + 6 H₂O ⇌ 2 Al(OH)₃ + 3 H₂Se

• According to the Sonoda method, H₂Se is generated from the reaction of H₂O and CO on Se in the presence of Et₃N.
• H₂Se can by synthesized by the direct reaction of H₂ with elemental selenium at >350°C.

Selenium metal can be recovered from H₂Se through a reaction with aqueous sulfur dioxide (SO₂).

2 H₂Se + SO₂ ⇌ 2 H₂O + Se + S

Applications

• H₂Se is commonly used in the synthesis of Se-containing compounds; it adds across alkenes. Illustrative is the synthesis of selenoureas from nitriles.

• H₂Se gas is used to dope semiconductors with selenium.

References

1. Patnaik, Pradyot Handbook of Inorganic Chemicals. McGraw-Hill Handbooks New York (2003).
2. V.I. Cohen, Synthesis 1:60-3 (1980).
3. Greenwood, N. N., Earnshaw, A. Chemistry of the Elements Second Ed. Butterworth-Heinemann, Boston (1997).
4. CRC Handbook of Chemistry and Physics Ed. 82. Chapman and Hall (2001-2002).

• Feher, F. In "Handbook of Preparative Inorganic Chemistry"; Brauer, E., Ed.; Academic: New York, 1963; 1, p 418.
• Sonoda, N.; Kondo K.; Nagano, K.; Kambe, N.; Morimoto, F. Angewandte Chemie, International Edition English 1980, vol. 19, page 308