Hydrogen selenide is an inorganic compound with the formula H<sub>2</sub>Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium. H<sub>2</sub>Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit of 0.05 ppm over an 8-hour period. Even at extremely low concentrations, this compound has a very irritating smell resembling that of decayed horseradish or "leaking gas", but smells of rotten eggs at higher concentrations.

Structure and properties

H<sub>2</sub>Se 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&nbsp;cm<sup>−1</sup>.

The properties of H<sub>2</sub>S and H<sub>2</sub>Se are similar, although the selenide is more acidic with pK<sub>a</sub> = 3.89 and the second pK<sub>a</sub> = 11,

Preparation

Industrially, it is produced by treating <em>elemental selenium</em> at T&nbsp;>&nbsp;300&nbsp;°C with hydrogen gas.

::Al<sub>2</sub>Se<sub>3</sub> + 6 H<sub>2</sub>O ⇌ 2 Al(OH)<sub>3</sub> + 3 H<sub>2</sub>Se

H<sub>2</sub>Se can also be prepared by means of different methods based on the in situ generation in aqueous solution using boron hydride, Marsh test and Devarda's alloy. According to the Sonoda method, H<sub>2</sub>Se is generated from the reaction of H<sub>2</sub>O and CO on Se in the presence of Et<sub>3</sub>N. H<sub>2</sub>Se can be purchased in cylinders.

Reactions

Elemental selenium can be recovered from H<sub>2</sub>Se through a reaction with aqueous sulfur dioxide (SO<sub>2</sub>).

::2 H<sub>2</sub>Se + SO<sub>2</sub> ⇌ 2 H<sub>2</sub>O + 2 Se + S

Its decomposition is used to prepare the highly pure element.

Applications

H<sub>2</sub>Se is commonly used in the synthesis of Se-containing compounds. It adds across alkenes. Illustrative is the synthesis of selenoureas from cyanamides:

:273px|Selenourea reaction

H<sub>2</sub>Se gas is used to dope semiconductors with selenium.

Safety

Hydrogen selenide is hazardous, being the most toxic selenium compound

Despite being extremely toxic, no human fatalities have yet been reported. It is suspected that this is due to the gas' tendency to oxidise to form red selenium in mucous membranes; elemental selenium is less toxic than selenides are.