HYDROGEN SELENIDE (H2SE): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The selenhídrico acid or hydrogen selenide is an inorganic compound with the chemical formula H ₂ Se. It is covalent in nature, and under ordinary conditions of temperature and pressure it is a colorless gas; but with a strong smell recognizable to its minor presence. Chemically, it is a chalcogenide, so selenium has a valence of -2 (Se ^2-).

Of all the selenides, H ₂ Se is the most toxic because its molecule is small and its selenium atom is less sterically hindered when reacting. On the other hand, its odor allows those who work with it to detect it immediately in case of a leak outside the laboratory hood.

Hydrogen selenide can be synthesized by the direct combination of its two elements: molecular hydrogen, H ₂, and metallic selenium. It can also be obtained by dissolving selenium-rich compounds, such as iron (II) selenide, FeSe, in hydrochloric acid.

On the other hand, hydrogen selenide is prepared by dissolving hydrogen selenide in water; that is, the former is dissolved in water, while the latter consists of gaseous molecules.

Its main use is to be a source of selenium in organic and inorganic synthesis.

Structure of hydrogen selenide

Hydrogen selenide molecule. Source: Ben Mills

The image above shows that the H ₂ Se molecule has angular geometry, although its 91 ° angle makes it look more like an L than a V. In this model of spheres and rods, the hydrogen atoms and that of selenium are the white and yellow spheres, respectively.

This molecule, as shown, is the one in the gas phase; that is, for hydrogen selenide. When dissolved in water, it releases a proton and in solution we have the pair HSe ^- H ₃ O ⁺; this pair of ions comes to hydrogen selenide, denoted H ₂ Se (aq) to differentiate it from hydrogen selenide, H ₂ Se (g).

Therefore, the structures between H ₂ Se (ac) and H ₂ Se (g) are very different; the first is surrounded by an aqueous sphere and has ionic charges, and the second consists of an agglomerate of molecules in the gas phase.

H ₂ Se molecules can barely interact with each other by very weak dipole-dipole forces. Selenium, although it is less electronegative than sulfur, concentrates a higher electron density by "taking it away" from the hydrogen atoms.

Selenium hydrides tablets

If the H ₂ Se molecules are subjected to extraordinary pressure (hundreds of GPa), theoretically they are forced to solidify through the formation of Se-H-Se bonds; These are bonds of three centers and two electrons (3c-2e) where hydrogen participates. Therefore, the molecules begin to form polymeric structures that define a solid.

Under these conditions, the solid can be enriched with more hydrogen, which completely modifies the resulting structures. Furthermore, the composition becomes of the H _n Se type, where n varies from 3 to 6. Thus, the selenium hydrides compressed by these pressures, and in the presence of hydrogen, have chemical formulas H ₃ Se to H ₆ Se.

These hydrogen-enriched selenium hydrides are believed to have superconducting properties.

Properties

Physical appearance

Colorless gas that at low temperatures smells like rotten radish and rotten eggs if its concentration increases. Its smell is worse and more intense than that of hydrogen sulfide (which is already quite unpleasant). However, this is a good thing, as it helps in its easy detection and reduces the risks of prolonged contact or inhalation.

When it burns, it gives off a bluish flame produced by electronic interactions in the selenium atoms.

Molecular mass

80.98 g / mol.

Boiling point

-41 ° C.

Melting point

-66 ° C.

Vapor pressure

9.5 atm at 21 ° C.

Density

3.553 g / L.

pK

3.89.

Water solubility

0.70 g / 100 mL. This corroborates the fact that the selenium atom in H ₂ Se cannot form appreciable hydrogen bonds with water molecules.

Solubility in other solvents

-Soluble in CS ₂, which is not surprising from the chemical analogy between selenium and sulfur.

-Soluble in phosgene (at low temperatures, as it boils at 8 ° C).

Nomenclature

As already explained in previous sections, the name of this compound varies depending on whether H ₂ Se is in the gaseous phase or dissolved in water. When it is in water, it is referred to as hydrogen selenhydric acid, which is nothing more than a hydracid in inorganic terms. Unlike gaseous molecules, its acid character is greater.

However, whether as a gas or dissolved in water, the selenium atom maintains the same electronic characteristics; for example, its valence is -2, unless it undergoes an oxidation reaction. This valence of -2 is the reason why Seleni called uro hydrogen selenide as the anion is is ^2-; which is more reactive and reducing than S ^2-, sulfur.

If you use systematic nomenclature, you have to specify the number of hydrogen atoms in the compound. Thus, the H ₂ is called: selenide di hydrogen.

Selenide or hydride?

Some sources refer to it as a hydride. If it really were, selenium would be positively charged +2, and hydrogen negatively charged -1: SeH ₂ (Se ²⁺, H ^-). Selenium is a more electronegative atom than hydrogen, and therefore ends up “hoarding” the highest electron density in the H ₂ Se molecule.

However, as such the existence of selenium hydride cannot be theoretically ruled out. In fact, with the presence of the H ^- anions, it would facilitate the Se-H-Se bonds, responsible for the solid structures formed at enormous pressures according to computational studies.

Applications

Metabolic

Although it seems contradictory, despite the great toxicity of H ₂ Se, it is produced in the body in the metabolic pathway of selenium. However, as soon as it is produced, cells use it as an intermediate in the synthesis of selenium proteins, or it ends up being methylated and excreted; one of the symptoms of this is the taste of garlic in the mouth.

Industrial

H ₂ is used mainly to add selenium atoms to solid structures, such as semiconductor materials; to organic molecules, such as alkenes and nitriles for the synthesis of organic selenides; or to a solution to precipitate metal selenides.

References

1. Wikipedia. (2018). Hydrogen selenide. Recovered from: en.wikipedia.org
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3. Atomistry. (2012). Hydrogen Selenide, H ₂ Se. Recovered from: selenium.atomistry.com
4. Tang Y. & col. (2017). Hydrogen Selenide (H ₂ Se) Dopant Gas for Selenium Implantation. 21st International Conference on Ion Implantation Technology (IIT). Tainan, Taiwan.
5. Chemical formulation. (2018). Hydrogen selenide. Recovered from: formulacionquimica.com
6. PubChem. (2019). Hydrogen selenide. Recovered from: pubchem.ncbi.nlm.nih.gov
7. Zhang, S. et al. (2015). Phase Diagram and High-Temperature Superconductivity of Compressed Selenium Hydrides. Sci. Rep. 5, 15433; doi: 10.1038 / srep15433.
8. Acids.Info. (2019). Selenhydric acid: properties and applications of this hydracid. Recovered from: acidos.info/selenhidrico