BERYLLIUM HYDRIDE (BEH2): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The beryllium hydride is a covalent compound formed between the metal beryllium and hydrogen alkaline. Its chemical formula is BeH ₂, and being covalent, it does not consist of Be ²⁺ or H ^- ions. It is, together with LiH, one of the lightest metal hydrides capable of being synthesized.

It is produced by treating dimethyl beryllium, Be (CH ₃) ₂, with lithium aluminum hydride, LiAlH ₄. However, the purest BeH ₂ is obtained from the pyrolysis of di-tert-butylberyl, Be (C (CH ₃) ₃) ₂ at 210 ° C.

Source: Ben Mills, from Wikimedia Commons

As an individual molecule in the gaseous state it is linear in geometry, but in the solid and liquid state it polymerizes in arrays of three-dimensional networks. It is an amorphous solid under normal conditions, and it can turn crystalline and exhibit metallic properties under enormous pressure.

It represents a possible method of hydrogen storage, either as a source of hydrogen when decomposing, or as a solid absorbing gas. However, BeH ₂ is very toxic and polluting given the highly polarizing nature of beryllium.

Chemical structure

BeH molecule

The first image shows a single molecule of beryllium hydride in a gaseous state. Note that its geometry is linear, with the H atoms separated from each other by an angle of 180º. To explain this geometry, the Be atom must have sp hybridization.

Beryllium has two valence electrons, which are located in the 2s orbital. According to the valence bond theory, one of the electrons in the 2s orbital is energetically promoted to the 2p orbital; and as a consequence, you can now form two covalent bonds with the two sp hybrid orbitals.

And what about the rest of the Be's free orbitals? Two other pure, non-hybridized 2p orbitals are available. With them empty, BeH ₂ is an electron-deficient compound in gaseous form; and therefore, as its molecules cool and clump together, they condense and crystallize into a polymer.

BeH chains

Source: YourEyesOnly, from Wikimedia Commons

When the BeH ₂ molecules polymerize, the surrounding geometry of the Be atom stops being linear and becomes tetrahedral.

Previously, the structure of this polymer was modeled as if they were chains with BeH ₂ units linked by hydrogen bonds (upper image, with the spheres in white and grayish tones). Unlike the hydrogen bonds of dipole-dipole interactions, they have a covalent character.

In the Be-H-Be bridge of the polymer, two electrons are distributed between the three atoms (bond 3c, 2e), which theoretically should be located with greater probability around the hydrogen atom (because it is more electronegative).

On the other hand, the Be surrounded by four H's manages to relatively fill its electronic vacancy, completing its valence octet.

Here the valence bond theory pales to give a relatively accurate explanation. Why? Because hydrogen can only have two electrons, and the -H- bond would involve four electrons.

Thus, to explain the Be-H ₂ -Be bridges (two gray spheres joined by two white spheres) other complex models of the bond are needed, such as those provided by the molecular orbital theory.

It has been found experimentally that the polymeric structure of BeH ₂ is not actually a chain, but a three-dimensional network.

BeH three-dimensional networks

Source: Ben Mills, from Wikimedia Commons

The upper image shows a section of the three-dimensional network of BeH ₂. Note that the yellowish green spheres, the Be atoms, form a tetrahedron as in the chain; However, in this structure there are a greater number of hydrogen bonds, and in addition, the structural unit is no longer BeH ₂ but BeH ₄.

The same structural units BeH ₂ and BeH ₄ indicate that there is a greater abundance of hydrogen atoms in the lattice (4 H atoms for each Be).

This means that beryllium within this network manages to supply its electronic vacancy even more than within a chain-like polymeric structure.

And as the most obvious difference of this polymer with respect to the individual BeH ₂ molecule, is that Be must necessarily have an sp ³ hybridization (usually) to explain the tetrahedral and non-linear geometries.

Properties

Covalent character

Why is beryllium hydride a covalent and non-ionic compound? The hydrides of the other elements of group 2 (Mr. Becamgbara) are ionic, that is, they consist of solids formed by one M ²⁺ cation and two hydride anions H ^- (MgH ₂, CaH ₂, BaH ₂). Therefore, BeH ₂ does not consist of Be ²⁺ or H ^- interacting electrostatically.

The Be ²⁺ cation is characterized by its high polarizing power, which distorts the electronic clouds of the surrounding atoms.

As a result of this distortion, the H ^- anions are forced to form covalent bonds; links, which are the cornerstone of the structures just explained.

Chemical formula

BeH ₂ or (BeH ₂) n

Physical appearance

Colorless amorphous solid.

Water solubility

It decomposes.

Solubility

Insoluble in diethyl ether and toluene.

Density

0.65 g / cm3 (1.85 g / L). The first value can refer to the gas phase, and the second to the polymeric solid.

Reactivity

It reacts slowly with water, but is rapidly hydrolyzed by HCl to form beryllium chloride, BeCl ₂.

Beryllium hydride reacts with Lewis bases, specifically trimethylamine, N (CH ₃) ₃, to form a dimeric adduct, with bridged hydrides.

Also, it can react with dimethylamine to form trimeric beryllium diamide, _3, and hydrogen. Reaction with lithium hydride, where the H ^- ion is the Lewis base, sequentially forms LIBeH ₃ and Li ₂ BeH ₄.

Applications

Beryllium hydride could represent a promising way to store molecular hydrogen. As the polymer decomposes, it would release H ₂, which would serve as rocket fuel. From this approach, the three-dimensional network would store more hydrogen than the chains.

Likewise, as can be seen in the image of the network, there are pores that would allow the H ₂ molecules to be accommodated.

In fact, some studies simulate what such physical storage would be like in crystalline BeH ₂; that is, the polymer subjected to enormous pressure, and what would be its physical properties with different amounts of adsorbed hydrogen.

References

1. Wikipedia. (2017). Beryllium hydride. Recovered from: en.wikipedia.org
2. Armstrong, DR, Jamieson, J. & Perkins, PG Theoret. Chim. Acta (1979) The electronic structures of polymeric beryllium hydride and polymeric boron hydride. 51: 163. doi.org/10.1007/BF00554099
3. Chapter 3: Beryllium Hydride and its Oligomers. Recovered from: shodhganga.inflibnet.ac.in
4. Vikas Nayak, Suman Banger, and UP Verma. (2014). Study of Structural and Electronic Behavior of BeH ₂ as Hydrogen Storage Compound: An Ab Initio Approach. Conference Papers in Science, vol. 2014, Article ID 807893, 5 pages. doi.org/10.1155/2014/807893
5. Shiver & Atkins. (2008). Inorganic chemistry. In The elements of group 1. (Fourth edition). Mc Graw Hill.