The beryllium hydride It is a covalent compound formed between the alkaline earth metal beryllium and hydrogen. Its chemical formula is BeHtwo, and being covalent, it does not consist of Be ionstwo+ nor H-. It is, together with LiH, one of the lightest metal hydrides capable of being synthesized.
It is produced by treating dimethylberyllium, Be (CH3)two, with lithium aluminum hydride, LiAlH4. However, the BeHtwo purer is obtained from the pyrolysis of di-tert-butylberyl, Be (C (CH3)3)two at 210 ºC.
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 can turn crystalline and exhibit metallic properties under enormous pressure..
It represents a possible method of storing hydrogen, either as a source of decomposing hydrogen, or as a solid absorbing gas. However, the BeHtwo it is very toxic and polluting given the highly polarizing nature of beryllium.
Article index
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, the BeHtwo It is an electron-deficient compound in gaseous form; and therefore, as its molecules cool and clump together, they condense and crystallize into a polymer.
When the BeH moleculestwo polymerize, the surrounding geometry of the Be atom ceases to be linear and becomes tetrahedral.
Previously, the structure of this polymer was modeled as if they were chains with BeH unitstwo linked by hydrogen bonds (upper image, with the spheres in white and gray 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 (link 3c, 2e), which theoretically should be located with greater probability around the hydrogen atom (because they are 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 imply the participation of four electrons.
Thus, to explain the Be-H bridgestwo-Be (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 BeHtwo not actually a chain, but a three-dimensional network.
The upper image shows a section of the three-dimensional network of BeHtwo. 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 the BeHtwo but the BeH4.
The same structural units BeHtwo and BeH4 indicate that there is a greater abundance of hydrogen atoms in the network (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 molecule of BeHtwo, is that the Be must necessarily have an sp hybridization3 (usually) to explain tetrahedral and nonlinear geometries.
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 a cation Mtwo+ and two hydride anions H- (MgHtwo, CaHtwo, Bahtwo). Therefore, the BeHtwo does not consist of Betwo+ nor H- interacting electrostatically.
The cation Betwo+ It is characterized by its high polarizing power, which distorts the electronic clouds of the surrounding atoms.
As a result of this distortion, the anions H- they are forced to form covalent bonds; links, which are the cornerstone of the structures just explained.
BeHtwo or (BeHtwo) n
Colorless amorphous solid.
It decomposes.
Insoluble in diethyl ether and toluene.
0.65 g / cm3 (1.85 g / L). The first value can refer to the gas phase, and the second to the polymeric solid.
Reacts slowly with water, but is rapidly hydrolyzed by HCl to form beryllium chloride, BeCltwo.
Beryllium hydride reacts with Lewis bases, specifically trimethylamine, N (CH3)3, to form a dimeric adduct, with bridging hydrides.
Also, it can react with dimethylamine to form trimeric beryllium diamide, [Be (N (CH3)two)two]3 and hydrogen. The reaction with lithium hydride, where the H ion- is the Lewis base, sequentially forms LIBeH3 and LitwoBeH4.
Beryllium hydride could represent a promising way to store molecular hydrogen. As the polymer decomposes, it would release Htwo, 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 molecules to be housedtwo.
In fact, some studies simulate what such physical storage would be like in BeHtwo crystalline; that is, the polymer subjected to enormous pressures, and what would its physical properties be with different amounts of adsorbed hydrogen.
Yet No Comments