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Beryllium hydride (systematically named poly eryllane(2)'' and beryllium dihydride) is an
Isolated molecules of (sometimes called dihydridoberyllium and written to emphasize the differences with the solid state) are only stable as a dilute gas. When condensed, unsolvated will spontaneously autopolymerise.
Free molecular BeH2 produced by high-temperature electrical discharge has been confirmed to have linear geometry with a Be-H bond length of 133.376 pm. Its hybridisation is ''sp''.
BeH2 is usually formed as an amorphous white solid, but a hexagonal crystalline form with a higher density (~0.78 g cm−3) was reported, prepared by heating amorphous BeH2 under pressure, with 0.5-2.5% LiH as a catalyst.
A more recent investigation found that crystalline beryllium hydride has a body-centred
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...
with the chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
()''n'' (also written ()''n'' or ). This alkaline earth hydride is a colourless solid that is insoluble in solvents that do not decompose it. Unlike the ionically bonded hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...
s of the heavier Group 2 The term Group 2 may refer to:
* Alkaline earth metal, a chemical element classification
* Astronaut Group 2, also known as The New Nine, the second group of astronauts selected by NASA in 1962
* Group 2 (racing)
The Group 2 racing class referred ...
elements, beryllium hydride is covalently bonded (three-center two-electron bond
A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one ''non''-bonding, and one ''anti''-b ...
).
Synthesis
Unlike the othergroup 2 The term Group 2 may refer to:
* Alkaline earth metal, a chemical element classification
* Astronaut Group 2, also known as The New Nine, the second group of astronauts selected by NASA in 1962
* Group 2 (racing)
The Group 2 racing class referred ...
metals, beryllium does not react with hydrogen.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier , p. 1048 Instead, BeH2 is prepared from preformed beryllium(II) compounds. It was first synthesised in 1951 by treating dimethylberyllium
Organoberyllium chemistry involves the synthesis and properties of organometallic compounds featuring the group 2 alkaline earth metal beryllium (Be). Beryllium is best known to have a +2 oxidation state and one of the smallest atoms and it is un ...
, Be(CH3)2, with lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li Al H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic ...
, LiAlH4.
Purer BeH2 forms from the pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements '' ...
of di-tert-butylberyllium, Be(C(CH3)3)2 at 210 °C.
A route to highly pure samples involves the reaction of triphenylphosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists ...
, PPh3, with beryllium borohydride, Be(BH4)2:
:Be(BH4)2 + 2 PPh3 → BeH2 + 2 Ph3PBH3
Structure
Gaseous form
Isolated molecules of (sometimes called dihydridoberyllium and written to emphasize the differences with the solid state) are only stable as a dilute gas. When condensed, unsolvated will spontaneously autopolymerise.
Free molecular BeH2 produced by high-temperature electrical discharge has been confirmed to have linear geometry with a Be-H bond length of 133.376 pm. Its hybridisation is ''sp''.
Condensed Beryllium hydride
BeH2 is usually formed as an amorphous white solid, but a hexagonal crystalline form with a higher density (~0.78 g cm−3) was reported, prepared by heating amorphous BeH2 under pressure, with 0.5-2.5% LiH as a catalyst.
A more recent investigation found that crystalline beryllium hydride has a body-centred orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
, containing a network of corner-sharing BeH4 tetrahedra, in contrast to the flat, hydrogen-bridged, infinite chains previously thought to exist in crystalline BeH2.
Studies of the amorphous form also find that it consists of a network of corner shared tetrahedra.
Chemical properties
Reaction with water and acids
Beryllium hydride reacts slowly with water but is rapidly hydrolysed by acid such ashydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chlorid ...
to form beryllium chloride
Beryllium chloride is an inorganic compound with the formula BeCl2. It is a colourless, hygroscopic solid that dissolves well in many polar solvents. Its properties are similar to those of aluminium chloride, due to beryllium's diagonal relatio ...
.
:BeH2 + 2 H2O → Be(OH)2 + 2 H2
:BeH2 + 2 HCl → BeCl2 + 2 H2
Reaction with Lewis bases
The two-coordinate hydridoberyllium group can accept an electron-pair donatingligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
(L) into the molecule by adduction:
: + L →
Because these reactions are energetically favored, beryllium hydride has Lewis-acidic character.
The reaction with lithium hydride
Lithium hydride is an inorganic compound with the formula Li H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not solub ...
(in which the hydride ion is the Lewis base), forms sequentially LiBeH3 and Li2BeH4. The latter contains the tetrahydridoberyllate(2-) anion .
Beryllium hydride reacts with trimethylamine
Trimethylamine (TMA) is an organic compound with the formula N(CH3)3. It is a colorless, hygroscopic, and flammable tertiary amine. It is a gas at room temperature but is usually sold as a 40% solution in water. (It is also sold in pressurized ...
, N(CH3)3 to form a dimeric adduct with bridging hydrides. However, with dimethylamine
Dimethylamine is an organic compound with the formula (CH3)2NH. This secondary amine is a colorless, flammable gas with an ammonia-like odor. Dimethylamine is commonly encountered commercially as a solution in water at concentrations up to arou ...
, HN(CH3)2 it forms a trimeric beryllium diamide, e(N(CH3)2)2sub>3, and hydrogen.
References
{{Hydrides by group Beryllium compounds Metal hydrides Substances discovered in the 1950s