Beryllium hydride
Encyclopedia
Beryllium hydride, BeH2, is a chemical compound
of beryllium
and hydrogen
commonly used in rocket fuel. Unlike the ionically bonded
hydride
s of the heavier Group 2
elements, beryllium hydride is covalently bonded
.
, LiAlH4.
Purer BeH2 forms from the pyrolysis
of di-tert-butylberyllium, Be(C(CH3)3)2 at 210 °C.
The purest beryllium hydride is obtained by the reaction of triphenylphosphine
, PPh3, with beryllium borohydride, Be(BH4)2:
Note that unlike the other elements in group 2
where the hydride can be prepared by reaction of the elements, the reaction of the metal with hydrogen to produce beryllium hydride has not proved possible.
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
unit cell, 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.
Molecular BeH2 has been confirmed in a recent study as linear with a Be-H bond length of 133.376 pm.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
of beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
commonly used in rocket fuel. Unlike the ionically bonded
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
s of the heavier Group 2
Alkaline earth metal
The alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
elements, beryllium hydride is covalently bonded
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
.
Synthesis
BeH2 was first synthesised in 1951 by reacting dimethylberyllium, Be(CH3)2, with lithium aluminium hydrideLithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
, LiAlH4.
Purer BeH2 forms from the pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of di-tert-butylberyllium, Be(C(CH3)3)2 at 210 °C.
The purest beryllium hydride is obtained by the reaction of triphenylphosphine
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
, PPh3, with beryllium borohydride, Be(BH4)2:
- Be(BH4)2 + 2PPh3 → 2Ph3PBH3 + BeH2
Note that unlike the other elements in group 2
Alkaline earth metal
The alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
where the hydride can be prepared by reaction of the elements, the reaction of the metal with hydrogen to produce beryllium hydride has not proved possible.
Structure
BeH2 is usually formed as an amorphous white solid, but a hexagonal crystalline formHexagonal crystal system
In crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families...
with a higher density (~0.78 g cm−3) was reported, prepared by heating amorphous BeH2 under pressure, with 0.5-2.5% LiH
Lithium hydride
Lithium hydride is the inorganic compound with the formula LiH. It is a colorless solid, although commercial samples are gray. Characteristic of a salt-like, or ionic, hydride, it has a high melting point and is not soluble in any solvent with which it does not react...
as a catalyst.
A more recent investigation found that crystalline beryllium hydride has a body-centred orthorhombic
Orthorhombic crystal system
In crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
unit cell, 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.
Molecular BeH2 has been confirmed in a recent study as linear with a Be-H bond length of 133.376 pm.