Magnesium hydride is the chemical compound with the molecular formula MgH
2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.
Preparation
In 1951 preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500 °C) with
MgI2 catalyst:
:Mg + H
2 → MgH
2
Lower temperature production from Mg and H
2 using nano crystalline Mg produced in
ball mills has been investigated. Other preparations include:
* the hydrogenation of
magnesium anthracene under mild conditions:
:Mg(anthracene) + H
2 → MgH
2
* the reaction of diethylmagnesium with
lithium aluminium hydride
* product of complexed MgH
2 e.g. MgH
2.THF by the reaction of
phenylsilane and dibutyl magnesium in ether or hydrocarbon solvents in the presence of
THF or
TMEDA as ligand.
Structure and bonding
The room temperature form α-MgH
2 has a
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visib ...
structure. There are at least four high pressure forms: γ-MgH
2 with α-PbO
2 structure, cubic β-MgH
2 with Pa-3
space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...
, orthorhombic HP1 with Pbc2
1 space group and orthorhombic HP2 with Pnma space group. Additionally a non stoichiometric MgH
(2-δ) has been characterised, but this appears to exist only for very small particles
(bulk MgH
2 is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies).
The bonding in the rutile form is sometimes described as being partially covalent in nature rather than purely ionic; charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated.
Molecular forms of magnesium hydride, MgH, MgH
2, Mg
2H, Mg
2H
2, Mg
2H
3, and Mg
2H
4 molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10 K, formed following laser ablation of magnesium in the presence of hydrogen.
The Mg
2H
4 molecule has a bridged structure analogous to dimeric
aluminium hydride, Al
2H
6.
Reactions
MgH
2 readily reacts with water to form hydrogen gas:
:MgH
2 + 2 H
2O → 2 H
2 + Mg(OH)
2
At 287 °C it decomposes to produce H
2 at 1 bar pressure.
Extract of page 65
/ref> The high temperature required is seen as a limitation in the use of MgH2 as a reversible hydrogen storage medium:
References
{{Magnesium compounds
Magnesium compounds
Metal hydrides