Magnesium hydride
# Magnesium hydride

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 $MgI_2$ catalyst:

    Mg + $H_2$ → $MgH_2$

Lower temperature production from Mg and $H_2$ using nanocrystalline 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 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, 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. The high temperature required is seen as a limitation in the use of $MgH_2$ as a reversible hydrogen storage medium:

    $MgH_2$ → Mg + $H_2$
