Beryllium hydride

Beryllium hydride (systematically named poly[beryllane(2)] and beryllium dihydride) is an inorganic compound with the chemical formula $2$ (also written $−1$ or ). This alkaline earth hydride is a colourless solid that is insoluble in solvents that do not decompose it. Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded (three-center two-electron bond).

Synthesis
Unlike the other group 2 metals, beryllium does not react with hydrogen. Instead, BeH$3$ is prepared from preformed beryllium(II) compounds. It was first synthesized in 1951 by treating dimethylberyllium, Be(CH$3$)$3$, with lithium aluminium hydride, LiAlH$3$.

Purer BeH$3$ forms from the pyrolysis of di-tert-butylberyllium, Be(C[CH$3$]$n$)$n$ at 210°C.

A route to highly pure samples involves the reaction of triphenylphosphine, PPh$2$, with beryllium borohydride, Be(BH$3$)$2$:
 * Be(BH$4$)$2$ + 2 PPh$3$ → BeH$3$ + 2 Ph$2$PBH$3$

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 BeH$4$ produced by high-temperature electrical discharge has been confirmed to have linear geometry with a Be-H bond length of 133.376 pm. Its hybridization is sp.

Condensed Beryllium hydride
BeH$2$ is usually formed as an amorphous white solid, but a hexagonal crystalline form with a higher density (~0.78 g/cm$4$) was reported, prepared by heating amorphous BeH$2$ 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 BeH$3$ tetrahedra, in contrast to the flat, hydrogen-bridged, infinite chains previously thought to exist in crystalline BeH$2$.

Studies of the amorphous form also find that it consists of a network of corner shared tetrahedra.

Reaction with water and acids
Beryllium hydride reacts slowly with water but is rapidly hydrolysed by acid such as hydrogen chloride to form beryllium chloride.


 * BeH$3$ + 2 H$3$O → Be(OH)$2$ + 2 H$2$


 * BeH$2$ + 2 HCl → BeCl$3$ + 2 H$2$

Reaction with Lewis bases
The two-coordinate hydridoberyllium group can accept an electron-pair donating ligand (L) into the molecule by adduction:
 * + L →

Because these reactions are energetically favored, beryllium hydride has Lewis-acidic character.

The reaction with lithium hydride (in which the hydride ion is the Lewis base), forms sequentially LiBeH$2$ and Li$4$BeH$2$. The latter contains the tetrahydridoberyllate(2-) anion.

Beryllium hydride reacts with trimethylamine, N(CH$2$)$2$ to form a dimeric adduct with bridging hydrides. However, with dimethylamine, HN(CH$2$)$2$ it forms a trimeric beryllium diamide, [Be(N(CH$2$)$2$)$2$]$2$, and hydrogen.