Beryllium hydride
Chemical compound
From Wikipedia, the free encyclopedia
Beryllium hydride (systematically named poly[beryllane(2)] and beryllium dihydride) is an inorganic compound with the chemical formula (BeH
2)n (also written ([BeH
2])n or BeH
2). 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[1] (three-center two-electron bond).
| |
| Names | |
|---|---|
| Other names
Beryllium dihydride Beryllium hydride Beryllane | |
| Identifiers | |
3D model (JSmol) |
|
| ChEBI | |
| ChemSpider | |
PubChem CID |
|
| UNII | |
CompTox Dashboard (EPA) |
|
| |
| |
| Properties | |
| BeH2 | |
| Molar mass | 11.03 g mol−1 |
| Appearance | white solid[1] |
| Density | 0.65 g/cm3 |
| Melting point | 250 °C (482 °F; 523 K) decomposes[1] |
| decomposes | |
| Solubility | insoluble in diethyl ether, toluene |
| Structure[2] | |
| orthorhombic | |
| Ibam, no. 72 | |
a = 4.1600 Å, b = 9.0820 Å, c = 7.7070 Å | |
Lattice volume (V) |
291.18 Å3 |
Formula units (Z) |
12 |
| Thermochemistry | |
Heat capacity (C) |
30.124 J/mol K |
| Hazards | |
| NIOSH (US health exposure limits): | |
PEL (Permissible) |
TWA 0.002 mg/m3 C 0.005 mg/m3 (30 minutes), with a maximum peak of 0.025 mg/m3 (as Be)[3] |
REL (Recommended) |
Ca C 0.0005 mg/m3 (as Be)[3] |
IDLH (Immediate danger) |
Ca [4 mg/m3 (as Be)][3] |
| Related compounds | |
Other cations |
lithium hydride, sodium hydride, magnesium hydride, calcium hydride, boron hydrides, aluminium hydride |
Related compounds |
beryllium fluoride |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
Synthesis
Unlike the other group 2 metals, beryllium does not react with hydrogen.[4] Instead, BeH2 is prepared from preformed beryllium(II) compounds. It was first synthesized in 1951 by treating dimethylberyllium, Be(CH3)2, with lithium aluminium hydride, LiAlH4.[5]
Purer BeH2 forms from the pyrolysis of di-tert-butylberyllium, Be(C[CH3]3)2 at 210°C.[6]
A route to highly pure samples involves the reaction of triphenylphosphine, PPh3, with beryllium borohydride, Be(BH4)2:[1]
- Be(BH4)2 + 2 PPh3 → BeH2 + 2 Ph3PBH3
Structure
Gaseous form
Isolated molecules of BeH
2 (sometimes called dihydridoberyllium and written [BeH
2] to emphasize the differences with the solid state) are only stable as a dilute gas. When condensed, unsolvated BeH
2 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 hybridization is sp.[7]
Condensed beryllium hydride
BeH2 most often is formed as an amorphous white powder by decomposing various organo-beryllium compounds[2], which has been shown to consist of a network of corner sharing tetrahedra.[8]
Early work showed that there are multiple phases of BeH2, with the low temperature, higher-density phase shown to have a hexagonal crystalline form by selected-area electron diffraction (SAED)[9], but this structure has not been solved to date. The other phase was argued to have either a monoclinic or tetragonal cell.[9]
Further synchrotron Powder X-ray diffraction studies of the higher-density phase of BeH2 have shown crystalline beryllium hydride to have a body-centered orthorhombic unit cell (space group Ibam), containing a network of corner-sharing BeH4 tetrahedra, in contrast to the flat, hydrogen-bridged, infinite chains previously thought to exist in crystalline BeH2.[2][9] The Be-H bond length is significantly shorter in for the orthorhomibic crystal shown than would be assumed in the hydrogen-bridged infinite chain model, with greater bond angles found in the BeH2 crystal structure compared to the 90° angles expected in the hydrogen-bridged model.[2] Before the structure was solved by synchrotron X-ray diffraction, it was thought that there was hydrogen-bridging.
Chemical properties
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.[4]
- 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 donating ligand (L) into the molecule by adduction:[10]
- [BeH
2] + L → [BeH
2L]
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 LiBeH3 and Li2BeH4.[4] The latter contains the tetrahydridoberyllate(2-) anion BeH2−
4.
Beryllium hydride reacts with trimethylamine, N(CH3)3 to form a dimeric adduct with bridging hydrides.[11] However, with dimethylamine, HN(CH3)2 it forms a trimeric beryllium diamide, [Be(N(CH3)2)2]3, and hydrogen.[4]