Chemistry:Beryllium hydride

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Beryllium hydride
Beryllium-hydride-3D-balls.png
Names
Other names
Beryllium dihydride
Beryllium hydride
Beryllane
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
UNII
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
decomposes
Solubility insoluble in diethyl ether, toluene
Thermochemistry
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)[2]
REL (Recommended)
 Ca C 0.0005 mg/m3 (as Be)[2]
IDLH (Immediate danger)
 Ca [4 mg/m3 (as Be)][2]
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).
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Infobox references
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Beryllium hydride (systematically named poly[beryllane(2)] and beryllium dihydride) is an inorganic compound with the chemical formula (BeH2)n (also written ([BeH2])n or BeH2). 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).

Synthesis

Unlike the other group 2 metals, beryllium does not react with hydrogen.[3] Instead, BeH2 is prepared from preformed beryllium(II) compounds. It was first synthesised in 1951 by treating dimethylberyllium, Be(CH3)2, with lithium aluminium hydride, LiAlH4.[4]

Purer BeH2 forms from the pyrolysis of di-tert-butylberyllium, Be(C(CH3)3)2 at 210 °C.[5]

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

Structure of gaseous BeH2.

Isolated molecules of BeH2 (sometimes called dihydridoberyllium and written [BeH2] to emphasize the differences with the solid state) are only stable as a dilute gas. When condensed, unsolvated BeH2 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 hybridisation is sp.[6]

Condensed Beryllium hydride

Subunit of structure of BeH2. Each Be is tetrahedral and each H is doubly bridging.[7]

BeH2 is usually formed as an amorphous white solid, but a hexagonal crystalline form with a higher density (~0.78 g cm−3) was reported,[8] 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.[7]

Studies of the amorphous form also find that it consists of a network of corner shared tetrahedra.[9]

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.[3]

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]

[BeH2] + L → [BeH2L]

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.[3] 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.[3]

References

  1. ↑ 1.0 1.1 1.2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 115. ISBN 978-0-08-037941-8. 
  2. ↑ 2.0 2.1 2.2 NIOSH Pocket Guide to Chemical Hazards. "#0054". National Institute for Occupational Safety and Health (NIOSH). https://www.cdc.gov/niosh/npg/npgd0054.html. 
  3. ↑ 3.0 3.1 3.2 3.3 Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN:0-12-352651-5, p. 1048
  4. ↑ Glenn D. Barbaras; Clyde Dillard; A. E. Finholt; Thomas Wartik; K. E. Wilzbach; H. I. Schlesinger (1951). "The Preparation of the Hydrides of Zinc, Cadmium, Beryllium, Magnesium and Lithium by the Use of Lithium Aluminum Hydride". Journal of the American Chemical Society 73 (10): 4585–4590. doi:10.1021/ja01154a025. 
  5. ↑ G. E. Coates; F. Glockling (1954). "Di-tert.-butylberyllium and beryllium hydride". Journal of the Chemical Society: 2526–2529. doi:10.1039/JR9540002526. 
  6. ↑ Peter F. Bernath; Alireza Shayesteh; Keith Tereszchuk; Reginald Colin (2002). "The Vibration-Rotation Emission Spectrum of Free BeH2". Science 297 (5585): 1323–1324. doi:10.1126/science.1074580. PMID 12193780. Bibcode: 2002Sci...297.1323B. 
  7. ↑ 7.0 7.1 Gordon S. Smith; Quintin C. Johnson; Deane K. Smith; D. E. Cox; Robert L. Snyder; Rong-Sheng Zhou; Allan Zalkin (1988). "The crystal and molecular structure of beryllium hydride". Solid State Communications 67 (5): 491–494. doi:10.1016/0038-1098(84)90168-6. Bibcode: 1988SSCom..67..491S. 
  8. ↑ G. J. Brendel; E. M. Marlett; L. M. Niebylski (1978). "Crystalline beryllium hydride". Inorganic Chemistry 17 (12): 3589–3592. doi:10.1021/ic50190a051. 
  9. ↑ Sujatha Sampath; Kristina M. Lantzky; Chris J. Benmore; Jörg Neuefeind; Joan E. Siewenie (2003). "Structural quantum isotope effects in amorphous beryllium hydride". J. Chem. Phys. 119 (23): 12499. doi:10.1063/1.1626638. Bibcode: 2003JChPh.11912499S. 
  10. ↑ Sharp, Stephanie B.; Gellene, Gregory I. (23 November 2000). "σ Bond Activation by Cooperative Interaction with ns2 Atoms: Be + n H2, n = 1−3". The Journal of Physical Chemistry A 104 (46): 10951–10957. doi:10.1021/jp002313m. 
  11. ↑ Shepherd Jr., Lawrence H.; Ter Haar, G. L.; Marlett, Everett M. (April 1969). "Amine complexes of beryllium hydride". Inorganic Chemistry 8 (4): 976–979. doi:10.1021/ic50074a051. 



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