Beryllium hydride

Beryllium hydride
Names
Other names
Beryllium dihydride
Beryllium hydride
Beryllane
Identifiers
CAS Number
3D model (JSmol)
ChEBI
ChemSpider
PubChem CID
UNII
CompTox Dashboard (EPA)
InChI
  • InChI=1S/Be.2H checkY
    Key: RWASOQSEFLDYLC-UHFFFAOYSA-N checkY
  • InChI=1/Be.2H/rBeH2/h1H2
    Key: RWASOQSEFLDYLC-JICJMJRQAQ
SMILES
  • [BeH2]
Properties
Chemical formula
 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]
Solubility in water
 decomposes
Solubility insoluble in diethyl ether, toluene
Structure[2]
Crystal structure
 orthorhombic
Space group
 Ibam, no. 72
Lattice constant
a = 4.1600 Å, b = 9.0820 Å, c = 7.7070 Å
α = 90°, β = 90°, γ = 90°[2]
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).
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Infobox references

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).

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

Structure of gaseous BeH2.

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

Subunit of orthorhombic BeH2 structure. Each Be is tetrahedral and each H is doubly bridging.[2]

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]

References

  1. ^ a b c d Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 115. doi:10.1016/C2009-0-30414-6. ISBN 978-0-08-037941-8.
  2. ^ a b c d e f 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. Bibcode:1988SSCom..67..491S. doi:10.1016/0038-1098(84)90168-6.
  3. ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0054". National Institute for Occupational Safety and Health (NIOSH).
  4. ^ a b c d Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5, p. 1048
  5. ^ 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.
  6. ^ G. E. Coates & F. Glockling (1954). "Di-tert.-butylberyllium and beryllium hydride". Journal of the Chemical Society: 2526–2529. doi:10.1039/JR9540002526.
  7. ^ Peter F. Bernath; Alireza Shayesteh; Keith Tereszchuk; Reginald Colin (2002). "The Vibration-Rotation Emission Spectrum of Free BeH2". Science. 297 (5585): 1323–1324. Bibcode:2002Sci...297.1323B. doi:10.1126/science.1074580. PMID 12193780. S2CID 40961746.
  8. ^ 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. Bibcode:2003JChPh.11912499S. doi:10.1063/1.1626638.
  9. ^ a b c G. J. Brendel; E. M. Marlett & L. M. Niebylski (1978). "Crystalline beryllium hydride". Inorganic Chemistry. 17 (12): 3589–3592. doi:10.1021/ic50190a051.
  10. ^ Sharp, Stephanie B.; Gellene, Gregory I. (23 November 2000). "σ Bond Activation by Cooperative Interaction with ns2 Atoms: Be + n H
    2    , 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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