Beryllium hydride

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.[3] Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded[1] (three-center two-electron bond).

For the monohydride, see Beryllium monohydride.
Beryllium hydride
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 amorphous 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, calcium hydride, boron hydrides
Related compounds
 beryllium fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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 synthesised 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

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,[7] 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.[8]

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.[4]

BeH2 + 2 H2O → Be(OH)2 + 2 H2
BeH2 + 2 HCl → BeCl2 + 2 H2

Reaction with Lewis bases

Beryllium hydride reacts with trimethylamine, N(CH3)3 to form a dimeric adduct, with bridging hydrides.[10] However, with dimethylamine, HN(CH3)2 it forms a trimeric beryllium diamide, [Be(N(CH3)2)2]3 and hydrogen.[4] The reaction with lithium hydride where the hydride ion is the Lewis base, forms sequentially LiBeH3 and Li2BeH4.[4]

Dihydridoberyllium
Structure of gaseous BeH2.

Dihydridoberyllium is a related compound with the chemical formula BeH
2 (also written [BeH
2]). It is a gas that cannot persist undiluted. Unsolvated dihydridoberyllium will spontaneously autopolymerise to oligomers. Free molecular BeH2 produced by electrical discharge at high temperature has been confirmed as linear with a Be-H bond length of 133.376 pm. Its hybridisation is sp. [11]

Chemical properties

In theory, the two-coordinate hydridoberyllium group (-BeH) in hydridoberylliums such as dihydridoberyllium can accept an electron-pair donating ligand into the molecule by adduction:[12]

[BeH
2] + L → [BeH
2L]

Because of this acceptance of the electron-pair donating ligand (L), dihydridoberyllium has Lewis-acidic character. Dihydridoberyllium can accept two electron pairs from ligands, as in the case of the tetrahydridoberyllate(2-) anion (BeH2−
4).

References
  1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8., p. 115
  2. NIOSH Pocket Guide to Chemical Hazards. "#0054". National Institute for Occupational Safety and Health (NIOSH).
  3. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  4. 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". J. Am. Chem. Soc. 73 (10): 4585–4590. doi:10.1021/ja01154a025.
  6. G. E. Coates & F. Glockling (1954). "Di-tert.-butylberyllium and beryllium hydride". J. Chem. Soc.: 2526–2529. doi:10.1039/JR9540002526.
  7. G. J. Brendel; E. M. Marlett & L. M. Niebylski (1978). "Crystalline beryllium hydride". Inorg. Chem. 17 (12): 3589–3592. doi:10.1021/ic50190a051.
  8. 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.
  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. Bibcode:2003JChPh.11912499S. doi:10.1063/1.1626638.
  10. 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.
  11. 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.
  12. 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.
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