Beryllium hydride

For the monohydride, see Beryllium monohydride.
Beryllium hydride
Names
Other names
Beryllium dihydride
Beryllium hydride
Beryllane
Identifiers
7787-52-2 
ChEBI CHEBI:33787 Yes
ChemSpider 17215712 Yes
Jmol-3D images Image
PubChem 139073
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
Specific
heat capacity (C)
 30.124 J/mol K
Related compounds
Other cations
 lithium hydride, calcium hydride, boron hydrides
Related compounds
 beryllium fluoride
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Beryllium hydride (systematically named beryllium dihydride) is an inorganic compound with the chemical formula (BeH
2)n (also written ([BeH
2])n or BeH
2). It is a colourless solid that is insoluble in solvents that do not decompose it.[2] 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 involve the reaction of triphenylphosphine, PPh3, with beryllium borohydride, Be(BH4)2:[1]

Be(BH4)2 + 2 PPh3 → 2 Ph3PBH3 + BeH2

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,[6] 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.[8]

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]

Reaction with Lewis bases

Beryllium hydride reacts with trimethylamine, N(CH3)3 to form a dimeric aduct, with bridging hydrides.[9] However with dimethylamine, HN(CH3)2 it forms a trimeric beryllium diamide, [Be(N(CH3)2)2]3 and hydrogen.[3] The reaction with lithium hydride where the hydride ion is the Lewis base, forms sequentially LiBeH3 and Li2BeH4.[3]

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. [10]

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:[11]

[BeH
2] + L → [BeH
2L]

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

References

  1. ↑ 1.0 1.1 1.2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419., p. 115
  2. ↑ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  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, and 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.
  5. ↑ G. E. Coates and F. Glockling (1954). "Di-tert.-butylberyllium and beryllium hydride". J. Chem. Soc.: 2526–2529. doi:10.1039/JR9540002526.
  6. ↑ G. J. Brendel, E. M. Marlett, and L. M. Niebylski (1978). "Crystalline beryllium hydride". Inorg. Chem. 17 (12): 3589–3592. doi:10.1021/ic50190a051.
  7. ↑ Gordon S. Smith, Quintin C. Johnson, Deane K. Smith, D. E. Cox, Robert L. Snyder, Rong-Sheng Zhou and 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.
  8. ↑ Sujatha Sampath, Kristina M. Lantzky, Chris J. Benmore, Jörg Neuefeind, and Joan E. Siewenie (2003). "Structural quantum isotope effects in amorphous beryllium hydride". J. Chem. Phys. 119 (23): 12499. doi:10.1063/1.1626638.
  9. ↑ Shepherd Jr., Lawrence H.; Ter Haar, G. L.; Marlett, Everett M. (April 1969). "Amine complexes of beryllium hydride" (PDF). Inorganic Chemistry (American Chemical Society) 8 (4): 976–979. doi:10.1021/ic50074a051. Retrieved 16 October 2013.
  10. ↑ 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.
  11. ↑ 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 (ACS Publications) 104 (46): 10951–10957. doi:10.1021/jp002313m.