Beryllium hydride

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Beryllium hydride
Beryllium-hydride-3D-balls.png
Identifiers
CAS number 7787-52-2 N
PubChem 139073
ChemSpider 17215712 YesY
ChEBI CHEBI:33787 YesY
Jmol-3D images Image 1
Properties
Molecular formula BeH2
Molar mass 11.03 g mol−1
Appearance amorphous white solid[1]
Density 0.65 g/cm3
Melting point 250 °C with decomp.[1]
Solubility in water decomposes
Related compounds
Other cations lithium hydride, calcium hydride, boron hydrides
Related compounds beryllium fluoride
Except where noted otherwise, 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 beryllium dihydride) is an inorganic compound with the chemical formula (BeH
2
)n (also written ([BeH
2
]
)n or BeH
2
). It is a solid that is insoluble in solvent that do not decompose it.[2] Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded.[1]

Synthesis[edit]

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[edit]

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[edit]

Reaction with water and acids[edit]

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[edit]

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]

Molecular BeH2 (Dihydridoberyllium)[edit]

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]

Structure of gaseous BeH2.

References[edit]

  1. ^ a b c d 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. ^ a b c d 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.