For the monohydride, see Beryllium monohydride.

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.[3] 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.[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 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.[6]

Condensed Beryllium hydride

BeH2 is usually formed as an amorphous white solid, but a hexagonal crystalline form with a higher density (~0.78 g/cm3) was reported,[7] prepared by heating amorphous BeH2 under pressure, with 0.5-2.5% LiH as a catalyst.

Subunit of structure of BeH2. Each Be is tetrahedral and each H is doubly bridging.[8]

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.[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]

[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.[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. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 115. ISBN 978-0-08-037941-8.
  2. ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0054". National Institute for Occupational Safety and Health (NIOSH).
  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 & 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. Bibcode:2002Sci...297.1323B. doi:10.1126/science.1074580. PMID 12193780. S2CID 40961746.
  7. ^ G. J. Brendel; E. M. Marlett & L. M. Niebylski (1978). "Crystalline beryllium hydride". Inorganic Chemistry. 17 (12): 3589–3592. doi:10.1021/ic50190a051.
  8. ^ a b 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. ^ 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.