Diborane(6), generally known as diborane, is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless, pyrophoric gas with a repulsively sweet odor. Synonyms include boroethane, boron hydride, and diboron hexahydride. Diborane is a key boron compound with a variety of applications. It has attracted wide attention for its electronic structure. Its derivatives are useful reagents.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||27.67 g·mol−1|
|Odor||repulsive and sweet|
|Melting point||−164.85 °C (−264.73 °F; 108.30 K)|
|Boiling point||−92.49 °C (−134.48 °F; 180.66 K)|
|Vapor pressure||39.5 atm (16.6 °C)|
|Tetrahedral (for boron)|
Heat capacity (C)
Std enthalpy of
|Main hazards||highly flammable, reacts with water|
|Safety data sheet||See: data page|
|GHS Signal word||Danger|
|H220, H280, H314, H318, H330, H370, H372|
|P210, P260, P264, P270, P271, P280, P284, P301+330+331, P303+361+353, P304+340, P305+351+338, P307+311, P310, P314, P320, P321, P363, P377, P381, P403, P403+233, P405, P410+403, P501|
|NFPA 704 (fire diamond)|
|38 °C (100 °F; 311 K)|
|Lethal dose or concentration (LD, LC):|
LC50 (median concentration)
|40 ppm (rat, 4 h)|
29 ppm (mouse, 4 h)
40–80 ppm (rat, 4 h)
159–181 ppm (rat, 15 min)
LCLo (lowest published)
|125 ppm (dog, 2 h)|
50 ppm (hamster, 8 h)
|NIOSH (US health exposure limits):|
|TWA 0.1 ppm (0.1 mg/m3)|
|TWA 0.1 ppm (0.1 mg/m3)|
IDLH (Immediate danger)
Related boron compounds
|Supplementary data page|
|Refractive index (n),|
Dielectric constant (εr), etc.
|UV, IR, NMR, MS|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Structure and bondingEdit
The structure of diborane has D2h symmetry. Four hydrides are terminal, while two bridge between the boron centers. The lengths of the B–Hbridge bonds and the B–Hterminal bonds are 1.33 and 1.19 Å respectively. This difference in bond lengths reflects the difference in their strengths, the B–Hbridge bonds being relatively weaker. The weakness of the B–Hbridge compared to B–Hterminal bonds is indicated by their vibrational signatures in the infrared spectrum, being ≈2100 and 2500 cm−1 respectively.
The model determined by molecular orbital theory describes the bonds between boron and the terminal hydrogen atoms as conventional 2-center 2-electron covalent bonds. The bonding between the boron atoms and the bridging hydrogen atoms is, however, different from that in molecules such as hydrocarbons. Each boron uses two electrons in bonding to the terminal hydrogen atoms and has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. The B2H2 ring is held together by four electrons forming two 3-center 2-electron bonds. This type of bond is sometimes called a "banana bond".
Of the other elements in group IIIA, gallium is known to form a similar compound digallane, Ga2H6. Aluminium forms a polymeric hydride, (AlH3)n; although unstable, Al2H6 has been isolated in solid hydrogen and is isostructural with diborane.
Production and synthesisEdit
Extensive studies of diborane have led to the development of multiple syntheses. Most preparations entail reactions of hydride donors with boron halides or alkoxides. The industrial synthesis of diborane involves the reduction of BF3 by sodium hydride, lithium hydride or lithium aluminium hydride:
- 8 BF3 + 6 LiH → B2H6 + 6 LiBF4
- 4 BCl3 + 3 LiAlH4 → 2 B2H6 + 3 LiAlCl4
- 4 BF3 + 3 NaBH4 → 2 B2H6 + 3 NaBF4
- 2 BH4− + 2 H+ → 2 H2 + B2H6
Similarly, oxidation of borohydride salts has been demonstrated and remains convenient for small-scale preparations. For example, using iodine as an oxidizer:
- 2 NaBH
4 + I
2 → 2 NaI + B
6 + H
Another small-scale synthesis uses potassium hydroborate and phosphoric acid as starting materials.
Diborane is a highly reactive and versatile reagent. Its dominating reaction pattern involves formation of adducts with Lewis bases. Often such initial adducts proceed rapidly to give other products. It reacts with ammonia to form the diammoniate of diborane, DADB, with lesser quantities of ammonia borane, depending on the conditions used. Diborane also reacts readily with alkynes to form substituted alkene products, which readily undergo further addition reactions.
but discarded because back then it was too expensive and dangerous to handle.
Diborane also reacts violently with water to form hydrogen and boric acid:
- B2H6 + 6 H2O → 2 B(OH)3 + 6 H2 (ΔHr = −466 kJ/mol = −16.82 kJ/g)
Diborane also reacts with methanol to give hydrogen and trimethoxyborate ester:
- B2H6 + 6 MeOH → 2 B(OMe)3 + 6 H2
- B2H6 + 2 LiH → 2 LiBH4
- B2H6 + HX → B2H5X + H2 (X = Cl, Br)
Reagent in organic synthesisEdit
This section does not cite any sources. (February 2021) (Learn how and when to remove this template message)
Diborane and its variants are central organic synthesis reagents for hydroboration, whereby alkenes add across the B–H bonds to give trialkylboranes. Diborane is used as a reducing agent roughly complementary to the reactivity of lithium aluminium hydride. The compound readily reduces carboxylic acids to the corresponding alcohols, whereas ketones react only sluggishly.
Diborane was first synthesised in the 19th century by hydrolysis of metal borides, but it was never analysed. From 1912 to 1936, Alfred Stock, the major pioneer in the chemistry of boron hydrides, undertook his research that led to the methods for the synthesis and handling of the highly reactive, volatile, and often toxic boron hydrides. He proposed the first ethane-like structure of diborane. Electron diffraction measurements by S. H. Bauer initially appeared to support his proposed structure.
Because of a personal communication with L. Pauling (who supported the ethane-like structure), H. I. Schlessinger and A. B. Burg did not specifically discuss 3-center 2-electron bonding in their then classic review in the early 1940s. The review does, however, discuss the bridged D2h structure in some depth: "It is to be recognized that this formulation easily accounts for many of the chemical properties of diborane..."
In 1943, H. Christopher Longuet-Higgins, while still an undergraduate at Oxford, was the first to explain the structure and bonding of the boron hydrides. The article reporting the work, written with his tutor R. P. Bell, also reviews the history of the subject beginning with the work of Dilthey. Shortly afterwards, the theoretical work of Longuet-Higgins was confirmed in an infrared study of diborane by Price.  The structure was re-confirmed by electron-diffraction measurement in 1951 by K. Hedberg and V. Schomaker, with the confirmation of the structure shown in the schemes on this page.
William Nunn Lipscomb Jr. further confirmed the molecular structure of boranes using X-ray crystallography in the 1950s and developed theories to explain their bonding. Later, he applied the same methods to related problems, including the structure of carboranes, on which he directed the research of future 1981 Nobel Prize winner Roald Hoffmann. The 1976 Nobel Prize in Chemistry was awarded to Lipscomb "for his studies on the structure of boranes illuminating problems of chemical bonding".
Traditionally, diborane has often been described as electron-deficient, because the 12 valence electrons can only form 6 conventional 2-centre 2-electron bonds, which are insufficient to join all 8 atoms. However, the more correct description using 3-centre bonds shows that diborane is really electron-precise, since there are just enough valence electrons to fill the 6 bonding molecular orbitals. Nevertheless, some leading textbooks still use the term "electron-deficient".
Diborane has been tested as a rocket propellant. Complete combustion is strongly exothermic. However, combustion is not complete in the rocket engine, as some boron monoxide, B2O, is produced. This mirrors the incomplete combustion of hydrocarbons, to produce carbon monoxide, CO.
Diborane has been used as a rubber vulcaniser, as a catalyst for hydrocarbon polymerisation, as a flame-speed accelerator, and as a doping agent for the production of semiconductors. It is also an intermediate in the production of highly pure boron for semiconductor production. It is also used to coat the walls of tokamaks to reduce the amount of heavy-metal impurities in the core plasma.
This section does not cite any sources. (February 2021) (Learn how and when to remove this template message)
The toxic effects of diborane are primarily due to its irritant properties. Short-term exposure to diborane can cause a sensation of tightness of the chest, shortness of breath, cough, and wheezing. These signs and symptoms can occur immediately or be delayed for up to 24 hours. Skin and eye irritation can also occur. Studies in animals have shown that diborane causes the same type of effects observed in humans.
- Haynes, p. 4.52.
- NIOSH Pocket Guide to Chemical Hazards. "#0183". National Institute for Occupational Safety and Health (NIOSH).
- Haynes, p. 5.6.
- "Diborane". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
- "CDC – DIBORANE – International Chemical Safety Cards – NIOSH". Archived from the original on 2018-10-07. Retrieved 2018-10-07.
- Cooper, C. B., III; Shriver, D. F.; Onaka, S. (1978). "Ch. 17. Vibrational spectroscopy of hydride-bridged transition metal compounds". Transition Metal Hydrides. Advances in Chemistry. 167. pp. 232–247. doi:10.1021/ba-1978-0167.ch017. ISBN 9780841203907.CS1 maint: multiple names: authors list (link)
- Rasul, G.; Prakash, G. K. S.; Olah, G. A. (2005). "Comparative ab initio Study of the Structures and Stabilities of the Ethane Dication C2H62+ and Its Silicon Analogues Si2H62+ and CSiH62+". Journal of Physical Chemistry A. 109 (5): 798–801. Bibcode:2005JPCA..109..798R. doi:10.1021/jp0404652. PMID 16838949.
- Laszlo, P. (2000). "A Diborane Story". Angewandte Chemie International Edition. 39 (12): 2071–2072. doi:10.1002/1521-3773(20000616)39:12<2071::AID-ANIE2071>3.0.CO;2-C. PMID 10941018.
- Andrews, L.; Wang, X. (2003). "The Infrared Spectrum of Al2H6 in Solid Hydrogen". Science. 299 (5615): 2049–2052. Bibcode:2003Sci...299.2049A. doi:10.1126/science.1082456. PMID 12663923. S2CID 45856199.
- Brauer, Georg (1963). Handbook of Preparative Inorganic Chemistry. 1 (2nd ed.). New York: Academic Press. p. 773. ISBN 978-0121266011.
- Norman, A. D.; Jolly, W. L.; Saturnino, D.; Shore, S. G. (1968). Diborane. Inorganic Syntheses. 11. pp. 15–19. doi:10.1002/9780470132425.ch4. ISBN 9780470132425.
- Mikhailov, B. M. (1962). "The Chemistry of Diborane". Russian Chemical Reviews. 31 (4): 207–224. Bibcode:1962RuCRv..31..207M. doi:10.1070/RC1962v031n04ABEH001281.
- Gammon, Benson E.; Genco, Russell S.; Gerstein, Melvin (1950). A preliminary experimental and analytical evaluation of diborane as a ram-jet fuel (PDF). National Advisory Committee for Aeronautics.
- Tower, Leonard K.; Breitwieser, Roland; Gammon, Benson E. (1958). Theoretical Combustion Performance of Several High-Energy Fuels for Ramjet Engines (PDF). National Advisory Committee for Aeronautics.
- "LIQUID HYDROGEN AS A PROPULSION FUEL, 1945–1959. Part II: 1950–1957. Chapter 5. NACA Research on High-Energy Propellants". history.nasa.gov.
- Housecroft, C. E.; Sharpe, A. G. (2008). "Chapter 13: The Group 13 Elements". Inorganic Chemistry (3rd ed.). Pearson. p. 336. ISBN 978-0-13-175553-6.
- Stock, A. (1933). The Hydrides of Boron and Silicon. New York: Cornell University Press.
- Bauer, S. H. (1937). "The Structure of Diborane". Journal of the American Chemical Society. 59 (6): 1096–1103. doi:10.1021/ja01285a041.
- Bauer, S. H. (1942). "Structures and Physical Properties of the Hydrides of Boron and of their Derivatives". Chemical Reviews. 31 (1): 43–75. doi:10.1021/cr60098a002.
- Schlesinger, H. I.; Burg, A. B. (1942). "Recent Developments in the Chemistry of the Boron Hydrides". Chemical Reviews. 31 (1): 1–41. doi:10.1021/cr60098a001.
- Longuet-Higgins, H. C.; Bell, R. P. (1943). "64. The Structure of the Boron Hydrides". Journal of the Chemical Society (Resumed). 1943: 250–255. doi:10.1039/JR9430000250.
- Dilthey, W. (1921). "Über die Konstitution des Wassers". Angewandte Chemie. 34 (95): 596. doi:10.1002/ange.19210349509.
- Price, W. C. (1948). "The absorption spectrum of diborane". J. Chem. Phys. 16 (9): 894. Bibcode:1948JChPh..16..894P. doi:10.1063/1.1747028.
- Hedberg, K.; Schomaker, V. (1951). "A Reinvestigation of the Structures of Diborane and Ethane by Electron Diffraction". Journal of the American Chemical Society. 73 (4): 1482–1487. doi:10.1021/ja01148a022.
- "The Nobel Prize in Chemistry 1976". Nobelprize.org. Retrieved 2012-02-01.
- Longuet-Higgins, H. C. (1957). "The structures of electron-deficient molecules". Quarterly Reviews, Chemical Society. 11 (2): 121–133. doi:10.1039/qr9571100121. Retrieved 15 July 2020.
- Murrell, J. N.; Kettle, S. F. A.; Tedder, J. M. (1965). Valence theory. John Wiley and Sons. p. 243.
- Lipscomb, William N. (11 December 1976). "The Boranes and their relatives (Nobel lecture)" (PDF). nobelprize.org. Nobel Foundation. pp. 224–245. Retrieved 16 July 2020.
One of the simple consequences of these studies was that electron deficient molecules, defined as having more valence orbitals than electrons, are not really electron deficient.
- Housecroft, Catherine E.; Sharpe, Alan G. (2005). Inorganic Chemistry (2nd ed.). Pearson Prentice-Hall. p. 326. ISBN 0130-39913-2.
An electron-deficient species possesses fewer valence electrons than are required for a localized bonding scheme.
- Bilstein, Roger. "Stages to Saturn". chapter 5: NASA Public Affairs Office. p. 133. Retrieved 14 November 2015.CS1 maint: location (link)