borane

BH₃ is a trigonal planar molecule with D_3h symmetry. The experimentally determined B–H bond length is 119 pm.{{cite journal|last1=Kawaguchi|first1=Kentarou|title=Fourier transform infrared spectroscopy of the BH₃ ν₃ band|journal=The Journal of Chemical Physics|volume=96|issue=5|year=1992|pages=3411–3415|issn=0021-9606|doi=10.1063/1.461942|bibcode=1992JChPh..96.3411K}}

In the absence of other bases, it dimerizes to form diborane. Thus, it is an intermediate in the preparation of diborane according to the reaction:

:BX₃ +BH₄⁻ → HBX₃⁻ + (BH₃) (X=F, Cl, Br, I)

:2 BH₃ → B₂H₆

The standard enthalpy of dimerization of BH₃ is estimated to be −170 kJ mol⁻¹.{{cite journal | last1 = Page | first1 = M. | last2 = Adams | first2 = G.F. | last3 = Binkley | first3 = J.S. | last4 = Melius | first4 = C.F. | year = 1987 | title = Dimerization energy of borane | journal = J. Phys. Chem. | volume = 91 | issue = 11| pages = 2675–2678 | doi = 10.1021/j100295a001 }}

The boron atom in BH₃ has 6 valence electrons. Consequently, it is a strong Lewis acid and reacts with any Lewis base ('L' in equation below) to form an adduct:{{cite book |ref=Carey |year=2007 |title=Advanced Organic Chemistry: Part B: Reactions and Synthesis |edition=5th |publisher=Springer |place=New York |author1=Carey, Francis A. |author2=Sundberg, Richard J. |isbn=978-0387683546|page=337}}

:BH₃ + L → L—BH₃

in which the base donates its lone pair, forming a dative covalent bond. Such compounds are thermodynamically stable, but may be easily oxidised in air. Solutions containing borane dimethylsulfide and borane–tetrahydrofuran are commercially available; in tetrahydrofuran a stabilising agent is added to prevent the THF from oxidising the borane.Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell {{ISBN|978-0471417828}} A stability sequence for several common adducts of borane, estimated from spectroscopic and thermochemical data, is as follows:

:PF₃ < CO< Et₂O< Me₂O< C₄H₈O < C₄H₈S < Et₂S< Me₂S< Py < Me₃N< H⁻

BH₃ has some soft acid characteristics as sulfur donors form more stable complexes than do oxygen donors.{{Greenwood&Earnshaw}} Aqueous solutions of BH₃ are extremely unstable.{{cite journal|last1=Finn|first1=Patricia|last2=Jolly|first2=William L.|title=Asymmetric cleavage of diborane by water. The structure of diborane dihydrate|journal=Inorganic Chemistry|date=August 1972|volume=11|issue=8|pages=1941–1944|doi=10.1021/ic50114a043|url=https://escholarship.org/uc/item/7nb9k0rm}}{{cite journal|last1=D'Ulivo|first1=Alessandro|title=Mechanism of generation of volatile species by aqueous boranes|journal=Spectrochimica Acta Part B: Atomic Spectroscopy|date=May 2010|volume=65|issue=5|pages=360–375|doi=10.1016/j.sab.2010.04.010}}

:{{Chem|BH|3}} + 3{{H2O}} → {{Chem|B(OH)|3}} + {{Chem|3 H|2}}

Molecular species BH₃ is a very strong Lewis acid. It can be isolated in the form of various adducts, such as borane carbonyl, BH₃(CO).{{Cite journal|last=Burg|first=Anton B.|author2=Schlesinger, H. I.|title=Hydrides of boron. VII. Evidence of the transitory existence of borine ({{Chem|BH|3}}): Borine carbonyl and borine trimethylammine|journal=Journal of the American Chemical Society|date=May 1937|volume=59|issue=5|pages=780–787|doi=10.1021/ja01284a002}}

Molecular BH₃ is believed to be a reaction intermediate in the pyrolysis of diborane to produce higher boranes:

:B₂H₆ ⇌ 2BH₃

:BH₃ +B₂H₆ → B₃H₇ +H₂ (rate determining step)

:BH₃ + B₃H₇ ⇌ B₄H₁₀

:B₂H₆ + B₃H₇ → BH₃ + B₄H₁₀

:::::⇌ B₅H₁₁ + H₂

Further steps give rise to successively higher boranes, with B₁₀H₁₄ as the most stable end product contaminated with polymeric materials, and a little B₂₀H₂₆.

Borane ammoniate, which is produced by a displacement reaction of other borane adducts, eliminates elemental hydrogen on heating to give borazine (HBNH)₃.{{ cite book | author1 = Housecroft, C. E. | author2 = Sharpe, A. G. | chapter = Chapter 13: The Group 13 Elements | title = Inorganic Chemistry | url = https://archive.org/details/inorganicchemist00hous_159 | url-access = limited | edition = 3rd | year = 2008 | page = [https://archive.org/details/inorganicchemist00hous_159/page/n374 336] | publisher = Pearson | isbn = 978-0-13-175553-6 }}

Borane adducts are widely used in organic synthesis for hydroboration, where BH₃ adds across the C=C bond in alkenes to give trialkylboranes:{{cite book |ref=Carey |year=2007 |title=Advanced Organic Chemistry: Part B: Reactions and Synthesis |edition=5th |publisher=Springer |place=New York |author1=Carey, Francis A. |author2=Sundberg, Richard J. |isbn=978-0387683546|page=337}}

:(THF)BH₃ + 3 CH₂=CHR → B(CH₂CH₂R)₃ + THF

This reaction is regioselective.{{cite book |ref=Carey |year=2007 |title=Advanced Organic Chemistry: Part B: Reactions and Synthesis |edition=5th |publisher=Springer |place=New York |author1=Carey, Francis A. |author2=Sundberg, Richard J. |isbn=978-0387683546|page=338}} Other borane derivatives can be used to give even higher regioselectivity. The product trialkylboranes can be converted to useful organic derivatives. With bulky alkenes one can prepare species such as [HBR₂]₂, which are also useful reagents in more specialised applications. Borane dimethylsulfide which is more stable than borane–tetrahydrofuran may also be used.{{cite journal |last= Kollonitisch |first= J. |title= Reductive Ring Cleavage of Tetrahydrofurans by Diborane |journal= J. Am. Chem. Soc. |year= 1961 |volume= 83 |issue= 6 |pages= 1515 |doi= 10.1021/ja01467a056}}{{cite journal|last1=Burkhardt|first1=Elizabeth R.|last2=Matos|first2=Karl|title=Boron reagents in process chemistry: Excellent tools for selective reductions|journal=Chemical Reviews|date=July 2006|volume=106|issue=7|pages=2617–2650|doi=10.1021/cr0406918|pmid=16836295}}

Hydroboration can be coupled with oxidation to give the hydroboration-oxidation reaction. In this reaction, the boryl group in the generated organoborane is substituted with a hydroxyl group.{{cite book |ref=Carey |year=2007 |title=Advanced Organic Chemistry: Part B: Reactions and Synthesis |edition=5th |publisher=Springer |place=New York |author1=Carey, Francis A. |author2=Sundberg, Richard J. |isbn=978-0387683546|page=344}}

Phosphine-boranes, with the formula R_3−nH_nPBH₃, are adducts of organophosphines and borane.

Borane adducts with amines are more widely used.{{cite journal|author1=Carboni, B. |author2=Mounier, L. |title=Recent developments in the chemistry of amine- and phosphine-boranes|journal=Tetrahedron|year=1999| volume=55|issue=5 |page=1197|doi=10.1016/S0040-4020(98)01103-X}} Borane makes a strong adduct with triethylamine; using this adduct requires harsher conditions in hydroboration. This can be advantageous for cases such as hydroborating trienes to avoid polymerization. More sterically hindered tertiary and silyl amines can deliver borane to alkenes at room temperature.

File:Examples of amine borane complexes.png

{{anchor|Borane(5)}}

{{Redirect-for|BH5|the edition of the Hebrew Bible known as BH5|Biblia Hebraica Quinta}}

Borane(5) is the dihydrogen complex of borane. Its molecular formula is BH₅ or possibly BH₃(η²-H₂).{{cite journal|title=The stability of η2-H2 borane complexes – a theoretical investigation|first1=Dénes|last1=Szieberth|first2=Tamás|last2=Szpisjak|first3=Gábor|last3=Turczel|first4=László|last4=Könczöl|date=19 August 2014|journal=Dalton Transactions|volume=43|issue=36|pages=13571–13577|doi=10.1039/C4DT00019F|pmid=25092548}} It is only stable at very low temperatures and its existence is confirmed in very low temperature.{{cite journal|title=Reactions of Pulsed-Laser Evaporated Boron Atoms with Hydrogen. Infrared Spectra of Boron Hydride Intermediate Species in Solid Argon|first1=Thomas J.|last1=Tague|first2=Lester|last2=Andrews|date=1 June 1994|journal=Journal of the American Chemical Society|volume=116|issue=11|pages=4970–4976|doi=10.1021/ja00090a048}}{{cite journal|title=The structure and stability of BH₅. Does correlation make it a stable molecule? Qualitative changes at high levels of theory|first1=Peter R.|last1=Schreiner|first2=Henry F. |last2=Schaefer III|first3=Paul von Ragué|last3=Schleyer|date=1 June 1994|journal=The Journal of Chemical Physics|volume=101|issue=9|pages=7625|doi=10.1063/1.468496|bibcode = 1994JChPh.101.7625S}} Borane(5) and methanium (CH₅⁺) are isoelectronic.A Life of Magic Chemistry: Autobiographical Reflections Including Post-Nobel Prize Years and the Methanol Economy, 159p Its conjugate base is the borohydride anion.

• Butterfly cluster compound

{{Reflist}}

{{Boron compounds}}

{{Hydrides by group}}