Borane
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Trihydridoboron, also known as borane or borine, is an unstable and highly reactive
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
with the
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
. The preparation of
borane carbonyl Borane carbonyl is the inorganic compound with the formula H3BCO. This colorless gas is the adduct of borane and carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slig ...
, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. Consequently, it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen.


Structure and properties

BH3 is a trigonal planar molecule with D3h symmetry. The experimentally determined B–H bond length is 119  pm. In the absence of other chemical species, it reacts with itself to form
diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
. 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. The boron atom in BH3 has 6 valence electrons. Consequently, it is a strong
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
and reacts with any Lewis base ('L' in equation below) to form an adduct: :BH3 + L → L—BH3 in which the base donates its lone pair, forming a dative
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
. Such compounds are thermodynamically stable, but may be easily oxidised in air. Solutions containing
borane dimethylsulfide Borane dimethylsulfide (BMS) is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS ...
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 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. Aqueous solutions of BH3 are extremely unstable. : + 3 → +


Reactions

Molecular BH3 is believed to be a reaction intermediate in the pyrolysis of
diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
to produce higher
boranes Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bond ...
: :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 Borazine, also known as borazole, is a non-polar inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For thi ...
(HBNH)3. Borane adducts are widely used in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
for
hydroboration In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon (, , , and ). This chemical reaction is useful in the organic synthesis of organic compounds. Hydroboration p ...
, where BH3 adds across the C=C bond in alkenes to give trialkylboranes: :(THF)BH3 + 3 CH2=CHR → B(CH2CH2R)3 + THF This reaction is
regioselective In chemistry, regioselectivity is the preference of chemical bonding or breaking in one direction over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong Base ( ...
, 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 BR2sub>2, which are also useful reagents in more specialised applications.
Borane dimethylsulfide Borane dimethylsulfide (BMS) is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS ...
which is more stable than borane–tetrahydrofuran may also be used. 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.


As a Lewis acid

Phosphine-boranes, with the formula R3−nHnPBH3, are adducts of
organophosphine Organophosphines are organophosphorus compounds with the formula PR''n''H3−''n'', where R is an organic substituent. These compounds can be classified according to the value of ''n'': primary phosphines (''n'' = 1), secondary phosphines ...
s and borane. Borane(5) is the
dihydrogen complex Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. They are a subset of sigma complexes. The prototypical complex is W(CO)3( PCy3)2(H2). This class of compounds represent intermediates in metal-catalyzed reactions ...
of borane. Its molecular formula is BH5 or possibly BH32-H2). It is only stable at very low temperatures and its existence is confirmed in very low temperature. Borane(5) and
methanium In chemistry, methanium is a complex positive ion with formula []+, namely a molecule with one carbon atom covalent bond, bonded to three hydrogen atoms and one hydrogen molecule, bearing a +1 electric charge. It is a superacid and one of the ...
(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 Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example cyanoborohydride or cyanotrihyd ...
anion.


See also

* Butterfly cluster compound


References

{{Hydrides by group