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 In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...

molecule with D_3h 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. 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

and reacts with any

Lewis base 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 ...

('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 and

borane–tetrahydrofuran Borane–tetrahydrofuran is a dipolar bond charge-transfer complex composed of borane and tetrahydrofuran (THF). These solutions are used for reductions and hydroboration, reactions that are useful in synthesis of organic compounds.Marek Zaidlewi ...

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 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 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 this ...

(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, 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 which is more stable than

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-borane In chemistry, phosphine-boranes are organophosphorus compounds with the formula R3−nHnPBH3. They are Lewis acid-Lewis base adducts derived from organophosphines (PR3−nHn) and borane (BH3). They are generally colorless or white solids. Since ...

s, with the formula R_3−nH_nPBH₃, 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 i ...

of borane. Its molecular formula is BH₅ or possibly BH₃(η²-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 o ...

(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.

References

{{Hydrides by group

Structure and properties

Reactions

As a Lewis acid

See also

References