Boron Hydrides

The binary compounds of boron with hydrogen are called Boron hydrides.

These hydrides of boron are called boranes.These hydrides can be divided into several series:

1) B_nH_n+4  (called nido borane) such as B₂H₆ (diborane), B₅H₉ (pentaborane) , B₆H₁₀ (hexaborane) , B₈H₁₂ (octaborane) , B₁₀H₁₄ (decaborane).

2) B_nH_n+6  (called arachno borane) such as B₄H₁₀ (tetraborane) , B₅H_{9 (}pentaborane), B₆H₁₀ (hexaborane),  B₈H₁₄ ( octaborane), B₉H₁₅ ( nonaborane).

Preparation

1) By reduction of boron trifluoride etherate with Lithium aluminium hydride in diethyl ether.

4 BF₃.Et₂O + 3 LiAlH₄ ——> 2 B₂H₆ + 3 LiF + 3 AlF₃ + 4 Et₂O

2) Diborane is prepared in the laboratory by the oxidation of sodium borohydride with iodine in diglyme as solvent.

NaBH₄ + I₂ ———-> B₂H₆ + 6 NaF

3) On the industrial scale, diborane is obtained by the reduction of BF₃ with sodium hydride.

BF₃ + 6 NaH ——–> B₂H₆ + 6 NaF

Preparation of higher boranes

Higher boranes are obtained when B₂H₆ is heated at 373-523 K.

 

Properties

1) Physical state

Diborane is a colourless ,highly toxic gas, b.p. 180 K.

2) Stability

Diborane is stable only at low temperature. When diborane is heated in a sealed tube,  between 373- 533 K, a complex reaction occurs and various higher boranes are formed.

B₂H₆ ——-> B₄H₁₀ , B₅H₁₁, B₆H₁₂

By careful control of temperature ,pressure and reaction time , different individual boranes can be obtained.

2 B₂H₆ ——> B₄H₁₀ + H₂

3) Combustibility

It catches fire spontaneously upon exposure to air. It burns in oxygen evolving an enormous amount of heat.

B₂H₆ + 3 O₂ ——–> B₂O₃ + 3 H₂O ; Δ_cH° = -1976 KJ/mol

Like diboranes, higher boranes spontaneously burn in air.

4) Hydrolysis

Boranes are readily hydrolysed by water to form boric acid.

B₂H₆ + 6 H₂O ——> 2 H₃BO₃ + 6 H₂

With methanol ,trimethylborate is formed.

B₂H₆ + 6CH₃OH —–> 2 B(OCH₃)₃ + 6 H₂

5) Reaction with Lewis bases – cleavage reaction

Diborane on treatment with Lewis bases first undergoes cleavage to form borane which then reacts with Lewis bases to form adducts.

B₂H₆ + 2 NMe₃ ——> 2 BH₃.NMe₃

B₂H₆ + 2 CO —> 2 BH₃.CO

6) Reaction with Ammonia

 

Diborane combines with ammonia to form an addition product, B₂H₆.2NH₃ formulated as [BH₂(NH₃)₂]⁺ [BH₄]‾ , which when heated to 473 K decompose to give a volatile compound called borazine or borazole.

3 B₂H₆ + 6 NH₃ ——–> 3 [BH₂ (NH₃)₂]⁺ [BH₄]‾ ——–> 2 B₃N₃H₆ + 12 H₂

Borazine is isoelectronic and isosteric with benzene and its structure is similar to that of benzene expect that in benzene the π-electrons are completely delocalised but in borazine, they are only partially delocalized.

Because of its similarity with benzene , borazine is also called inorganic benzene.

7) Formation of complex borohydride

Several metal hydrides react with diborane to from tetrahydridoborates known as borohydride. All these contain the tetrahedral [BH₄]‾ ion.

2 NaH + B₂H₆ ——> 2 Na⁺ [BH₄]‾

2LiH + B₂H₆ ——>2 Li⁺ [BH₄]‾

Both sodium borohydride and lithium borohydride are used as reducing agent in organic synthesis. They also serve as starting material for many other borohydrides.

8) Reaction with alkalies

Diborane dissolves in strong alkali to produce metaborates and dihydrogen gas.

B₂H₆ + 2 KOH + 2 H₂O ——–> 2 KBO₂ + 6 H₂

9) Action of halogen acids

Diborane reacts with halogen acids to give halodiboranes evolving H₂ gas.

The reactivity of halogen  acids follow the order: HI> HBr > HCl

Thus HI react at about 323 K in absence of catalyst while HBr and HCl reacts in presence of their aluminium halide as catalyst.

B₂H₆ + HI ——> B₂H₅I + H₂

B₂H₆ + HBr ——> B₂H₅Br + H₂

B₂H₆ + HCl ——> B₂H₅Cl + H₂

 

10) Action of halogens

Halogens also react with diborane to form the corresponding halodiboranes.

The reactivity decreases in the order: Cl₂ > Br₂ > I₂.

Thus Cl₂ reacts explosively at room temperature, Br₂ reacts rapidly at 373 K while I₂ reacts slowly at still higher temperature.

B₂H₆ + Cl₂ ——->B₂H₅Cl + HCl

Uses of Boron Hydrides

1) Dibornae is used for preparing a number of boron hydrides such as LiBH₄ , NaBH₄ .

2) It is used as a reducing agent in organic reactions.

3)It is used as a fuel for supersonic rockets.

Structure of Diborane

1) In diborane, each boron atom has 3 Valence electrons for sharing. If we assume that each boron forms 3 covalent bonds with 3 hydrogen atoms, then there are no electrons left with boron atom for sharing with other boron atoms. Therefore the two boron atoms cannot be linked.

There are two types of hydrogen atom :

1) The four hydrogen atoms (two on left and two on right shown by thick lines) are called terminal hydrogens.

These four terminal hydrogens and the two boron atoms lie in the same plane while the remaining two hydrogen atoms- one lying above and one lying below this plane form bridges and hence are called which bridged hydrogen.

 

bana are two types of bonds in B₂H₆ molecules.

1) The four terminal B-H bonds are normal covalent bonds and hence are quite strong. Each bond is formed by sharing a pair of electron between B and H and hence are also called two centre electron pair bonds or two centre two electron bond(2c-2e)

2) The two bridge bonds i.e. B—–H—–B are quite different from normal covalent bonds. Each bridge hydrogen is bonded to two boron atoms by a pair of of electrons. These are three centre electron pair bonds or three centre 2 electron bonds and hence are quiet weak.

Because of their semblance to a banana, these three centre electron pair bonds are also called banana Bond.

Structure of diborane on the basis of hybridization

The boron atom in the excited state is 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z⁰ .

It undergoes sp³ hybridisation

The two half filled hybrid orbitals of each boron atom overlap with the half filled orbitals of hydrogen atom forming normal covalent bonds whereas the third half filled hybrid orbital of one boron atom and the vacant hybrid orbital of the second boron atoms overlap simultaneously with the half filled orbitals of hydrogen atom The electron cloud contains only two electrons but spreads over 3 atoms. That is why this bond is called three centre electron pair bond.

Because of the shape of the electron cloud formed, it is called banana bond. Molecules like B₂H₆ which do not have sufficient number of electrons to form normal covalent bonds are called electron deficient molecules.