Unit - 1

Chemistry of Boranes

Structure and bonding in boranes and boron cage compounds- Closo, nido, arachno and
carboranes – Styx notation – Wade’s rule – Electron count in polyhedral boranes – Synthesis
of polyhedral boranes – Isolobal analogy between main group and transition metal fragments–
Boron halides – Phosphine- Boron heterocycles – Borazine.

1. Boranes:

Preparation:

Diborane can also be obtained in small quantities by the reaction of iodine with sodium borohydride
in diglyme.

2NaBH4 + I2 → B2H6 + 2NaI + H2

On heating magnesium boride with HCl a mixture of volatile boranes are obtained.

2Mg3B2 + 12HCl → 6MgCl2 + B4H10 + H2

B4H10 + H2 → 2B2H6

PHYSICAL PROPERTIES

● Colourless and highly inflammable gas at room temperature.

● At high concentrations, it ignites rapidly in the presence of moist air at room temperature.
● It smells sweet.
● It has boiling point of about 180 K
● It is toxic gas
● It releases huge amount of energy when burnt in the presence of O 2.
● It readily hydrolysed in the water to give hydrogen gas and boric acid.

Chemical Properties:

i) Diboranes reacts with water and alkali to give boric acid and metaborates respectively.

B2H6 + 6H2O → 2H3BO3 + 6H2

B2H6 + 2NaOH + 2H2O → 2NaBO2 + 6H2

ii) Action of air:

At room temperature pure diborane does not react with air or oxygen but in impure form it gives
B2O3 along with large amount of heat.
B2H6 + 3O2 → B2O3 + 3H2O

ΔH = -2165 KJ mol-1

iii) Diborane reacts with methyl alcohol to give trimethyl Borate.

B2H6 + 6CH3OH → 2B(OCH3)3+ 6H2

iv) Hydroboration:

Diborane adds on to alkenes and alkynes in ether solvent at room temperature. This reaction is
called hydroboration and is highly used in synthetic organic chemistry, especially for anti
Markovnikov addition.

B2H6 + 6RCH=CHR → 2B(RCH-CH2R)3

v) Reaction with ionic hydrides

When treated with metal hydrides it forms metal borohydrides

vi) Reaction with ammonia:

When treated with excess ammonia at low temperatures diborane gives diboranediammonate. On
heating at higher temperatures it gives borazole.

Structure and Bonding in diborane:

In diborane two BH2 units are linked by two bridged hydrogens. Therefore, it has eight B-H bonds.
However, diborane has only 12 valance electrons and are not sufficient to form normal covalent
 bonds. The four terminal B-H bonds are normal covalent bonds (two centre - two electron bond or
2c-2e bond).

The remaining four electrons have to be used for the bridged bonds. i.e. two three centred B-H-B
bonds utilise two electrons each. Hence, these bonds are three centre- two electron bonds (3c-2e).
The bridging hydrogen atoms are in a plane as shown in the figure 2.3. In diborne, the boron is
sp3 hybridised.

Fig.1. Bonding in diborane

Three of the four sp3 hybridised orbitals contains single electron and the fourth orbital is empty.
Two of the half filled hybridised orbitals of each boron overlap with the two hydrogens to form
four terminal 2c-2e bonds, leaving one empty and one half filled hybridised orbitals on each boron.
The Three centre - two electron bonds), B-H-B bond formation involves overlapping the half filled
hybridised orbital of one boron, the empty hybridised orbital of the other boron and the half filled
1s orbital of hydrogen.

Uses of diborane:

i) Diborane is used as a high energy fuel for propellant

ii) It is used as a reducing agent in organic chemist
iii) It is used in welding torches

2. Borazine:

● Borazine, also known as borazole.

● It is a polar inorganic compound with the chemical formula B3H6N3.
● In this cyclic compound, three BH units and three NH units alternate.
● The compound is isoelectronic and isostructural with benzene.
● Borazine has polar hexagonal structure containing 6 membered ring, in which B and N atoms are
arranged alternately.
● Because the similarity between the structures of borazine and benzene that borazine is called
inorganic benzene.
● In borazine, both Boron and Nitrogen are sp2 hybridized.
● Each N-atom has an empty p-orbital.
● B-N bond in borazine is dative bond, while arises from the sidewise overlap between the filled p-
orbitals of N-atom and empty p-orbitals of B-atom.
● Since borazine is isoelectronic with benzene, both the compounds have aromatic electron cloud.
● Due to greater difference in electronegativity values of B and N - atoms, the electron cloud in B3N3
ring of the borazine molecule is partially delocalized. While in the case of benzene ring, the electron
cloud is completely delocalized.
● MO calculations have indicated that electron drift from N to B is less than the electron drift from B to
N due to the greater electronegativity of the N-atom.
● In benzene molecules, C=C bonds are non-polar, while in case of B3H6N3, due to difference in
electronegativities between B and N atoms, the B-N bond is polar.
● It is due to the partial delocalization of electron clouds that bonding in the B3N3 ring is weakened.
● N-atoms retain its basicity and B-atoms retain its acidity.
● In borazine, B-N bond length is equal to 1.44 Ao, which is between calculated single B-N bonds (1.54
Ao).
● B=N bond length is 1.36 Ao.
● The angles are equal to 120o.

Preparation:

3B2H6 + 6NH3 —-> 3[BH2(NH3)2][BH4] —heat—> 2B3 N3H6 +12H2

NH4Cl + BCl3 ——> Cl3 B3N3H9 —NaBH4—> B3 N3H6

NH4Cl + NaBH4 ——> B3N3H6 + H2 + NaCl

PROPERTIES

 Colorless liquid
 Volatile liquid
 boiling point 84.5 0C
 Melting point -58 0C
 decomposes at -80 0C
 Structure:

Chemical properties

2B3N3H6 + 6HCl ——> 2Cl3 B3 N3H9 —6NaBH4 —> 2B3 N3H12 + 3B2 H6 + 6NaCl

USES

● Borazines are also starting materials for other potential ceramics such as boron carbonides
● Borazine can also be used as precursor to growing boron nitride thin films on surfaces, such as
nanomesh structure which is formed on rhodium

3. BORON HETEROCYCLES

If one or more CH group in Benzene and cyclopentadienyl are replaced by BH group then it is called
Boron heterocycles.

PREPARATION

● When stanno-hydration of diethynyl methane with dibutyl tin hydride gives 1,4-dihydro - 1,4-dibutyl
stanno benzene . It reacts with renal Boron Bromide to give 1-phenyl-1,4-dihydroxy-bromobenzene.
● When compound is converted to its anion by dipronatio with 30 -butyl / lithium which on reaction
with FeCl2 ,which gives bis(1-phenyl borate benzene ) iron.
● Boron containing 𝞹 e- system have recently attracted much attention in research activities.
● 3 coordinate boron is isoelectronic and iso structural with positively carbocation with its vacant Pz
orbital.
● Boron is inherently electron deficient with strong e- acceptor nature allowing conjugation of organic
𝞹 - systems.
● Hence, it shows unique electronic and physical properties with lewis acid boron forms unique
complexes.
● This complex is important for turning the electronic structure of 3 - dimensional molecules.
● Boron takes trigonal planar geometry which can be used as a building block for constructing complex
molecules.

DIFFERENT TYPES OF HETEROCYCLES

 4. ISOLOBAL ANALOG

Hoffmann developed the concept of isolobal analog to show the connection between an
organic or inorganic compound. According to this concept the molecular fragments of a cluster is said
to be isolobal when they possess some number of frontier molecular orbital with,

1. Similar shape, Symmetry, Radical extent

2. Approximately same energy
3. Same no of electrons are available for cluster bonding.
4. Isolobal fragments may not be isostructural and isoelectronic.
5. The relationship between isolobal fragments is shown by <r>

FIND OUT THE ISOLOBAL PAIRS FROM THE FOLLOWING

1) Mn(𝐶𝑂)5 →7+10→17
𝐶𝐻3 →4+3=7 Isolobal

2) Fe(𝐶𝑂)4 → 8 + 8 → 16
O→ 6 → 6 Isolobal

3) CO(𝐶𝑂)6 → 9 + 6 → 15
𝑅2 Si→ 2 + 4 → 6 Non Isolobal

4) Mn(𝐶𝑂)5 → 7 + 10 → 17
Rs→ 1 + 6 → 7 Isolobal

5. BORON HALIDES

It is represented as BX3 where X= F, Cl, Br, I

SYNTHESIS

1.) When B2O3 is treated with halogens in presence of carbon, Boron halides are formed.

B2O3 + 3 C + 3 Cl2 → 2 BCl3 + 3 CO

2.) It is synthesized by reactions of CaF2 and boric oxide in the presence of H2SO4

3 CaF2+ B2O3+ 3 H2SO4 → 2 BF3+ 3CaSO4+ 3 H2O

PROPERTIES

● They are small in size with high charge density.

● They are covalent in nature.
● They act as non- electrolytes in liquid state.
● The boiling point is ranging from 13-3000C and it decreases when the atomic number increases.
● Boron halides are electron deficient because B atom has an incomplete octet structure.
● Boron atom accepts 6 electrons from 3 BX bonds and 2 more electrons from the donor compound.

CHEMICAL PROPERTIES:

i) BCl3 hydrolyzes readily to give hydrochloric acid and boric acid:

BCl3 + 3 H2O → B(OH)3 + 3 HCl

ii) Reduction of BCl3 to elemental boron is conduct commercially (see below). In the laboratory,
when boron trichloride can be converted to diboron tetrachloride by heating with copper metal: [5]
2 BCl3 + 2 Cu → B2Cl4 + 2 CuCl

6. Polyboranes:

● 𝐵4 𝐻10

● 𝐵5 𝐻11
● 𝐵6 𝐻10

 Wade provided the rule to correlate the no of frame work with e − with boron, cluster.
 Later, Mingo extended the rule for the transition metal clusters.
 General method for applying wade’s rule is as follows
 Determine the total no of the valence electrons from the chemical formula.
 Subtract 2 electrons for each B - H unit.
 Divide the no of remaining electrons by 2 to get the no of skilled electron pairs (SEP)
 A cluster with ‘n’ vertices and (n+1) SEP for bonding has closo structure.
 A cluster with ‘B’ atom (n-1) vertices and (n+2) SEP for bonding has Nido structure.
 A cluster with (n-2) vertices (i.e) (n-2) boron atoms and (n+3) SEP for bonding has Arachno
structure. (i.e) n-3 Boron atoms with (n+4)

STYX NUMBER OR NOTATION

The overall bonding in boranes can be represented by a 4digit number called Styx number.

s = no.of B-H-B bonds (1st digit)

t = no.of B-H-B bonds (2nd digit)

y = no.of B-B bonds (3rd digit)

x = no.of B𝐻2 group (4th digit)

EXAMPLE

1) 𝐵2 𝐻6 (Diborane) Styx number Styx Code Skeletal

s=2 2002 2 x 2 = 4𝑒 −
t=0 0x2=0
y=0 0x0=0
x=2 2 x 2 = 4𝑒 −
___
8𝑒 − s

FRAMEWORK ELECTRONS :

The electrons involved in bonding in the formation of Boron cluster in boranes is known as
Framework electrons (or) electron

B-H-B 2𝑒 −
B-B 2𝑒 −
B-B-B 2𝑒 −
-B𝐻2 2𝑒 −

3) 𝐵5 𝐻9 (Pentaborane 9) Styx number Styx Code Skeletal

s=4(B H B) 4120 9 x 2 = 8𝑒 −
t=1 (B B B) 1 x 2 = 2𝑒 −
y=2(B B) 2 x 2 = 8𝑒 −
x=3(-B𝐻2 ) 0x2=0
________
14𝑒 − 𝑠
4) 𝐵5 𝐻11 (Pentaborane 11) Styx number Styx Code
Skeletal

s=3(B H B) 3203 3 x 2 = 6𝑒 −
t=2(B B B) 2 x 2 = 4𝑒 −
y=0(B B) 0x2=0
x=3(-B𝐻2 ) 3 x 2 = 6𝑒 −
________
16𝑒 −
 WADE’s - MNGO’S RULE :
● This theory provides electrons counting rules to predict the structure of clusters such as boranes &
carbones
● In case of transition metal cluster, for each transition metal to additional electrons are subtracted from
the total electron count
● This rule is otherwise called as PSEPT theory (Polyhedral skeletal electron pair theory )

𝐹 𝑇𝑜𝑡𝑎𝑙 𝑣𝑎𝑙𝑒𝑛𝑐𝑒 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 − 12 𝑥 𝑛

=
2 2

𝐹 6 𝑥 9 + 16 𝑥 2 − 12 𝑥 6
Eg: 𝑅ℎ6 (𝐶𝑜)16 = 2 = 2
86 − 72
= 2
= 7
n =6
n +1 = 7 => closo
𝑅ℎ6 (𝐶𝑜)16 => close (n+1) vertex

References

1.https://www.brainkart.com/article/Diborane_38581/#:~:text=Diborane%20is%20agas%20at%20roo
m,boric%20acid%20and%20metaborates%20respectively.
2. https://edubuzznotes.com/borazine-preparation-properties-structure/