Structure of Boron nitride (Inorganic Graphite):

Diborane react with excess NH₃ at temperature to form boron nitride (BN) _x.while when diborane and NH₃ react in 1:2 ratios at low temperature give Borazole.

Boron Nitrides exist two forms just like allotropic forms of carbon (Graphite and Diamond) and both have formula (BN)_x.

(i) Boron Nitride (Inorganic Graphite):

(1) Boron nitride is a hexagonal 2D planar giant covalent network , slippery  and a white solid with a layered structure like graphite. Doe to similar structure with graphite it know as “Inorganic graphite” and due to white colour  it is also called “white graphite”

(2) The thermodynamically stable phase of boron nitride, BN, consists of planar sheets of atoms like those in graphite The planar sheets of alternating B and N atoms consist of edge shared hexagons and, as in graphite, the B-N distance within the sheet (145 pm) is much shorter than the distance between the sheets (333 pm,). The difference between the structures of graphite and boron nitride, however, lies in the register of the atoms of neighboring sheets:

(3) The B-N-B or N-B-N bond angle is 120^oc . It may be expected for perfect hexagonal ring bond network just like graphite. And boron and nitrogen atoms are sp2 hybridized.

(4) Boron nitride (Inorganic graphite ) is a very good insulator (thermal and electrical) and chemically very inert , chemically posses great stability due to the  very strong B-N bonding  in the 2D layers structure. It melts under pressure at  3000^oc  so it is great thermal stability.

(5) In (BN)_x the hexagonal rings are stacked directly over each other, with B and N atoms alternating in successive layers; in graphite, the hexagons are staggered. Molecular orbital calculations suggest that the stacking

(6) In (BN)_x stems from a partial positive charge on B and a partial negative charge on N. This charge distribution is consistent with the electronegativity difference of the two elements.

 (7)In  Boron nitride(BN)x the Vander Waals forces holding the sheet in line with each other are stronger, so boron nitride is not as good a good lubricant as graphite. However , the use of boron nitride as a lubricant is noted as high temperature due to its chemical stability.

(8) As with impure graphite, layered boron nitride is a slippery material that is used as a lubricant. Unlike graphite, however, it is a colorless electrical insulator, as there is a large energy gap between the filled and vacant π bands

Uses:

(1) Boron nitride ceramics us in high temperature (range 2700-3000^oc)  equipment due to excellent thermal stability, thermal shock stability and chemical stability.

(2) Boron nitride based ceramics are stable in air at 1000^oc while carbon-graphite based materials ignited at that temperature.

(3) Hexagonal boron nitride can be made in single layers and can also be formed into nanotubes. And that nanotubes are used for wire sieving and a catalyst support.

(4) Hexagonal boron nitride can be incorporated in ceramics, alloys, resins, plastics, rubbers to give them self-lubricating properties. And plastics based hexagonal boron nitride  decrease thermal expansion, increased thermal conductivity, increased electrical insulation. 

Structure of "Borazon" (Cubic boron nitride):

Structure of "Inorganic Benzene":

DIBORANE-PREPARATION:

Boranes are hydride of Boron and diborane is famous borane. It is gas and is highly inflammable in air and poisonous Diborane is used for preparing substances such as high energy fuel and propellants.

Other Methods:

Structure of “Borazine/Borazole”/inorganic Benzene:

(1) Borazine is an inorganic compound with the chemical formula  (B₃N₃H₆).and also called Borazole. It is a heterocyclic compound, containing the 3-(BH) units and 3-(NH) units alternate.

(2) Borazine formed by reaction of B₂H₆ and NH₃ in the ratio of 1:2 at room temperature.

(3) Borazine is isoelectronic and isostructural with benzene. For this reason borazine is known as “inorganic benzene”. Like benzene borazine has delocalized electrons and aromatic character. And it is a colourless liquid.

(4) In Borazine molecule nitrogen is more electro negative than the boron. Nitrogen acquires partial negative charge and boron acquires partial positive charge and back bonding take place between boron and nitrogen.

(5) As compared with Benzene, Borazole/borazine is less stable and more reactive toward Electrophilic Aromatic Substitution reactions due to presence of polar B-N bond in it while in benzene all the C-C bonds are non polar.

(6) Borazine is a highly polar molecule due to high Electronegativity difference between Boron and Nitrogen.

Even though Borazine/Borazole  and Benzene have same stricture their chemical properties are different.

(1) Organic benzene is C₆H₆ while Inorganic benzene is Borazine having chemical formula B₃N₃H₆

(2) The pi bonds in borazine are highly polarized than pi bonds in benzene due to high polarity (B-N polar bond) of Borazine molecules. Thus borazine is more nucleophillic (Negative) hence more reactive than benzene with respect to “Electrophic addition reactions”.

Here protonation(H⁺) take place at nitrogen atoms due to more electron density (more negative)  and  chlorine attack at boron atoms.

(3) Aromaticity of borazine is less than benzene because some delocalization in Borazine/Borazole is not complete as benzene. One reason is that nitrogen atom has more electronegativity than boron ,the electron density is higher at nitrogen atoms then boron. The electron density is determined by both sigma and pi bonds, both of which have polarity, but opposite directions,   hence it is less reactive  toward “Electrophilic  substitution reactions” than Benzene. 

(4) Borazine undergo polymerization when strongly heated under vacuum and yield biborazonyl and naphthazine whose structure are similar to biphenyl and naphthalene.

Structure of "Borazon" (Cubic boron nitride):

Structure of "Inorganic Gaphite";

Related Questions: 

Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

Why Ga has small size than Al exceptionally

Why aqueous solution of borax reacts with two moles of acids ?

What is structure of solid Ortho Boric acid ?

What is the structure of trimetaboric acid and trimetaborate ion?  

Why Borazine is more reactive than benzene towards Electrophic Aromatic substitution reactions ?

Why Borazine (B3N3H6) is also known as inorganic benzene ?.

Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

Why B-F do not exist as dimer?. Explain.

Although anhydrous aluminium chloride is covalent but its aqueous solution is ionic in nature. Why?

Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

Four-center two-electron bond (4C-2e Bond): Structure of AlCl3:

What is the molecular formula of Borax ?

What is the difference between the structure of AlCl3 and diborane?

STRUCTURE OF DIBORANE :

(1)  B₂H₆ contains 4-Terminal H are bonded by Sigma bond and  remaining 2-H are bridging hydrogen’s and of these are broken then dimer become monomer.

(2) Boron undergoes sp3 hybridisation 3 of its sp³ hybridised orbitals contain one( e¯) each and fourth sp³ hybrid orbital is vacant.

(3) 3-(Three) of these sp³ hybrid orbitals get overlapped by s orbitals of 3-hydrogen atoms.

(4) One of the sp3 hybrid orbitals which have been overlapped by s orbital of hydrogen gets overlapped by vacant sp³ hybrid orbital. Of 2^nd Boron atom. And it’s vice versa.

(5) By this two types of overlapping take place 4 (sp3– s) overlap bonds and 2(sp² – s – sp³) overlap bonds.

(6) H is held in this bond by forces of attraction from B and This bond is called 3 centred two electron bonds (3C-2e bond) . Also called Banana bonds. Due to repulsion between the two hydrogen nuclei, the delocalised orbitals of bridges are bent away from each other on the middle giving the shape of banana.

(7) The two bridging hydrogens are in a plane and perpendicular to the rest four hydrogen..

ILLUSTRATED EXAMPLE (1): In Diborane

(A) 4 bridged hydrogens and two terminal hydrogen are present

(B) 2 bridged hydrogens and four terminal hydrogen are present

(C) 3 bridged and three terminal hydrogen are present

(D)None of the above

ILLUSTRATED EXAMPLE (2): Which one of the following statements is not true regarding diborane?

(A) It has two bridging hydrogens and four perpendicular to the rest.

(B) When methylated, the product is Me₄B₂H₂.

(C) The bridging hydrogens are in a plane and perpendicular to the rest.

(D ) All the B–H bond distances are equal

ILLUSTRATED EXAMPLE (3): The structure of diborane (B₂H₆) contains

(A) Four (2C–2e–) bonds and two (2C–3e–) bonds

(B) Two (2C–2e–) bonds and two (3C–2e–) bonds

(C) Four (2C–2e–) bonds and four (3C– 2e–) bonds

(D )None of these

ILLUSTRATED EXAMPLE (4): The molecular shapes  of diborane is shown:

Consider the following statements for diborane:

1. Boron is approximately sp3 hybridised

2. B–H–Bangle is 180°

3. There are two terminal B–H bonds for each boron atom

4. There are only12 bonding electrons available

Of these statements:

(A ) 1, 3 and 4 are correct                  (B) 1, 2 and 3 are correct

(C) 2, 3 and 4 are correct                    (D) 1, 2 and 4 are correct

STRUCTURE OF DIAMOND :

(1) Each carbon is linked to another atom and there is very closed packing in structure of Diamond.

(2) Density and hardness is very much greater for diamond because of closed packing in diamond due to sp3 hybrid and are tetrahedrally arranged around it. And C-C distance is 154pm

(3) Diamond has sharp cutting edges that's why it is employed in cutting of glass.

(4) Diamond crystals are bad conductor of electricity because of absence of mobile electron.

(5) 1 carat of diamond = 200 mg.

(6) Diamond powder if consumed is fatal and causes death in minutes.