DIBORANEDIBORANE

Methods Of PreparetionMethods Of Preparetion It is prepared by treating boron Trifluoride with LiAlH4

in diethyl ether 4BF3 + 3LiAlH4 2B2H6 +3LiF +

3ALF3

In a laboratory the diborane is prepared by the oxidation of sodium borobhydride with iodine

2NaBH4 + I2 B2H6 +2NaI +H2

In industry it is prepared by the reaction of BF3 with sodium hydride

2 BF3 + 6 NaH 450 K B2H6 + 6 NaF

PropertiesProperties

1) Diborane is a colorless, highly toxic gas. It has boiling point ok 180 K

2) Diborane catches fire spontaneously when it is exposed to atmospheric air

it burns with oxygen. The reaction is exothermic releasing a lager amount of energy

B2H6 + 3O2 B2O3 + 3H2O ΔH= - 1976 K J mol-1

3) it is readily hydrolyzed by water to form boric acid B2H6 + 6H2O 2B(OH)3 + 6H2

4) With Lewis bases, diborane first undergoes cleavage (breaking) to form borane (BH3) which then reacts to form adducts

B2H6 + 2NMe 2BH3 . NMe B2H6 + 2CO 2BH3 . CO

5) with ammonia an addition product B2H6.NH3 which then decomposes on heating at 473K to give a volatile compound called borazole (Borazine)

B2H6 + NH3 B2H6.NH3

3 B2H6 + 6NH3 473K 2B3N3H6 + 12H2

( borazole)

B2H6 + 6NH3 (BN) n Boron nitride

B undergoes sp3 hybridization and 3 hybridized orbitals have one electron each and one orbital is vacant. Each Boron form two sigma bonds by overlapping with s orbital of H and a normal 2e2c covalent bond is formed. These are coplanar.

Two more bonds are formed by overlapping of one boron sp3 orbital with one electron, one hydrogen s orbital with one electron and another boron sp3 with no electron. Thus a 2e3c bond is formed , also called banana or tau bond.

Structure

B in GS ES

Sp3

H

2s 2p

B B

H

H

HHH

H

1S

Types of Bonds in Boranes

1. Normal covalent bonds -2c-2e- B-H

2. Normal covalent bonds - 2c-2e- B-B

3. Bridge bonds -3c-2e- B-H-B

4. Bridge bonds - 3c-2e- B-B-B

5. Closed bridge bonds - 3c-2e-

B

B B

Structure of Higher Boranes

Structure of Higher Boranes

1. Tetraborane – B 4H10

Bonds –(i)Four bridging (3c-2e) B-H-B bonds viz, B1 –H-B3 , B2-H –B3, B1-H-B4 and B2- H-B4

(ii)One direct (2c-2e) B-B bond (B1-B2 bond)

(iii)Six terminal (2c-2e) B-H bonds namely B3-H, B3-H, B2-H, B2-H, B4-H and B4-H bonds.

2.Pentaborane – B5H9

Bonds – 1. Five terminal B-H bonds (viz., B1-H, B4-H, B2-H, B3-H and B5-H bonds) 2.Four bridging B-H-B bonds (namely B1-H-B2, B2-H-B3, B3-H-B4, and B4-H-B1 bonds), 3. Two B-B bonds (B1-B5 and B5-B4 bonds) 4.One closed (3c-2e) B5-B3-B2 bonds.

3. Decaborane – B10H14Bonds – 1.Four bridging (3c-2e) B-H-B bonds namely B5-H-B6, B6-H-B7, B8-H-B9 and B9-H-B10.

2. Each of B-atoms is linked with one H-atom by terminal B-H bonds. Thus the molecule has ten terminal B-H bonds viz., B1-H, B2-H, B3-H, B4-H, B5-H, B6-H, B7-H, B8-H, B9-H and B10-H bonds. 3.Four B-B bonds (B2-B5, B2-B7, B4-B8, B4-B10) 4.Four closed (3c-2e) B-B-B bonds (B1-B2-B3, B1-B3-B4, B1-B5-B10 and B3-B7-B8 bonds) +

BORAZINE – BORAZOLE – INORGANIC BENZENE

(HBNH)3

PREPARATION

PROPERTIES