Tuesday, December 25, 2018

Diborane preparation and its bonding


What is diborane ?

The simplest and the most important hydride of boron is called diborane, which is a colourless gas with a foul smell.
Diborane is also known as boron hydride or boroethane or diboron hexahydride. It is a covalent compound of boron and hydrogen.

3D STRUCTURE OF DIBORANE


Preparation of diborane :


Diborane may be prepared by the interaction of iodine and sodium borohydride in the presence of diglyme.


Diborane is also be prepared by the reduction of boron trichloride with lithium aluminium hydride.

Structure of diborane :


Diborane is an example of electron deficient compound. Boron atom in excited state has three half filled orbitals with three electrons available for sharing. Therefore,it can link to itself three hydrogen atoms.Thus, while each boron atom in diborane can link to itself three hydrogen atoms, there are no electrons left to form a bond between two boron atoms, as shown below.



The electron diffraction and I.R spectroscopic experiments suggest the following structure of diborane. 



In diborane, four hydrogen atoms, two on the left and two on the right,known as terminal hydrogens and other two hydrogen atoms known as bridging atoms. The two boron atoms and the four terminal hydrogen atoms lie in the same plane while the two boron atoms and the two bridging hydrogen atoms one above and the other below, lie in a plane perpendicular to this plane.


Bonding in diborane :


The bonding in diborane may be explained on the basis of an idea of three centre-two electron,         BーHーB types of bond.
In simple molecular orbital theory of covalent bonding two atomic orbitals of the same symmetry and similar energy linearly combine with each other resulting in the formation of two molecular orbitals by the LCAO method.

Valence orbitals of each boron atom in diborane are sp3 hybridised. The two terminal BH2 groups are present in one plane and the bridged part of the molecule is present in a plane perpendicular to the plane containing the two BH2 groups. 

Since two valence electrons of each boron atom are used up for BH2 bonding, the remaining valence electron of each boron atom is used in B-H-B bonding.Thus each boron atom contributes one valence electron towards the 3c-2e B-H-B bond.
The bonding molecular orbitals involved in diborane and the structure of the molecule are given below.



What is three centre-two electron bond ( 3c-2e ) ?


The three centre-two electron ( 3c-2e ) bond is a chemical bond in which two electrons are shared between three atoms. This happens usually with elements that can form electron deficient bonds. The most common example of this type of bond is diborane.

What is banana bond ? 

The chemically covalent bent bond like banana, is known as banana bond. The most common example of banana bond is diborane molecule.


Practice:

What is diborane ? explain the bonding in diborane .
What is boron hydride ?
What is 3c-2e bond ? give example.
What is banana bond ? give example.

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