Explain the structure of diborane.

Explanation

Structure of diborane : Electron diffraction studies have shown that diborane contains two coplanar `BH_2` groups . Its structure can be represented as

The four H atoms present in the `BH_2` groups are known as terminal hydrogen atoms. The remaining two H atoms are called Bridge hydrogens. The two bridge hydogens lie in a plane perpendicular to the plane of the two `BH_2` groups. Out of the two bridge hydrogens , one H atom lies above the plane and the other H atom lies below the plane
Orbital structure of Diborane :
In Diborane each boron atom undergoes `sp^3` hybridisation resulting in four equivalent `sp^3` hybrid orbitals . Three of these orbitals have one electron each and the fourth hybrid orbital is vacant . In the formation of B-H-B bridge, `sp^3` hybird orbital with one electron from one boron atom, 1s orbital of one bridge hydrogen and the vacant `sp^3` hybrid orbital of the second boron atom overlap as shown below.

The two B-H-B bridges present in Diborane are abnormal bonds. They are considered as three centered two electron bonds. This type of bond is called banana bond or tau bond . In this type of bond a pair of electrons holds three atoms . In Diborane there are two such bridges. This diboranes structure can also be represented as shown here.