Ncert Exercises & Solutions

In the structure of diborane,

1.	All hydrogen atoms lie in one plane and boron atoms lie in a plane perpendicular to this plane.
2.	2 boron atoms and 4 terminal hydrogen atoms lie in the same plane and 2 bridging hydrogen atoms lie in the perpendicular plane.
3.	4 bridging hydrogen atoms and boron atoms lie in one plane and two terminal hydrogen atoms lie in a plane perpendicular to this plane.
4.	All the atoms are in the same plane.

Hint: Follow the structure of Diborane

Boron is trivalent, we would accept a simple hydride BH₃. However, BH₃ is not stable. The boron possesses incomplete octet and BH₃ dimerises to form B₂H₆ molecule with covalent and three centre 2-electron bond.

The simplest boron hydride is diborane B₂H₆.

As seen from the structure dawn, 6 electrons are required for the formation of the conventional covalent bond structure by B-atom, whereas in diborane, there are 12 valence electrons, three from each boron atom and six from the six hydrogen atoms.

The geometry of B₂H₆ can be represented as

and

The four-terminal hydrogen atoms and two boron atoms lie in one plane. Above and below the plane, there are two bridging hydrogen atoms. Each boron atom forms four bonds even though it has only three electrons.

The terminal B-H bonds are regular bonds but the bridge B-H bonds are different.

Each bridge hydrogen is bonded to the two boron atoms only by sharing of two electrons.

Such a covalent bond is called three center electron-pair bond or a multi-center bond or a banana bond.

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