Question

Describe the hybridisation in case of PCl₅. Why are the axial bonds longer as compared to equatorial bonds?

Answer 1

The ground state and excited state outer electronic configurations of phosphorus (Z = 15) are:

Phosphorus atom is sp³d hybridized in the excited state. These orbitals are filled by the electron pairs donated by five Cl atoms as:

PCl₅

The five sp³d hybrid orbitals are directed towards the five corners of the trigonal bipyramidals. Hence, the geometry of PCl₅ can be represented as:

There are five P–Cl sigma bonds in PCl₅. Three P–Cl bonds lie in one plane and make an angle of 120° with each other. These bonds are called equatorial bonds.

The remaining two P–Cl bonds lie above and below the equatorial plane and make an angle of 90° with the plane. These bonds are called axial bonds.

As the axial bond pairs suffer more repulsion from the equatorial bond pairs, axial bonds are slightly longer than equatorial bonds.