Question

Describe the shape of PCl₅ molecule. 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: 

Excited state: Ne 3s¹ 3px ¹ 3py ¹ 3p¹ _z 3d¹ 

Phosphorus atom is sp³ d hybridized in the excited state. These orbitals are filled by the electron pairs donated by five Cl atoms. The five sp³ d hybrid orbitals are directed towards the five corners of the trigonal bi-pyramidals. 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