Formation of PCl5 (sp3d hybridisation) : The ground state and the excited state outer electronic configurations of phosphorus (Z = 15) are represented here.
P (ground state) 3s :

↑↓

3p :

↑

↑

↑

3d :
P (excited state) 3s

↑

3p :

↑

↑

↑

3d :

↑

PCl5 : 3s :

↑↓

3p :

↑↓

↑↓

↑↓

3d :

↑↓

sp3d hybrid orbitals filled by electron pairs donated by five Cl atoms.

Now the five orbitals (i.e., one s, three p and one d orbital) are available for hybridisation to yield a set of five sp3d hybrid orbitals which are directed towards the five corners of a trigonal bipyramid as depicted in the figure.
All the bond angles in trigonal bipyramidal geometry are not equivalent. In PCl5 the five sp3d orbitals of phosphorus overlap with the singly occupied p orbitals of chlorine atoms to form five P–Cl sigma bonds. Three P–Cl bonds lie in one plane and make angle of 120° with each other; these bonds are termed as equatorial bonds. The remaining two P–Cl bonds, one lying above and the other lying below the equatorial plane, make an angle of 90° with the plane. These bonds are called axial bonds. As the axial bond pairs suffer more repulsive interaction from the equatorial bond pairs, therefore axial bonds have been found to be slightly longer and hence slightly weaker than the equatorial bonds.