Hybridisation in PCl5:
In phosphorus pentachloride (PCl5), the central phosphorus atom undergoes sp3d hybridisation.
The electronic configuration of phosphorus is:
3s2 3p3
In the excited state, one electron from the 3s orbital is promoted to the 3d orbital, giving:
3s1 3p3 3d1
These one s, three p and one d orbitals hybridise to form five equivalent sp3d hybrid orbitals.
These orbitals arrange themselves in a trigonal bipyramidal geometry.
Structure of PCl5:
• Three chlorine atoms occupy the equatorial positions (120° apart).
• Two chlorine atoms occupy the axial positions (180° apart and 90° from equatorial bonds).
Why are axial bonds longer than equatorial bonds?
Axial bonds experience greater repulsion compared to equatorial bonds.
Each axial bond is at 90° to three equatorial bonds, leading to strong bond-pair–bond-pair repulsions.
In contrast, equatorial bonds are separated by 120° and experience less repulsion.
Due to greater repulsion, the axial P–Cl bonds are pushed away and become longer and weaker than equatorial P–Cl bonds.
Conclusion:
Thus, PCl5 shows sp3d hybridisation with trigonal bipyramidal geometry, and the axial bonds are longer than equatorial bonds due to greater repulsive interactions.
Hybridisation and geometry of [Ni(CN)$_4$]$^{2-}$ are