Question

Describe the hybridisation in case of \( PCl_5\). Why are the axial bonds longer as compared to equatorial bonds?

Answer 1

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.

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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).

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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.

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Conclusion:

Thus, PCl₅ shows sp³d hybridisation with trigonal bipyramidal geometry, and the axial bonds are longer than equatorial bonds due to greater repulsive interactions.