Question
Wed January 04, 2012 By: Utkarsh Agarwal

WHY THE AXIAL BONDS IN PCL5 IS LONGER THAN EQUITORAL BONDS IN THE SAME ?

Expert Reply
Wed January 04, 2012

 Central atom:

P

 

P contributes:

5 e?

5 x Cl contibute:

5 e?

 

 
  • There are two P–Cl bonding environments in this molecule:
    • Each equatorial P–Cl bond makes two 90° and two 120° bond angles with the other bonds in the molecule.
    • Each axial P–Cl bond makes three 90° and one 180° bond angles with the other bonds in the molecule.
  • The difference in bond length is due to repulsive forces from electron pairs at other bond angle.

    In PCl5 there are electron pairs at 2 axial and 3 equatorial bonds.The e- pair at equatorial position are at an angle of 1200 from other two e- pairs at equatorial positions and 90o from the 2 pairs of e- at axial position. Thus it suffers repulsive forces from 2 e- pair at 900 and two at 1200.

    The e- pair at axial position is repelled by three e- pair at 900 that are at equatorial position and 1 e- pair at 1800 which is at other axial end.

    Repulsive force decreases with increase in bond angle. The repulsive force from 1200 and 1800 bond angle is almost negligible. So the axial position has 3 repulsions from 900 and equatorial has 2 repulsion from 900. As axial is repelled more it gets farther and the bond length increases.

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