Question

Explain why PCl₅ is trigonal bipyramidal whereas IF₅ is square pyramidal.

Answer 1

In PCl : Number of lone pair of electron on central atom = ½ (valence electron of central atom – oxidation state of central atom)    

= ½ (5-5) = 0 

Total electron pair (Valence Shell Electron Pair) on central atom= σ-bond + lone pair 

= 5+0 =5 

So hybridisation of central atom is sp³d², and structure is triagonal bipyramid. Final structure is triagonal bipyramid. 

In IF₅ : 

Number of lone pair of electron on central atom = ½ (valence electron of central atom – oxidation state of central atom)   

= ½ (7-5) =1 

Total electron pair (Valence Shell Electron Pair) on central atom= σ-bond + lone pair    = 5+1 =6 

So hybridisation of central atom is sp³d² , and spatial arrangement is structure is octahedral. Actual shape (without lone pair) is square pyramid. Final structure is square bipyramid.