Explain why `PCl_(5)` is trigonal bipyramidal whereas `IF_(5)` is square pyramidal ?

Explanation

Both these compounds have different structures due to the different state of hybridisation of the central atom i.e., phosphous and iodine.
`PCl_(5) : H=1//2[5+5-0+0]=5(sp^(3)d)`
`lF_(5) : H=1//2[7+5-0+0]=6(sp^(3)d^(2))`
`PCl_(5)` has trigonal bipyramidal shape. `IF_(5)` has distorted octahedral shape which is acatually square pyramidal.