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2015-12-25T14:12:57+05:30
Due to small size of Cl atom, five Cl atoms can be accommodated around P atom. But I is of large size and therefore, five I atoms cannot be accommodated around P atom. As a result, P - I bonds are weak and prefer to form PI3 rather than PI5.

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3 5 3
2015-12-25T18:07:19+05:30
Ya the reason is this that P is so small in size than I therefore PI5 not known.
While the Cl is also big in size than P but its less big in size than I therefore PCl5 is known.
I hope this is helpful for u;)
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