Question

Why PCl5 form and PH5 not?

Answer 1

In PCl5, Cl is more electronegative than P, P gets partial +ve charge , and hence orbital contraction occurs. s-d excitation takes place and P now has five unpaired electrons . So PCl5 exists. 

But in case of PH5 orbital intraction doesn't  occus coz D-ORBITAL of P interacts with s- orbital of H and it is not stable according to energy consideration.
it is very unstable
so PH5 doesn't exists