Question

PCl5 is stable bt PH5 is not

Answer 1

PH5 doesn't exist because d-orbital of P interacts with s-orbital of H.Bond formed isn't stable and not energeticallyfavorable.It depends on size and orientation of interaction.

PCl5 is stable because bond formed is d-d bond and lone pair of Cl does back-bonding with empty d-orbital of P.This makes bond more strong.So it lowers the energy of the molecule thus making it more stable.