why does PF5 forms but PH5 does not?
Phosphurus can form bonds to five ligands, as in PF5 or PCl5, by using its d-orbitals. However, this is easier if pi-overlap with p-orbitals occurs on the ligands. F and Cl have p-orbitals, H do not. Therefore PH5 is unstable, and splits very easily into the more stable PH3 + H2.