1Wikipedia:
A phosphorane (IUPAC nomenclature: λ5-phosphane) is a functional group in organophosphorus chemistry with pentavalent phosphorus. It has the general formula PR5. The parent compound is the unstable molecule PH5. The derivative pentaphenylphosphorane (Ph5P) is stable.
Phosphoranes adopt a trigonal bipyramidal molecular geometry with the two apical bonds longer than the three equatorial bonds. Bonding is described by three-center, 4-electron bonds, as also invoke for closely related molecule phosphorus pentafluoride.
1the bonds of P-H won't be able to sustain the repulsion from the equatorial bonds rendering the molecule unstable.
29the extent of overlapping of d orbital of P and 1s orbital of H is very less coz the energy difference between them is too high for favourable bond formation so at the most 1s of H can interact considerably with p orbitals that is why PH3 exists whereas u don't see PH5 as d-s overlapping is less so it dissociates into PH3 and H2....
39Small size of phosphor atom, small length of P-I bond and large size of iodine atom = instability. Add to that axial bonds in the hypothetical structure of PI5 which would face repulsions.
