Question

Why Ph5 molecule does not exist..?

Answer 1

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The extent of overlapping of d orbital of phosphorous and 1s orbital of hydrogen is very less because the energy difference between them is too high for favourable bond formation so that the most of 1s orbitals of 1s of hydrogens interact with p orbitals so that we can form PH3 but PH5 doesnt exists as d-s orbital overlapping is less and so it will dissociate into PH3 and H2.

Answer 2

Existence of compounds like PH5 is limited by the extent of d orbital's participation in hybridization. Usually d orbitals are too large and too high in energy to mix with s and p orbitals. Imagine s and p orbitals as beggars and d orbital as a rich businessman. Generally they would never have a lunch together on a same table. The radius of d orbital ia directly proportional to its energy. Only if d orbital contracts in size its energy will decrease and hybridization may occur.

(1)”If an atom carries a positive charge, then electrons will be pulled inside towards it. The effect is greatest for outer electrons.”

If central P atom is bonded to a higher electronegative element such as F, Cl, or O, then these atoms attract the shared electrons towards them. The P atom partially losses one of its shared electrons and gains a partial positive charge. Now go to (1). Electrons come close and d orbital contracts. As a result, its energy decrease and it gets hybridized with s and p orbitals. Hence, bonding occurs.

Now Hydrogen is a very less electronegative element and hence it cannot make the d orbital contract. Thus, no bonding occurs.