Why PF5 is known but neither NCl5 nor NF5 is known?

Answer 1

There are 5 outermost electrons in both phosphorus and nitrogen atoms but in nitrogen no more than three unpaired electrons are possible due to absence of d-orbitals in 2nd energy level so nitrogen may form only three covalent bonds (as NCl3 and NF3) but d-orbitals are present in 3rd energy level of Phosphorus so in excited state it may have five unpaired electrons and may form PCl5.

Answer 2

PF5 is known because phosphorus readily forms a stable compound due to the presence of empty d orbitals which can accommodate additional electrons. In contrast, NCl5 and NF5 are not known due to the high electronegativity of nitrogen and the resultant instability of compounds with high coordination numbers. Additionally, the large size of chlorine atoms makes it difficult for them to bond with multiple nitrogen atoms.