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.