Question

ncl5 is not formed why

Answer 1

ncl5 not fomed because it open is a 2nd period element which has no vacant d-orbital whereas Phosphorus is the element of 3rd period which has vacantd-orbital. Hence Phosphorus can form bonds with 5 Chlorine atoms but Nitrogen can't.

Answer 2

NCL 5 does not exist
because of absence of the d orbital in nitrogen to accomodate electrons from chlorine
nitrogen cannot accommodate more than 8 electrons in its valence shell