Question

Which of the following species does not exist?
A. $B{{F}_{3}}$
B. $N{{F}_{3}}$
C. $P{{F}_{5}}$
D. $N{{F}_{5}}$

Answer 1

The unstable nature of trihalides of nitrogen is due to low polarity of $N-X$bond and a large difference in the size of $N$ and $X$ atoms. Phosphorus can extend its covalency beyond 3 because of the presence of empty 3d orbitals.

Complete step by step solution:
Option A: Truly $B{{F}_{3}}$ exists. Indeed it's a Lewis acid. Since it doesn't have its octet complete it is called as hypovalent. Likewise there is Back bonding occurring in $B{{F}_{3}}$ as Fluorine has electrons to give and Boron has void orbital accessible and the back bonding is of the sort $2P\pi -2P\pi$ because of which there is fractional double bond character and thus it helps in making boron less electrophilic consequently expanding its stability.
Option B:
The valence of nitrogen is 3.It has 5 electrons in its outermost orbital, but take a look at the electronic configuration.
$N-1{{s}^{2}},2{{s}^{2}},2{{p}^{3}}$
It contains 3 electrons in $2p$orbital, because there is no vacant $2d$ orbital (it is not possible as it does not exist) to fill its octet, that means to expand so $N$ can only form $N{{F}^{3}}$ and not $N{{F}^{5}}$.
Option C: Nitrogen and phosphorus both have 5 electrons in their outermost shell. So they need 3 electrons to finish their octet. So $N{{F}_{3}}$ and $P{{F}_{5}}$ exist and both nitrogen and phosphorus show the co-valency of 3. Yet, Nitrogen doesn't have empty d orbitals however phosphorus has void 3d orbital. So it can acknowledge more electrons and can expand its covalency to 5 to make $P{{F}_{5}}$.
Option D: Nitrogen doesn't have any 2d orbitals in its valence shell. In this manner, it can't broaden its covalency upto five.

Hence, the correct option is D. $N{{F}_{5}}$.

Note: $B{{F}_{3}}$ is deficient but since of bigger size of 'F' atom, it can't go through dimerization, so it exists as $B{{F}_{3}}$with a fractional negative charge in boron and halfway positive sure charge on fluorine. For the existence of any compound it must be stable enough.