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

Hint: 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.
$$N-1s^2,2s^2,2p^3$$
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.