Question

Which of the following compound cannot act as Lewis base?

• A. ${\mathrm{ClF}}_5$
• B. ${\mathrm{PCl}}_5$
• C. ${\mathrm{NF}}_3$
• D. ${\mathrm{SF}}_4$

Answer 1

The correct option is B

${\mathrm{PCl}}_5$

The explanation of the correct answer

(B)${\mathrm{PCl}}_5$

1. In ${\mathrm{PCl}}_5$, Phosphorus have five bond pairs of $\mathrm{P}-\mathrm{Cl}$ the bond.
2. It does not have any lone pairs to donate and hence, cannot act as a Lewis base.
3. It can accept a pair of electrons from a Lewis base, so acts as a Lewis acid.

The Explanation for the incorrect answer

(A)${\mathrm{ClF}}_3$

1. In ${\mathrm{ClF}}_3$, Chlorine atom has three bond pairs of the bond $\mathrm{Cl}-\mathrm{F}$ and two lone pairs.
2. As it has two lone pairs to donate and hence, can act as Lewis base.

(C)${\mathrm{NF}}_3$

1. IN ${\mathrm{NF}}_3$, neither Nitrogen nor Fluorine has any vacant d orbital.
2. It cannot accept any electron and hence cannot acts as Lewis acid.
3. Thus it is a Lewis base.

(D)${\mathrm{SF}}_4$

1. In ${\mathrm{SF}}_4$, Sulphur is bonded with four fluorine atoms and Sulphur has six electrons in its valence shell.
2. Sulphur is left with a lone pair and can donate its electrons and hence can act as a Lewis base.

Therefore, option B is correct. ${\mathrm{PCl}}_5$ cannot act as a Lewis base.