Question

Which of these is least likely to act as a Lewis base?
(A)- $CO$
(B)- ${{F}^{-}}$
(C)- $B{{F}_{3}}$
(D)- $P{{F}_{3}}$

Answer 1

In order to determine the Lewis base, we look for the free non-bonded electron pair in the molecule, which it can donate easily to the other species. This can be done with the help of the Lewis structures.

Complete step by step solution:
In the given molecules, from its Lewis structures, we will see the number of unpaired electrons present in it, which has a tendency to form a dative bond with the electron-deficient species, that is, Lewis acids.
So, in the carbon monoxide molecule, having a linear structure with ten valence electrons in it. The valence shell configuration of oxygen is $2{{s}^{2}}2{{p}^{4}}$ , such that it forms two sigma bonds with the carbon atom. Then, the remaining two lone pairs, one of them forms a coordinate bond with the carbon atom to complete its octet. Thus, left with one non-bonded lone pair of electrons on the oxygen atom, which contributes to its basicity.
Similarly, in the fluoride ion, ${{F}^{-}}$ with valence configuration as $2{{s}^{2}}2{{p}^{6}}$, having a negative charge and four lone pairs of electrons present in it. Thus, being electron-rich, it can donate its electrons to electron-deficient species.
Also, in phosphorus trifluoride, $P{{F}_{3}}$ the phosphorus atom having valence configuration $3{{s}^{2}}3{{p}^{3}}$. It forms three sigma-bonds with the fluorine atom, leaving it with one non-bonded lone pair of electrons. Thus, it can share this lone pair with another electron-deficient species.
Whereas, in $B{{F}_{3}}$, the boron atom having valence configuration $2{{s}^{2}}2{{p}^{1}}$ , forms three sigma bonds with the fluorine atom. So, having only six electrons around it and is two electrons less to complete its octet. Thus, it acts as a Lewis acid, having an empty p-orbital to accept electrons.

Therefore, the species which is least likely to act as a Lewis base is option (C)- $B{{F}_{3}}$.

Note: The Lewis base are electron-rich species having non-bonded electrons present in it, with a tendency to donate these electrons. They also attack the electron-deficient species. Thus, known as nucleophiles.