Question

For the following question, an assertion (A) is followed by a corresponding Reason (R). Use the following keys to choose the appropriate answer.
Assertion (A): $N(Si{H}_3{)}_3$ is a weaker base than $N(C{H}_3{)}_3$.
Reason (R): Due to pπ-pπ back bonding in $N(Si{H}_3{)}_3$, the availability of electrons on the $N$ atom in $N(Si{H}_3{)}_3$ decreases.

• A. If both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. If both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. If (A) is correct , but (R) is incorrect
• D. If (A) is incorrect, but (R) is correct

Answer 1

The correct option is C If (A) is correct , but (R) is incorrect

Assertion suggests that $N(C{H}_3{)}_3$ is a stronger base while reasoning suggests that $N(Si{H}_3{)}_3$ is a stronger base. So, only one of these two statements can be right.
We need to check the availability of lone pair of $N$ on each of the two molecules. In $N(C{H}_3{)}_3$, we see $s{p}^3$ hybridization and hence tetrahedral geometry. However in $N(Si{H}_3{)}_3$, we see that there is a possibility of back bonding due to the availability of empty d-orbitals in $Si$. Thus, it is planar.
Now that we understand the geometry and shape, we can easily understand that the lone pair on $N(Si{H}_3{)}_3$ is less available for donation. Hence, $N(C{H}_3{)}_3$ is a stronger base. This was suggested by assertion. Hence, assertion is correct and reasoning isn't.