Question

Assertion (A): In $B_{2}H_{6}$, two bridging H atoms prevent the rotation between B atoms. Reason (R): Terminal B-H bonds are normal covalent bonds i.e. two centre two electron ($2C-2e$) bonds.}

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Bridges act like a "lock." Rotation is only possible in single (sigma) bonds without steric or structural constraints.

Answer 1

The Correct Option is B

Step 1: Concept Diborane ($B_{2}H_{6}$) has a bridged structure with two $3C-2e$ (banana) bonds and four terminal $2C-2e$ bonds.

Step 2: Meaning The bridge consists of two Boron atoms and two Hydrogen atoms in a plane perpendicular to the terminal bonds. This rigid cyclic structure prevents internal rotation.

Step 3: Analysis Assertion (A) is correct; the bridging system locks the molecule. Reason (R) is also a correct factual statement about terminal bonds. However, the nature of the terminal bonds does not explain why rotation is prevented; the bridging bonds ($3C-2e$) are responsible for that.

Step 4: Conclusion Both are correct, but (R) is not the explanation for (A). Final Answer: (B)