Question

$BC{l}_3$ does not exist as dimer but $B{H}_3$ exists as dimer $\left(B_2{H}_6\right)$ because:

• A. Chlorine is more electronegative than hydrogen
• B. There is $p_{\pi }-p_{\pi }$ back bonding in $BC{l}_3$, but $B{H}_3$ does not contain such multiple bonding
• C. Large-sized chlorine atoms do not fit in between the small boron atoms whereas small-sized hydrogen atoms fit between boron atoms
• D. None of these

Answer 1

The correct option is C Large-sized chlorine atoms do not fit in between the small boron atoms whereas small-sized hydrogen atoms fit between boron atoms

$B{H}_3$ is an electron-deficient compound which has an empty p orbital or we can say 6 electrons in its outermost shell. As the size of Hydrogen is small therefore it facilitates the dimerisation of $B{H}_3$ entered 2 electron bond.
This dimerisation results in the formation of diborane, here single hydrogen are shared between two boron atoms. On the other hand, $BC{l}_3$ does not dimerizes due to its small size. As the size of boron is small therefore it is unable to accommodate 4 large chlorine atoms at the same time.

Hence, the correct option is $C$