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Question

BCl3 does not exist as dimer but BH3 exists as dimer (B2H6) because:
  1. Chlorine is more electronegative than hydrogen
  2. There is pπpπ back bonding in BCl3, but BH3 does not contain such multiple bonding
  3. Large-sized chlorine atoms do not fit in between the small boron atoms whereas small-sized hydrogen atoms fit between boron atoms
  4. None of these

A
Chlorine is more electronegative than hydrogen
B
There is pπpπ back bonding in BCl3, but BH3 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
Solution
Verified by Toppr

BH3 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 BH3 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, BCl3 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

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