`BCl_(3)` does not exist as dimer but `BH_(3)` exist as dimer `(B_(2)H_(6))` because :-

Options

A. chlorine is more electronegative than hydrogen
B. there is `p pi-p pi` back bonding in `BCl_(3)` but `BH_(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 get fitted in between
D. none of the above

Answer

C

Explanation

Small sized boron cannot coordinate to four large sized chlorine atoms but can coordinate to four small sized hydrogen atoms (in `B_(2)H_(6)`).