Question

$BCl_3$ does not exist as dimer but $BH_3$ exists as dimer $(B_2H_6)$ because

• 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 where as small sized hydrogen atoms get fitted in between boron atoms
• D. none of the above

Answer 1

Answer: (C)

$BCl _{3}$ does not exist as dimer but $BH _{3}$ exists as dimer as dimer $B 2 H 6$ because, large sized chlorine atoms do not fit in between the small boron atoms where as small sized hydrogen atoms get fitted in between boron atoms Because of its instability it exists as a dimer, $B _{2} H_6$, where electrons are shared between each of the monomers. It is a gas, but is used in chemistry as solutions in THF, pyridine or dimethyl sulfide. In these solutions the lone pair of electrons from the oxyge, nitrogen or sulfur, are donated to the empty p orbital of boron, stabilising the $BH 3:$ solvent complex.