Question

${\mathrm{BCl}}_3$ does not exist as dimer but ${\mathrm{BH}}_3$ exists as dimer $\left({\mathrm{B}}_2{\mathrm{H}}_6\right)$. Explain Why?

Answer 1

• ${\mathrm{BH}}_3$ is an electron-deficient compound that has an empty p orbital.
• Hydrogen is smaller in size, therefore it facilitates the dimerisation of ${\mathrm{BH}}_3$ entered two electron bond.
• This dimerisation results in the formation of Diborane that is $\left({\mathrm{B}}_2{\mathrm{H}}_6\right)$.
• Here a single hydrogen atom is shared between two Boron atoms.
• The structure of Diborane is:

• On the other hand we have Chlorine which is bigger in size than both hydrogen and Boron.
• Thus Boron is unable to accommodate 4 large chlorine atoms at the same time.
• As a result ${\mathrm{BCl}}_3$ does not dimerize.