In the following reaction, compound (X) is :
${\mathrm{B}}_2{\mathrm{H}}_6+2\mathrm{X}⟶2{\mathrm{BH}}_3\mathrm{X}$

${\mathrm{B}}_2{\mathrm{H}}_6+2\mathrm{X}\stackrel{\text{ Symmetrical }}{\underset{\mathrm{Cleavage}}{\to }}2\left[{\mathrm{BH}}_3\mathrm{X}\right]$
X: Must be weak Lewis base or large sized strong Iewis base.
3° amine (Large size base) causes symmetrical cleavage in B₂H₆ (Diborane), while ${\ddot{\mathrm{NH}}}_3$, 1° amine and 2° amine causes unsymmetrical cleavage.