In the reaction  $\left[{\mathrm{Ph}}_4\mathrm{As}\right]\left[\mathrm{Cl}\right]+{\mathrm{Cl}}_2\stackrel{-{78}^{\circ }\mathrm{C}}{⟶}\left[{\mathrm{Ph}}_4\mathrm{As}\right]\left[{\mathrm{Cl}}_3\right]$

The reaction is,

$\left[{\mathrm{Ph}}_4\mathrm{As}\right]\left[\mathrm{Cl}\right]+{\mathrm{Cl}}_2\stackrel{-{78}^{\circ }\mathrm{C}}{⟶}\left[{\mathrm{Ph}}_4\mathrm{As}\right]\left[{\mathrm{Cl}}_3\right]$

Here Cl₂ acts as a Lewis acid as here it is accepting electron pair.