On adding $\mathrm{KI}$to benzenediazonium chloride, the product obtained is

Treating with $\mathrm{KI}$produces iodobenzene.  When benzene diazonium chloride reacts with potassium iodide, iodobenzene is obtained. Diazonium group is replaced with −I.

${\mathrm{C}}_6{\mathrm{H}}_5-{\mathrm{N}}_2^{+}{\mathrm{Cl}}^{-}+\mathrm{KI}\stackrel{\mathrm{\Delta }}{⟶}{\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{I}+{\mathrm{N}}_2\uparrow +\mathrm{KCl}$