H g C l 2 and I 2 both when dissolved in water containing I - ions the pair of species formed is:

In Solution containing $HgC{l}_2, I_2$ and $I^{-},$ both $HgC{l}_2$ and $I_2$ compete for $I^{-}.$
$\begin{array}{ccccc}HgC{l}_2+2I^{-}& \to & Hg{I}_2\downarrow & +& 2C{l}^{-}\\ & & \text{Red ppt}& & \end{array}$
$\begin{array}{ccc}Hg{I}_2+2I^{-}& \to & {\left[Hg{I}_4\right]}^{2-}\\ & & \text{soluble}\end{array}$
$I_2+I^{-}\to I_3^{-}$