When $\text{HgCl}_2$ is dissolved in water containing an excess of $\text{I}^-$ ions, it forms the soluble stable complex ion, tetraiodidomercurate(II) ion ($[\text{HgI}_4]^{2-}$). $\text{HgCl}_2 + 4\text{I}^- \rightarrow [\text{HgI}_4]^{2-} + 2\text{Cl}^-$ When $\text{I}_2$ is dissolved in water containing $\text{I}^-$ ions, it reacts to form the triiodide ion ($\text{I}_3^-$). Iodine is sparingly soluble in water but highly soluble in an aqueous solution of iodides (like $\text{KI}$) due to the formation of this $\text{I}_3^-$ complex ion . $\text{I}_2 + \text{I}^- \rightarrow \text{I}_3^-$ Therefore, the pair of species formed is $\text{HgI}_4^{2-}$ and $\text{I}_3^-$.