${\text{HgCl}}_2$ and ${\text{I}}_2$ both when dissolved in water containing ${\text{I}}^{-}$ ions the pair of species formed is:

1. Iodine reacts with iodide ions to form the triiodide complex: ${\text{I}}_2+{\text{I}}^{-}\to {\text{I}}_3^{-}$.2. Mercury(II) chloride reacts with excess iodide ions to form the soluble tetraiodomercurate(II) complex: ${\text{HgCl}}_2+4{\text{I}}^{-}\to {\text{HgI}}_4^{2-}+2{\text{Cl}}^{-}$.Thus, the species are ${\text{HgI}}_4^{2-}$ and ${\text{I}}_3^{-}$.