${\text{Hg}}_{}^{2+}\stackrel{{\text{I}}_{}^{-}}{\to }\underset{\mathrm{r}\mathrm{e}\mathrm{d}}{{\text{HgI}}_2^{}}\stackrel{{\text{I}}_{}^{-}}{\to }\underset{\mathrm{r}\mathrm{e}\mathrm{d}}{{[{\text{Hg}}_{}^{\text{II}}{\text{I}}_4^{}]}_{}^{2-}}$