A ⇏ B ⇊ {\displaystyle A \nRightarrow B\downdownarrows}
Hg 2 + → I − HgI 2 → I − [ Hg II I 4 ] 2 − {\displaystyle \ce{Hg^2+ ->[I-] HgI2 ->[I-] {[Hg^{II}I4]^2-}}}
