The tetrathionate anion, $S _4 O _6^{2-}$, is a sulphur oxoanion derived from the compound tetrathionic acid, $H _2 S _4 O _6$. Two of the sulphur atoms present in the ion are in oxidation state $0$ and two are in oxidation state $+5$.
${H_2}O\,\, + \,\,\,B{r_2}\,\,\,\, \to \,\,\,HOBr\,\,\, + \,\,\,HBr$