thiosulfurous acid

Thiosulfurous acid (HS-S(=O)-OH) or sulfurothionous O,O acid is a low oxidation state (+1) sulfur acid [ (+1) is the average oxidation state of the two structurally different sulfur atoms . The exterior atom has a oxidation number of (-1) while the central sulfur atom has the oxidation state of (+3) ]. It is termed a disulfur(I) acid. It is the equivalent acid for disulfur monoxide. Thiosulfurous acid is formed by reacting hydrogen sulfide and sulfur dioxide at room temperature. The thiosulfurous acid molecule is unstable when condensed, reacting with itself.
It decomposes to polysulfane oxide or polythionic acids in water which is termed Wakenroder's liquid.
Other possible isomers are dihydroxydislfane or hypodithionous acid HOSSOH, a linear chain, and thiothionyl hydroxide (S=S(OH)2) a tautomer where the hydrogen has moved from a sulfur to an oxygen. HOSSOH can have two different rotamers with symmetry C1 and C2. The isomer with one hydrogen on sulfur and one on oxygen is the most stable according to calculations.