S 2O2− 2, a reported sulfuroxyanion. However salts containing S 2O2− 2 and HS 2O− 2 are not well characterized; they would be conjugate bases derived from the parent thiosulfurous acid (H 2S 2O 2), which is also of doubtful existence.[1]
^Williamson, Kenneth L. (1989). "Reduction of Indigo: Sodium Hydrosulfite as a Reducing Agent". Journal of Chemical Education. 66 (4): 359. Bibcode:1989JChEd..66..359W. doi:10.1021/ed066p359.2.