As per recent studies, there is very little experimental evidence for low concentrations of $\ce{HSO3−}$ in aqueous medium, where the $\ce{S\bond{-}H}$ bond has been considered more favorable. In gas phase, however, formation of $\ce{S\bond{-}H}$ bond is considered energetically favorable, and is agreed upon by several researchers.
To add to the excellent answer by Oscar Lanzi, according to Golding R. M. (1960),$^\text{1}$ based on UV spectroscopy results in aqueous solutions, the equilibrium is dependent on concentration, and dimerization was also observed in aqueous solutions:
Controversy Surrounding Hydrogensulfite Structure
However, tautomerization remained, and remains, a topic of debate. For example, Baly E. C. C. and Bailey R. A. denied (1922) any isomerization of bisulfite.
No isomerism of sulphite molecules has been detected. The adsorption bands of sulphurous acid, bisulphite, and sulphite molecules lie in the extreme ultra-violet region.
Since, for long, tautomerization of bisulfite has been debated, I sought out for more recent works. The following are excerpts by Townsend, T. M., Allanic, A., Noonan, C., and Sodeau, J. R. (2012).$^\text{3}$
Initial calculations indicated that the hydrogen sulfonate form, $\ce{HSO3−}$, is the most stable form of the tautomers. However, Hoffmann has argued that the chemical reactivity of bisulfite ions in aqueous solutions is consistent with $\ce{HO\bond{−}SO2−}$ being the major reactive tautomer, which is in rapid equilibrium with $\ce{HSO3−}$. Baird and Taylor suggested that the $\ce{H\bond{−}S}$ bonded tautomer is the most energetically favorable. In contrast, ab-initio calculations by Stromberg indicated that the tautomeric form, bisulfite, is lower in energy. In a later study, Connick et al. proved the existence of the two tautomers in solution by examining the $\ce{S\bond{−}O}$ stretching region of Raman spectra of aqueous hydrogensulfite solutions.
Further confusion has arisen regarding the assignment of bands in the vibrational spectra of hydrogenosulfite solutions
containing both tautomers. There is a common misconception in the literature that, in the $\ce{S\bond{−}O}$ stretching region, the $\pu{1052 cm^{−1}}$ band is assignable to $\ce{S2O5^{2−}}$ and the $\pu{1021 cm^{−1}}$ band to $\ce{HOSO2−}$. However, utilizing both UV/vis and Raman spectroscopies, Connick et al. proved that the $\pu{1052 cm^{−1}}$ band is a composite of both $\ce{S2O5^{2−}}$ and one of the hydrogensulfite tautomers; the $\pu{1021 cm^{−1}}$ band is attributable to just one of the tautomers. However, the experiments were unable to distinguish between each tautomer. Risberg et al. concluded from an X-ray absorption and vibrational spectroscopic study that the $\pu{1052 cm^{−1}}$ peak was attributable to the $\ce{SO2}$ symmetric stretching mode of $\ce{HOSO2−}$, while the $\pu{1021 cm^{−1}}$ band was assigned to the $\ce{SO3}$ symmetric stretch of $\ce{HSO3−}$ only.
Zhang and Ewing have made a very tentative proposition for the existence of another dimer in solution with an $\ce{S\bond{−}S}$ bond, whose chemical formula is the same as Golding’s dimer ($\ce{H2S2O6^{2−}}$) but for which the structure is different. Like Golding’s dimer, the newly proposed complex is said to undergo condensation to form $\ce{S2O5^{2−}}$, whereas in contrast to Golding’s dimer, it is formed from two $\ce{HOSO2−}$ ions. It was stated that this structure is more plausible than the structure of Golding’s dimer because it can eliminate water from its structure more easily. However, this proposition is based on the assumption that the $\ce{HSO3−}$ concentration is very low, a statement for which there is little experimental evidence.
From this discussion, I infer that the major equilibrium existing in aqueous medium is the dimerization equilibrium, whatever the structure of dimer maybe:
$$
\ce{HO\bond{-}SO2- <=>[H2O] H2SO6^{2-}}
$$
And that in gas phase, the most stable isomer is $\ce{H\bond{-}SO3-}$.
References
- Golding, R.M., (1960). Ultraviolet Absorption Studies of the Bisulphite-Pyrosulphite Equilibrium. J. Chem. Soc., 3711-3716. 10.1039/JR9600003711
- Baly E. C. C., Bailey R. A. (1922). The equilibria in aqueous solutions of the alkali metal bisulphites. J. Chem. Soc., Trans., 121, 1813-1821. 10.1039/CT9222101813
- Townsend, T. M., Allanic, A., Noonan, C., and Sodeau, J. R. (2012). Characterization of Sulfurous Acid, Sulfite, and Bisulfite Aerosol Systems. J. Phys. Chem. A, 116, 4035-4046. 10.1021/jp212120h.
- Doctoral thesis by Mubarak Elsayed Osman at the University of London. Spectroscopic and thermodynamic properties of some oxy-sulphur species. Available as PDF.
