Transition metal sulfito complexes are coordination compounds containing sulfite (SO<sub>3</sub><sup>2-</sup>) as a ligand. The inventory is large. Few sulfito complexes have commercial applications, but sulfite is a substrate for the molybdoenzyme sulfite oxidase.
In principle, sulfite can bond to metal ions via S or O. To some extent, the sulfito ligand resembles nitrito (), which can bind through N or O. Monodentate, S-bonded sulfites are more common than O-bonded sulfito ligands. S-Bonded sulfite is a soft ligand with a strongly trans labilizing effect as indicated by the rapid aquation of to give .
In some cases, sulfite serves as a bridging ligand forming M-SO<sub>2</sub>-O-Mâ linkages.
Being dibasic, sulfito ligands are susceptible to O-alkylation and O-protonation. Some examples: