Transition metal sulfito complexes are coordination compounds containing sulfite () as a ligand. Many examples have been characterized. Few sulfito complexes have commercial applications, but sulfite is a substrate for the molybdoenzyme sulfite oxidase.
Bonding modes 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 .
Sulfite can function as a bridging ligand. One example is (en = ethylenediamine)
In some cases, sulfite simply serves as an anion. An example is the hexaaquo complex [Ni(H2O)6]SO3.
Preparation Sulfito complexes are typically prepared by substitution of metal complexes with sodium sulfite. The method is illustrated by the syntheses of the octahedral rhodium(III) and cobalt(III) complexes (M = Rh, Co), (where en = ethylenediamine). Some metal-sulfur dioxide complexes react with aqueous base to give sulfito derivatives:
Examples , octahedral cobalt(III) bound also to a pentadentate amine (tetren = ) , square planar palladium(II) , square planar gold(III) , illustrating a rare anion with a –10 charge consisting of three square planar platinum(II) centers where the Co-N bond trans to S is 205.5 pm in length, and Co-N bonds cis to S are 196 pm, ca. 5% shorter. This difference illustrates the high trans influence exerted by the sulfito ligand.
Complexes with modified sulfito ligands
Being dibasic, sulfito ligands are susceptible to O-alkylation and O-protonation. Some examples of such complexes:
References