Transition metal sulfito complex

Transition metal sulfito complexes are coordination compounds containing sulfite (SO₃^2-) as a ligand. The inventory is large. Few sulfito complexes have commercial applications, but sulfite is a substrate for the molybdoenzyme sulfite oxidase.^[2]

The structure of [Co(ethylenediamine)₂(SO₃)N₃]. Color code: blue = N, red = O, yellow = S.^[1]

Bonding modes

In principle, sulfite can bond to metal ions via S or O. To some extent, the sulfito ligand resembles nitrito (NO−2), 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 [Co(SO₃)(NH₃)₅]⁺ to give [Co(SO₃)(NH₃)₄(H₂O)]⁺.^[3]

In some cases, sulfite serves as a bridging ligand forming M-SO₂-O-M’ linkages.^[4]

Examples

• [Co(tetren)SO₃]^+[5] (tetren = HN(CH₂CH₂NHCH₂CH₂NH₂)₂)
• [M(SO₃)₂(en)₂]⁻ (M = Rh,^[6] Co,^[7][8] en = ethylenediamine)
• [Pd(en)(SO₃)₂]^{2− [9]}
• [Au(en)(SO₃)₂]^{− [10]}

Complexes with modified sulfito ligands

Structure of [PtCl₂(SO₃H)(SO₃)]^3−[11]

Being dibasic, sulfito ligands are susceptible to O-alkylation and O-protonation. Some examples:

• Ru(phen)₂(SO₃H)₂^[12]
• Pt[P(C₆H₅)₃]₂(SO₃CH₃)₂^[13]
• Pt₂(en)₂(mu−S,O−SO₃)₂ ^[14]
• [PtCl₂(SO₃H)(SO₃)]^3−[11]