Abstract In an attempt to produce substrate-Fe(II) complexes potentially relevant to the mechanism of action of sulfite reductase, several anaerobic reaction systems were investigated. The system Fe(TPP)+SO 2 in benzene afforded [(FE(TPP)) 2SO 4] ( 1), which was obtained in single crystal form as the solvate [(Fe- (TPP)) 2SO 4]·C 6H 6·2SO 2. This compound crystallizes in orthorhombic space group P2 122 1 with a= 12.647(2), b=13.483(3), c=23.409(6) Å and Z=2. Complex 1 contains five-coordinate Fe(III) units bridged by sulfate ion binding as a monodentate ligand to each metal. The structure of 1 is closely similar to that in a differently solvated, rhombohedral lattice reported earlier. The oxidized product results from traces of dioxygen and is probably formed from a bridged peroxide intermediate in a reaction demonstrated earlier. The system [Fe(Me 6- -4,11-dieneN 4)(MeCN) 2] 2++(n-Bu 4N)HSO 3 in acetonitrile yielded yellow, high-spin [Fe(Me 6- 4,11-dieneN 4)(S 2O 5)] ( 2). Compound 2 crystallizes as the acetonitrile monosolvate in monoclinic space group C2/ c with a=9.575(3), b=16.557(4), c= 15.409(4) Å, β=96.85(2)° and Z=4. Complex 2 contains a chelating disulfite, generated in the equilibrium 2HSO 3 −⇌S 2O 5 2−+H 2O. Dimensions of coordinated and free disulfite are not significantly different; however, the ligand is disordered around a C 2 axis passing through the Fe atom and bisecting the SS bond of the chelate ring. This is the first structure of a transition metal disulfite complex. The results suggest the conditions necessary to the formation of an authentic sulfite or bisulfite complex of a Fe(II) tetraaza macrocycle, species potentially suitable as models of the enzyme-substrate complex of sulfite reductase.