Substitution behaviour of an unusual seven-coordinate iron(III) complex in different solvents

In order to clarify the solution chemistry of 3d metal hepta-coordinate complexes, a detailed mechanistic study of the two-step substitution reaction between the “tweezer-like” iron(III) complex, [Fe(dapsox)(H2O)2]+ (H2dapsox=2,6-diacetylpyridine-bis-(semioxamazide)), and SCN− was performed in water and methanol as solvents. Based on the observed rate law and reported activation parameters (ΔH#, ΔS# and ΔV#) for both reaction steps in aqueous solution, the operation of an Ia mechanism for the diaqua, an Id mechanism for the aquathiocyanato, and an Id or D mechanism for the aquahydroxo forms of the Fe(III) complex, is proposed. The operation of a limiting D mechanism is proposed for both reaction steps in methanol as solvent.