Direct studies on 5-coordinate intermediates formed during substitution at tetrahedral Fe sites: role of bound nucleophile in labilisation of leaving group.

The substitution reactions of the tetrahedral Fe sites in [FeCl(4)](-), [Fe(2)S(2)Cl(4)](2-), [Fe(4)S(4)Cl(4)](2-) and [{MoFe(3)S(4)Cl(3)}(2)(micro-SEt)(3)](3-) with 4-RC(6)H(4)S(-) (R = MeO, Me, H, Cl or NO(2)) all involve rapid binding of the thiolate to a Fe site and formation of a kinetically and spectroscopically detectable intermediate. Kinetic studies allow calculation of the rate of Fe-Cl dissociation from the 5-coordinate site of the intermediate (k(2)(R)). The rate of Fe-Cl dissociation from the intermediate exhibits a marked dependence on the nature of the bound thiolate with log(10)(k(2)(R)) increasing in a linear manner with the calculated NBO charge on the sulfur atom of the coordinated thiolate. This behaviour indicates that Fe-Cl bond dissociation at the 5-coordinate intermediate involves a process in which Fe-thiolate bond shortening occurs prior to movement of the Fe-Cl bond.