REACTIONS OF [S2M(MU-S)(2)FECL2](2-) (M=MO OR W) WITH ARENETHIOLATE IONS - INFLUENCE OF NEIGHBORING METAL SITES ON THE RATES AND MECHANISMSOF SUBSTITUTION AT IRON

The kinetics of substitution of the chloro-groups in [S2Mo(mu-S)(2)FeC l2](2-) by 4-RC6H4S- (R=Cl, H, Me or MeO) to form [S2Mo(mu-S)(2)Fe(SC6 H4R-4)(2))(2-) has been studied in MeCN using stopped-flow spectrophot ometry. The mechanism involves initial binding of thiolate to Mo to ge nerate [S-2(4-RC6H4S)Mo(mu-S)(2)FeCl2](3-), which has been detected by spectrophotometry and H-1 NMR spectroscopy. Subsequently, this specie s undergoes substitution of the chloro-groups on the neighbouring Fe b y associative and dissociative pathways. The Mo-SC6H4R-4 residue affec ts these mechanisms in two ways: electron-withdrawing substituents fac ilitate the associative pathway (Hammett rho=+0.63) whilst electron-re leasing substituents facilitate the dissociative pathway (rho=-1.4).[S 2W(mu-S)(2)FeCl2](2-) and [O2Mo(mu-S)(2)FeCl2](2-) indicated similar m echanisms.