The cobalt(III) complexes of the hexadentate N2S2O2-coordinating ligand, 1,8-N-disalicylidene-3,6-dithiaoctane-1,8-diamine (III) and its 3,6-diaza N4O2-coordinating analog (IV) adopt structures of type VI, in which O-N-S or O-N-N sequences span meridional positions. In this comparison of the rates of reduction of the two complexes (to Co2+ in 0.1 M H+) using as reductants Cr(II), Eu(II), V(II), Ti(III), RU(NH3)(6)(2+), and the dihydro derivative of the N-methylphenazinium cation, a greatly enhanced reactivity (10(3)-10(6)) of the N2S2O2-bound oxidant is found to persist for both inner- and outer-sphere reductions and extends to the metal-free reductant as well. It is suggested that the higher rates for the S-2-bound system reflect the acquisition of a quasiaromatic character in the (S)(2)Co-III arm of this oxidant, within which electron transfer to the metal center requires less severe stretching of the metal-ligand bonds than is necessary for N- and O-ligated complexes. The augmentation in k(Cr(II)) values of carboxylato complexes of Co(III) on incorporation of a-thio substituents may, on the other hand, stem from an increased degree of conversion to a chelated Co(III)-Cr(II) precursor, VII, which is stabilized partially by back-bonding involving interaction of the e(g) electron of Cr(II) and a d-orbital of sulfur. In contrast to the reactions of oxidants featuring donor sulfur in the primary coordination sphere, this mode of acceleration does not extend to reductions by V2+, Eu2+ or Ti(III) (which are devoid of e(g) electrons) or to the outer-sphere reductions by Ru(NH3)6(2+). It is, however, observed with the d(10) reductant, Cu+.