Treatment of isolable, paramagnetic Cp2Ta(mu-CH2)(2)CoCp (1, Cp = eta(5)-C5H5) With aromatic disulfides RSSR affords diamagnetic monothiolate complexes Cp2Ta(mu-CH2)(2)Co(SR)Cp in quantitative yield. The structure of Cp2Ta(mu-CH2)(2)Co(SC6H3(CH3)(2))Cp (2a) has been characterized by X-ray crystallography. The mechanism of this disulfide S-S bond cleavage reaction has been probed using kinetic, substituent effect, and solvent effect techniques. A Hammett sigma/rho substituent constant analysis yields a nonlinear correlation. In contrast, application of the E- and C-based dual-parameter substituent model gives a good fit of the data and demonstrates that the reaction is sensitive to both the electrostatic and the covalent characteristics of the entering disulfide. All substituents examined (excluding p-(trifluoromethyl)phenyl) exhibited a solvent effect consistent with a modest amount of charge separation in the transition state. The reaction of 1 with the strongly electron withdrawing substituted bis[p-(trifluoromethyl)phenyl] disulfide is much more sensitive to solvent polarity changes. As a comparison system expected to proceed via a limiting outer-sphere electron-transfer mechanism, the reaction of aromatic disulfides with the 19-electron complex cobaltocene (CoCp2, 3), which leads to salts [CoCp2][SR] (4), was investigated using methods parallel to those employed in the study of Ta/Co complex 1. In the cobaltocene system, the kinetic, solvent effect, and substituent effect data showed that the reaction is especially sensitive to the electrostatic character of the disulfide and relatively insensitive to its covalent character and steric bulk. The comparison of cobaltocene with Ta/Co complex 1 shows that the latter reacts with most diaryl disulfides (perhaps with the exception of p-CF3 compound 2f, which may be a borderline case) via a transition state with substantially more covalent character, acid correspondingly less charge separation, than that involved in the outer-sphere reactions of Cp2Co.