Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)(3)Fe(mu-SH)(2)Fe(CO)(3) (Fe2S2H2) and (OC)(3)Fe(mu-SCH3)(mu-SH)Fe(CO)(3) (Fe2S2MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe2S2H2, Delta H double dagger and Delta S double dagger were determined to be 2.03 +/-0.56 kcal/mol and -19.3 +/- 1.7 cal/(mol K), respectively, giving k(abs) = (1.2 +/- 0.49) x 10(7) M-1 s(-1) at 25 degrees C. For Fe2S2MeH, Delta H double dagger and Delta S double dagger were determined to be 1.97 +/- 0.46 kcal/mol and -18.1 +/- 1.5 cal/(mol K), respectively, giving k(abs) = (2.3 +/- 0.23) x 10(7) M-1 s(-1) at 25 degrees C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (Delta H double dagger = 3.62 +/- 0.43 kcal/mol, Delta S double dagger = -21.7 +/- 1.3 cal/(mol K)) and H2S (Delta H double dagger = 5.13 +/- 0.99 kcal/mol, Delta S double dagger = -24.8 +/- 3.2 cal/(mol K)), giving k(abs) at 25 degrees C of (2.5 +/- 0.33) x 10(5) and (4.2 +/- 0.51) x 10(3) M-1 s(-1), respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe2S2H2 and Fe2S2MeH. Thus, Fe2S2MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (k(abs)/M-1 s(-1)/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe2S2H2 and Fe2S2MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe2S2H2 and Fe2S2MeH isomers. Derived radicals Fe2S2H center dot and Fe2S2Me center dot exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d(sigma)-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe2S2MeH was found to be 69.4 +/- 1.7 kcal/mol by determination of its pK(a) (16.0 +/- 0.4) and the potential for the oxidation of the anion, Fe2S2Me-, of -0.26 +/- 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe2S2Me center dot). This SBDFE for Fe2S2MeH corresponds to a gas-phase bond dissociation enthalpy (BDE) of 74.2 kcal/mol, in satisfactory agreement with the DFT value of 73.2 kcal/mol. Replacement of the Fe-Fe bond in Fe2S2MeH with bridging mu-S (Fe2S3MeH) or mu-CO (Fe2S2(CO)MeH) groups leads to (DFT) BDEs of 72.8 and 66.2 kcal/mol, the latter indicating dramatic effects of the choice of bridge structure on S-H bond strengths. These results provide a model for the reactivity of hydrosulfido sites of low-valent heterogeneous FeS catalysts.