Hydrogen uptake in hydrogenase enzymes can be assayed by H/D exchange reactivity in H-2/D2O or H-2/D-2/H2O mixtures. Diiron(I) complexes that serve as structural models for the active Site of iron hydrogenase are not active in such isotope scrambling but serve as precursors to (FeFeII)-Fe-II complexes that are functional models of [Fe]H(2)ase. Using the same experimental protocol as used previously for {(mu-H)(mu-pdt):Fe(CO)(2)(PMe3)](2)(+)}, 1-H+ (Zhao et al. J. Am. Chem. Soc. 2001, 123, 9710), we now report the results of studies of {(mu-SMe)(mu-pdt)[Fe(CO)(2)(PMe3)](2)(+)}, 1-SMe+, toward H/D exchange. The 1-SMe+ complex can take up H-2 and catalyze the H/D exchange reaction in D-2/H2O mixtures under photolytic, CO-loss conditions. Unlike 1-H+, it does not catalyze H-2/D-2 scrambling under anhydrous conditions. The molecular structure of 1-SMe+ involves an elongated Fe...Fe separation, 3.11 Angstrom, relative to 2.58 Angstrom in 1-H+. It is proposed that the strong SMe- bridging ligand results in catalytic activity localized on a single Fell center, a scenario that is also a prominent possibility for the enzyme active site. The single requirement is an open site on Fe-II available for binding of D-2 (or H-2), followed by deprotonation by the external base H2O (or D2O).