Hydrogenases catalyze the reversible oxidation of dihydrogen to protons and electrons. The structures of two Fe-only hydrogenases have been recently reported [Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853-1858. Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, E. C.; Fontecilla-Camps, J, C. Structure 1999, 7, 13-23], showing that the likely site of dihydrogen activation Is the so-called [2Fe](H) cluster, where each Fe ion is coordinated by CO and CN- ligands and the two metals are bridged by a chelating S-X-3-S ligand. Moreover, the presence of a water molecule coordinated to the distal Fe2 center suggested that the Fe2 atom could be a suitable site for binding and activation of H-2. In this contribution, we report a density functional theory investigation of the structural and electronic properties of complexes derived from the [(CO)(CH3S)(CN)Fe(II)(mu-PDT)Fe(II)(CO)(2)(CN)](-1) species, which is related to the [2Fe]H cluster observed in Fe-only hydrogenases. Our results show that the structure of the [2Fe](H) cluster observed in the enzyme does not correspond to a stable form of the isolated cluster, in the absence of the protein. As a consequence, the reactivity of [(CO)(CH3S)(CN)Fe(II) (mu-PDT)Fe(II)(CO)(2)(CN)](-1) derivatives in solution may be expected to be quite different from that of the active site of Fe-only hydrogenases. In fact, the most favorable path for H-2 activation Involves the two meta[ atoms and one of the bridging S atoms and is associated with a very low activation energy (5.3 kcal mol(-1)). The relevance of these observations for the catalytic properties of Fe-only hydrogenases is discussed in light of available experimental and theoretical data.