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The mechanism of the enzymatic hydrogen bond forming/breaking (2H(+) + 2e<==>H(2)) and the plausible charge and spin states of the catalytic diiron subcluster [FeFe](H) of the H cluster in Fe-only hydrogenases are probed computationally by the density functional theory. It is found that the active center [FeFe](H) can be rationally simulated as [[H](CH(3)S)(CO)(CN(-))Fe(p)(CO(b))(mu-SRS)Fe(d)(CO)(CN(-))L], where the monovalence [H] stands for the [4Fe4S](H)(2+) subcluster bridged to the [FeFe](H) moiety, (CH(3)S) represents a Cys-S, and (CO(b)) represents a bridging CO. L could be a CO, H(2)O, H(-), H(2), or a vacant coordination site on Fe(d). Model structures of possible redox states are optimized and compared with the X-ray crystallographic structures and FTIR experimental data. On the basis of the optimal structures, we study the most favorable path of concerted proton transfer and electron transfer in H(2)-forming/breaking reactions at [FeFe](H). Previous mechanisms derived from quantum chemical computations of Fe-only hydrogenases (Cao, Z.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3734; Fan, H.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3828) involved an unidentified bridging residue (mu-SRS), which is either a propanedithiolate or dithiomethylamine. Our proposed mechanism, however, does not require such a ligand but makes use of a shuttle of oxidation states of the iron atoms and a reaction site between the two iron atoms. Therefore, the hydride H(b)(-) (bridged to Fe(p) and Fe(d)) and eta(2)-H(2) at Fe(p) or Fe(d) most possibly play key roles in the dihydrogen reversible oxidation at the [FeFe](H) active center. This suggested way of H(2) formation/splitting is reminiscent of the mechanism of [NiFe] hydrogenases and therefore would unify the mechanisms of the two related enzymes.

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