| Summary: | Fe<sup>I</sup>Fe<sup>I</sup> Fe<sub>2</sub>(S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)(CO)<sub>6</sub>(µ-CO) (<b><sup>1</sup>a–CO</b>) and its Fe<sup>I</sup>Fe<sup>II</sup> cationic species (<b><sup>2</sup>a<sup>+</sup>–CO</b>) are the simplest model of the CO-inhibited [FeFe] hydrogenase active site, which is known to undergo CO photolysis within a temperature-dependent process whose products and mechanism are still a matter of debate. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations, the ground state and low-lying excited-state potential energy surfaces (PESs) of <b><sup>1</sup>a–CO</b> and <b><sup>2</sup>a<sup>+</sup>–CO</b> have been explored aimed at elucidating the dynamics of the CO photolysis yielding Fe<sub>2</sub>(S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)(CO)<sub>6</sub> (<b><sup>1</sup>a</b>) and [Fe<sub>2</sub>(S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)(CO)<sub>6</sub>]<sup>+</sup> (<b><sup>2</sup>a<sup>+</sup></b>), two simple models of the catalytic site of the enzyme. Two main results came out from these investigations. First, <b>a–CO</b> and <b><sup>2</sup>a<sup>+</sup>–CO</b> are both bound with respect to any CO dissociation with the lowest free energy barriers around 10 kcal mol<sup>−1</sup>, suggesting that at least <b><sup>2</sup>a<sup>+</sup>–CO</b> may be synthesized. Second, focusing on the cationic form, we found at least two clear excited-state channels along the PESs of <b><sup>2</sup>a<sup>+</sup>–CO</b> that are unbound with respect to equatorial CO dissociation.
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