Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to investigate how visible light photons can excite an asymmetrically substituted diiron hydride [Fe-2(pdt)(mu-H)(CO)(4)dppv](+)(1(+), dppv = cis-1,2- C2H2(PPh2)(2); pdt = 1,3-propanedithiolate), as well as the symmetric species [Fe-2(pdt)(mu-H)(CO)(4)(PMe3)(2)](+) (2(+)) which are the first photocatalysts of proton reduction operating without employing sensitizers (Wang, W.; Rauchfuss, T. B.; Bertini, L.; Zampella, G.; J. Am. Chem.. Soc., 2012, 134, 4525). Theoretical results illustrate that the peculiar reactivity associated to the -excited states of 1(+) and 2(+) is compatible with three different scenarios: (i) it can arise from the movement of the hydride ligand from fully bridging to semibridging/terminal coordination, which is expected to be more reactive toward protons, (ii) reactivity could be related to cleavage of a Fe-S bond, which implies formation of a-transient Fe penta-coordinate species that would trigger a facile turnstile hydride isomerization, if lifetime excitation is long. enough; (iii) also in line with a Fe-S bond cleavage is the possibility that after excited state decay, a highly basic S center is protonated so that a species simultaneously containing 5-H delta+ and Fe-H delta- moieties is formed,and, once reduced by a.suitable electron donor, it can readily afford H-2 plus an unprotonated form of the FeFe complex. This last possibility is consistent with P-31 NMR and IR solution data All the three possibilities are compatible with the capability of 1(+) and 2(+) to perform photocatalysis of hydrogen evolving reaction (HER) without sensitizer. Moreover, even though it turned out difficult to discriminate among the three scenarios, especially because of the lack of experimental excitation lifetimes, it is worth underscoring that all of the three pathways represent a novelty regarding diiron carbonyl photoreactivity, which is usually associated. with CO loss. Results provide also a rationale to the experimental observations which showed that the simultaneous presence of donor ligands (dppv in the case of 1(+)) and a H ligand in the coordination environment of diiron complexes is a key factor to prevent CO photodissociation and. catalyze HER. Finally, the comparison of photoexcitation behavior of 1(+) and 2(+) allows a sort of generalization about the functioning of such hydride species.