Density functional theory has been used to study diiron dithiolates [HFe2(xdt)(PR3)(n)(CO)(5-n)X] (n = 0, 2, 4; R = H, Me, Et; X = CH3S-, PMe3, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H(+) + 2e(-) <-> H-2 reaction. Our study is focused on the reduction step following protonation of the Fe-2(SR)(2) core. Fe(H)s detected in solution are terminal (t-H) and bridging (mu-H) hydrides. Although unstable versus mu-Hs, synthetic t-Hs feature milder reduction potentials than mu-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to mu-H. Herein, we present another strategy: in place of preventing isomerization, mu-H could be made a stronger oxidant than t-H (E degrees(mu-H) > E degrees(t-H)). The nature and number of PR3 unusually affect Delta E degrees(t-H-mu-H): 4PEt(3) models feature a mu-H with a milder E degrees than t-H, whereas the 4PMe(3) analogues behave oppositely. The correlation Delta E degrees(t-H-mu-H) <-> stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt(3) derivatives. One-electron reduction alleviates intramolecular repulsions only in y-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe-2 core. That implies that in 4-PEt3 species but not in 4PMe(3) analogues. Also determinant to observe E degrees(mu-H) > E degrees(t-H) is the presence of a Fe apical a-donor because its replacement with a CO yields E degrees(mu-H) > E degrees(t-H) even in 4PEt(3) species. Variants with neutral NHC and PMe3 in place of CH3S- still feature E degrees(mu-H) > E degrees(t-H). Replacing pdt with (Hadt)(+) lowers E degrees but yields E degrees(mu-H) > E degrees(t-H) indicating that mu-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe-2 core influences E degrees(mu-H-mu-H), provided that (i) the R size of PR3 must be greater than that of Me and (ii) an electron donor must be bound to Fe apically.