A series of iron azadithiolate complexes possessing an intramolecular secondary coordination sphere interaction and an ability to reduce HOAc at the potential near the first electron-transfer process are reported. A unique structural feature in which the aza nitrogen has its lone pair point toward the apical carbonyl carbon is observed in [Fe-2(mu-S(CH2)(2)NR-(CH2)(2)S)(CO)(6-x)L-x](2) (R = Pr-n, x = 0, la; R = Pr-i, x= 0, 1b; R = Pr-n L= PPh3, x= 1,2; R = Pr-n, L = (PBu3)-Bu-n, x = 1,3) as biomimetic models of the active site of Fe-only hydrogenase. The presence of this weak N center dot center dot center dot C(COap) interaction provides electronic perturbation at the Fe center. The distance of the N center dot center dot center dot C(COap) contact is 3.497 angstrom in 1a. It increases by 0.455 A in 2 when electronic density of the Fe site is slightly enriched by a weak a-donating ligand, PPh3. A longer distance (4.040 angstrom) is observed for the (PBu3)-Bu-n derivative, 3. This N center dot center dot center dot C(COap) distance is thus a dynamic measure of electronic nature of the Fe2 core. Variation of electronic richness within the Fe2 moiety among the complexes reflects on their electrochemical response. Reduction of 2 is recorded at the potential of -2.17 V, which is 270 mV more negative than that of 1. Complex 3 requires additional 150 mV for the same reduction. Such cathodic shift results from CO substitution by phosphines. Electrocatalytic hydrogen production from HOAc by both kinds of complexes (all-CO and phosphine-substituted species) requires the potential close to that for reduction of the parent molecules in the absence of acids. The catalytic mechanism of la is proposed to involve proton uptake at the (FeFeI)-Fe-0 redox level instead of the Fe Fe level. This result is the first observation among the all-CO complexes with respect to electrocatalysis of HOAc.