The anion [Fe-2(S2C3H6)(CN)(CO)(4)(PMe3)](-) (2(-)) is protonated by sulfuric or toluenesulfonic acid to give HFe2(S2C3H6)(CN)(CO)(4)(PMe3) (2H), the structure of which has the hydride bridging the Fe atoms with the PMe3 and CN- trans to the same sulfur atom. H-1, C-13, and P-31 NMR spectroscopy revealed that HFe2(S2C3H6)(CN)(CO)(4)(PMe3) is stereochemically rigid on the NMR time scale with four inequivalent carbonyl ligands, Treatment of 2(-) with (Me3O)BF4 gave Fe-2(S2C3H6)(CNMe)(CO)(4)(PMe3) (2Me). The Et4NCN-induced reaction of Fe-2(S2C3H6)(CO)(6) with P(OMe)(3) gave {Fe-2(S2C3H6)(CN)(CO)(4)[P(OMe)(3)]}(-) (4). Spectroscopic and electrochemical measurements indicate that 2H can be further protonated at nitrogen to give [HFe2(S2C3H6)(CNH)(CO)(4)(PMe3)](+) (2H(2)(+)). Electrochemical and analytical data show that reduction of 2H(2)(+) gives H-2 and 2(-), Parallel electrochemical studies on [HFe2(S2C3H6)(CO)(4)(PMe3)](+) (3H(+)) in acidic solutions led also to catalytic proton reduction, The 3H(+)/3H couple is reversible, whereas the 2H(2)(+)/2H(2) couple is not, because of the efficiency of the latter as a proton reduction catalyst. Proton reduction is proposed to involve protonation of reduced diiron hydrides. DFT calculations establish that the regiochemistry of protonation is subtly dependent on the coligands but is more favorable to occur at the Fe-Fe bond for [Fe-2(S2C3H6)(CN)(CO)(4)(PMe3)](-) than for [Fe-2(S2C3H6)(CN)(CO)(4)(PH3)](-) or {Fe-2(S2C3H6)(CN)(CO)(4)[P(OMe)(3)]}(-). The Fe2H unit stabilizes the conformer with eclipsed CN and PMe3 because of an attractive electrostatic interaction between these ligands.