Protonation of the symmetrical tetraphosphine complexes Fe-2(S2CnH2n)(CO)(2)(dppv)(2) afforded the corresponding terminal hydrides, establishing that even symmetrical diiron(l) dithiolates undergo protonation at terminal sites. The terminal hydride [HFe2(S2C3H6)(CO)(2)(dppv)(2)](+) was found to catalyze proton reduction at potentials 200 mV milder than the isomeric bridging hydride, thereby establishing a thermodynamic advantage for catalysis operating via terminal hydride. The azadithiolate protonates to afford, [Fe-2[(SCH2)(2)NH2YCO)(2)(dppv)(2)](+), [HFe2[(SCH2)(2)NH](CO)(2)-(dppv)(2)](+), and [HFe2[(SCH2)(2)NH2](CO)(2)(dppv)(2)](2+), depending on conditions.