A tris(N-pyrrolidinyl)phosphine (P(NC 4 H 8 ) 3 ) monosubstituted complex, [(μ-pdt)Fe 2 (CO) 5 P(NC 4 H 8 ) 3 ] (2) was synthesized as a functional model of the hydrogen-producing capability of the iron hydrogenase active site. The structure was fully characterized by X-ray crystallography. IR and electrochemical studies have indicated that the P(NC 4 H 8 ) 3 ligand has better electron-donating ability than that of those phosphine ligands, such as PMe 3 , PTA (1,3,5-triaza-7-phosphaadamantane), PMe 2 Ph PPh 3 , and P(OEt) 3 . The electrocatalytic activity of 2 was recorded in CH 3 CN in the absence and presence of weak acid, HOAc. The cathodic shift of potential at −1.98V and the dependence of current on acid concentration have indicated that complex 2 can catalyze the reduction of protons to hydrogen at its Fe 0 Fe I level in the presence of HOAc. IR spectroelectrochemical experiments are conducted during the reduction of 2 under nitrogen and carbon monoxide, respectively. The formation of a bridging CO group during the reduction of 2 at −1.98V has been identified using IR spectroelectrochemical techniques, and an electrocatalytic mechanism of 2 consistent with the spectroscopic and electrochemical results is proposed.