Novel thiolate-bridged diiron complexes bearing diazenido and diazene ligands in a side-on manner have been prepared and characterized by X-ray analysis. These sulfur-bridged diiron complexes work as effective catalysts toward the reduction of hydrazines into amines and ammonia via a sequential process of protonation and reduction on the sulfur-bridged diiron skeleton, as is likely observed at the active site of nitrogenase.