The recent elucidation of the structures of two iron hydrogenases isolated from different organisms [1,2] has provided a unique insight into the active site of these hydrogenproducing enzymes. This knowledge has fueled intense research aimed at the synthesis of close mimics of the active site that can achieve catalytic activities comparable to that found in the natural system. [3] The particular interest in this system arises from the fact that the enzyme relies exclusively on readily available iron cations in the active site. [4] Apart from a thiolate-linked [Fe 4 S 4 ] cluster, cyanide and carbonyl ligands populate the whole coordination sphere of the two iron nuclei in the site, which are within bonding distance of one another and are additionally connected by a nonproteic dithiolate bridge. [5][6][7] Although a subject of some controversy in the past, recent spectroscopic, crystallographic, and theoretical studies suggest the structure of this tether is S-CH 2 -NH-CH 2 -S (azadithiolate, ADT). [8,9] The importance of the nitrogen heteroatom in the disulfide bridge arises from its potential for protonation in its position close to the active site, which offers a thermodynamically and kinetically favorable pathway for hydrogen evolution in the natural system. Model complexes that exhibit catalytic features related to those of the iron hydrogenases have so far relied solely on propyldithiolate (PDT) bridges. For example, Rauchfuss and coworkers demonstrated that [(m-PDT)Fe 2 (CO) 4 PMe 3 (CN)] À serves as a catalyst for electrochemical hydrogen evolution, [10,11] and Darensbourg and co-workers have reported that [(m-H)(m-PDT)Fe 2 (CO) 4 (PMe 3 ) 2 ] + is a catalyst for H 2 /D 2 scrambling. [12] In these models, the electron-donating cyanide
[*] Dr.