Five new L-cysteinyl group-containing diiron/triiron azadithiolate complexes (3-6, 10), which could be regarded as the active site models of [FeFe]-hydrogenases, have been successfully synthesized. Treatment of L-cysteinyl sodium mercaptide CytSNa (1, Cyt = CH2CH(CO2Et)NH(CO2Bu-t) with complex [(mu-SCH2)(2)NCH2CH2Br]Fe-2(CO)(6) (2) in THF at room temperature resulted in formation of model complex [(mu-SCH2)(2)NCH(2)CH(2)SCyt]Fe-2(CO)(6) (3). Further treatment of 3 with decarbonylating agent Me3NO in MeCN at room temperature afforded model complex [(mu-SCH2)(2)NCH(2)CH(2)SCyt]Fe-2(CO)(5) (4). Similarly, treatment of 3 with an equimolar mixture of Me3NO and Ph3P gave model complex [(mu-SCH2)(2)NCH(2)CH(2)SCyt]Fe-2(CO)(5)(Ph3P) (5) and further treatment of 5 with Me3NO produced model complex [(mu-SCH2)(2)NCH(2)CH(2)SCyt]Fe-2(CO)(4)(Ph3P) (6). More interestingly, model complex [(mu-SCH2)(2)NCH-(CO2Et)CH2SFe(CO)(2)Cp]Fe-2(CO)(5) (10) could be synthesized by a "one pot" reaction of the in situ prepared (mu-HS)(2)Fe-2(CO)(6) (9) with 37% aqueous formaldehyde followed by treatment with the N-deprotected L-cysteinyl iron mercaptide Cp(CO)(2)FeSCH2CH(CO2Et)NH2 (8). Complex 8 is new, which was prepared by treatment of complex Cp(CO)(2)FeSCyt (7) with CF3CO2H followed by 25% aqueous NH3. All the new complexes 3-6, 8, and 10 were characterized by elemental analysis and various spectroscopic techniques, whereas complexes 5 and 10 were further characterized by X-ray crystallography.