A series of iron complexes based on the pentadentate ligand 4,7-bis(2'-methyl-2'-mercaptopropyl)-1-thia-4,7-diazacyclononane), (bmmp-TASN)(2-), have been synthesized and characterized as models of iron-containing nitrile hydratase (NHase). The chloro derivative [(bmmp-TASN)(FeCl)-Cl-III].0.5EtOH (1) contains a labile chloride which facilitates synthesis of related complexes via substitution reactions. Complex 1 is high-spin, g = 4.28. Addition of NEt4CN with 1 in CH2Cl2 results in the cyanide ligated complex [(bmmp-TASN)(FeCN)-C-III].0.5EtOH (2), which shows a single intense nu(CN) band at 2083 cm(-1) in the IR region. Complex 2 is low-spin, g(1) = 2.31, g(2) = 2.16, and g(3) = 1.96. Under basic conditions complex 1 affords a mu-oxo bridged climeric Fe(III) complex [(bmmp-TASN)Fe-III](2)O (3), which shows an intense band at 799 cm-1. Complex 3 was recrystallized from CH2Cl2/hexane solution in the triclinic space group P (1) over bar, with a = 10.5486(15) Angstrom, b = 13.0612(19) Angstrom, c = 8.1852(12) Angstrom, alpha = 96.923(2)degrees, beta = 112.729(2)degrees, gamma = 81.048(2)degrees, and Z = 1. Density functional theory (DFT) calculations of the previously communicated iron-nitrosyl complex [(bmmp-TASN)Fe-III(NO)][BPh4] (4) (Inorg. Chem. 2002, 41, 1039-1041) reveal that the HOMO region is dominated by Fe-S bonding. Complexes 1-4 display irreversible or quasi-reversible reductions in the cyclic voltammograms. All of the iron complexes and the zinc derivative, (bmmp-TASN)Zn (5), display an irreversible oxidation. Complex 5 was crystallized in the monoclinic space group P2(1)/n with a = 9.5759(6) Angstrom, b = 20.9790(13) Angstrom, c = 10.7113(7) Angstrom, beta = 91.283(1)degrees, and Z = 4.