A series of asymmetrically disubstituted models of the active site of [FeFe]-hydrogenase, (mu-pdt)[Fe(CO)(2)PMe3][Fe(CO)(2)NHC] (pdt = 1,3-propanedithiolate, NHC = IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene IMes (1), IMesMe, 1-methyl,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene (2) or IMe, 1,3-bis(methyl)imidazol-2-ylidene (3)), have been synthesized and characterized. The one-electron oxidation of these complexes to generate mixed valent models of the H-ox state of [FeFe]-hydrogenase, such as the previously reported (mu-pdt)(mu-CO)[Fe(CO)(2)PMe3][Fe(CO)IMes](+) (1(ox)) (Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008-7009) has been examined to explore the steric and electronic effects of different N-atom substituents on the stability and structure of the mixed valent cations. The differences in spectroscopic properties, structures, and relative stabilities of 1(ox) (mu-pdt)[Fe(CO)(2)PMe3][Fe(CO)(2)IMesMe](+) (2(ox)), and (mu-pdt)[Fe(CO)(2)PMe3]-[Fe(CO)(2)IMe](+) (3(ox)) are discussed in the context of both experimental and theoretical data. Of the three derivatives, only that with greatest steric bulk on the NHC ligand, 1(ox), shows a clear indication of mu-CO by solution nu(CO) IR and yields to crystallization as a rotated form, commensurate with the two-Fe subsite of H-ox. In addition, the reactivity of the complexes with extrinsic CO to form CO adducts and/or exchange with (CO)-C-13 is explored by experiment and by using density-functional theory calculations.