My research primarily focuses on biomimetics of the active sites of the [FeFe]- and [Fe]-hydrogenases (H2ase) and is classified into three parts. Part A: The one-electron oxidation of asymmetrically disubstituted FeIFeI models of the active site of the [FeFe]-H2ase, (mu-pdt)[Fe(CO)2PMe3][Fe(CO)2NHC] (pdt = 1,3- propanedithiolate, NHC = N-heterocyclic carbene) generates mixed valent FeIIFeI models of the Hox state of [FeFe]-hydrogenase. The spectroscopic properties, structures, reactivities and relative stabilities of the one-electron oxidized mixed valent complexes, (mu-pdt)(mu-CO)[FeII(CO)2PMe3][FeI(CO)NHC]+ are discussed in the context of experimental and theoretical data and biological relevance. Part B: DFT computations find the Fe-Fe bond in the FeIFeI diiron models ((mu- pdt)[Fe(CO)2L][Fe(CO)2L'] ( L, L' = CO, PPh3, or PMe3) is thermodynamically favored to produce the mu-oxo or oxidative addition product, FeII-O-FeII, nevertheless the sulfurbased HOMO-1 accounts for the experimentally observed mono- and bis-O-atom adducts at sulfur. The FeII(mu-H)FeII diiron model, (mu-pdt)(mu-H)[Fe(CO)2PMe3]2 (IV-5), for which the HOMO is largely of sulfur character, exclusively yields S-oxygenation. Deoxygenation with reclamation of the mu-pdt parent complexes occurs in a proton/electron coupled process. The possible biological relevance of oxygenation and deoxygenation studies is discussed. Comprehensive investigations of intramolecular CO site change and intermolecular CO/L (L = PMe3 or CN-) exchange of (mu-pst)[Fe(CO)3]2 (IV-1-O), (mu-pdt)[Fe(CO)3]2 (V-1), and their mono-CN-/PMe3 substituted derivatives indicated that the factors influencing the rate of the CO/L exchange reaction of such diiron carbonyls are intramolecular structural rearrangement (or fluxionality) and nucleophilic attack by the incoming ligand. Part C: X-ray diffraction and spectroscopic studies of a series of mono- and disubstituted complexes, FeI2(CO)xL4-x, x = 2 or 3, showed them to be rudimentary structural models of the [Fe]-H2ase active site in native (FeII(CO)2) or CO-inhibited (FeII(CO)3) states. Full characterization of the advanced model complexes ((NS)FeI(CO)2P, NS = 2-amidophenothiolate; P = phosphine) including x-ray diffraction, DFT computations, and Mossbauer studies revealed the interesting "noninnocent" character of these complexes due to the NS ligand. Ligand-based protonation with a strong acid, HBF4Et2O, interrupted the pi-delocalization over Fe and ligand of complex VII-1 and switched on CO uptake (1 bar) and 12CO/ 13CO exchange of VII-1. The intermediate, VII-1-H+, capable of CO uptake, was defined by DFT calculations.