The aerobic degradation of polycyclic aromatic compounds, which are widespread contaminants in soils and groundwaters, is carried-out in large part by various Fe-containing dioxygenases that perform the cis-dihydroxylation and oxidative cleavage of aromatic rings. Recently, a new Fe dioxygenase family emerged that catalyzes a remarkable set of transformations; the distinguishing feature of these enzymes is that their monoiron(II) centers are coordinated by three histidines residues (i.e., imidazole ligands) in a facial geometry - a departure from the canonical 2-histidine-1-carboxylate facial triad that is dominant among nonheme monoiron enzymes. Members of the 3His family are capable of oxidatively cleaving C-C bonds in substrates that are generally resistant to degradation, including β-diketones and monohydroxylated aromatics (e.g., salicylic acid). This thesis describes the design, synthesis, and characterization of novel transition-metal complexes with polyimidazole ligands that serve as faithful structural and functional models of these important metalloenzymes. Specifically, high-spin iron(II) β-diketonato complexes were synthesized with the PhTIP (tris(2-phenylimidazol-4-yl)phosphine), and tBuTIP ((tris-2-tert-butylimidazol-4-yl)phosphine) ligands. The complexes were analyzed with a combination of experimental and computational methods including X-ray crystallography, cyclic voltammetry, UV-vis absorption, 1H nuclear magnetic resonance, and density functional theory (DFT). The resulting geometric- and electronic-structure descriptions were compared with those obtained for analagous models with the anionic Me2Tp (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and Ph2Tp (hydrotris(3,5-diphenylpyrazol-1-yl)borate) ligands. A similar biomimetic approach was employed in the synthesis and characterization of models of the enzyme salicylate 1,2-dioxygenase