Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The fields of molecular biology and inorganic chemistry overlap in the study of metalloenzymes in the form of enzyme mimics among other. The advantage of indirect modeling or enzyme mimicry is high-resolution crystal structures and well-defined spectral data from which comparisons can be made to low-resolution crystal structures and poorly defined spectral data obtained from the native enzymes. Modes of action can be proposed by correlating spectral properties of the hydrogenase mimics with spectral properties of different enzyme states.The Fe-only hydrogenases are particularly common enzymes for synthetic organometallic chemists to mimic. This interest is motivated by the inclusion of high field ligands like cyano and CO (metal carbonyl) in the first coordination sphere of the pertinent di-iron cluster. Free cyano and carbonyl ligands are toxic to many biological systems so their inclusion in this system suggests they play pivotal roles.