The unique active site structure of nitrile hydratase (NHase) has a central metal ion (Co-III or Fe-III) coordinated by two amide nitrogens from the peptide backbone, one cysteine sulfur and two oxidized cysteine sulfurs (Cys-SO and Cys-SO2). In this review, the biological implications of the nitrile hydration mechanism are discussed in context of model complexes prepared with the aim of understanding the unique structure of nitrile hydratase.