Hexaamminecobalt(III), an octahedral, inert metal ion complex, has lately gained increasing attention of structural biologists and bioinorganic chemists due to its use in structure determination of nucleic acids. This complex mimics outer-sphere binding events of the physiologically relevant magnesium(II)hexaaqua ion; hexaamminecobalt(III) often finds usage either in NMR spectroscopy, where cross-peaks between the complex ammines and the nucleic acid protons near the binding site are observed, in X-ray spectroscopy as heavy metal derivative for phasing, or in other techniques. In this review, we discuss the basic hexaamminecobalt(III) binding modes and give an overview on the most recent findings on [Co(NH3)6]Cl3–nucleic acid complexes. The various techniques that are applied in combination with this complex are mentioned and briefly summarized. Special attention is given to the application of [Co(NH3)6]Cl3 in nuclear magnetic resonance spectroscopy of nucleic acids, where it is used to reveal potential outer-sphere magnesium binding sites.