Metal Complexes of Indigo and of Some Related Ligands

While the first coordination compounds of indigo were reported over 100 years ago, a systematic study of the coordination chemistry of this important dye and also its structural "relatives" remained silent for over half a century. Only in the last two decades of the 20th century research on this topic started again keeps on growing ever since. It could be established, mainly by X-ray crystallography, but also by many other modern spectroscopic techniques, that deprotonated indigo, and its deprotonated oxidized and reduced forms act as mono- and bidentate chelate ligands in metal complexes