Metal complexes with alizarin and alizarin red S: Electronic absorption spectra and structure of ligands

Most of the πl,π* bands of metal alizarinates are shown to be due to the 2,9- or 1,2-anthraquinoid structures. Bimetallic alizarinates with the 9,10-anthraquinoid structure are not formed because of the adjacent negatively charged oxygen atoms. The long-standing dispute concerning the structure of red metal alizarinates is solved: complexation always occurs at the peri- or ortho-hydroxycarbonyl group and involves the tautomeric anthraquinoid forms.