Electronic Absorption Spectra and Ligand Structure in the Metal Complexes of Quinizarin

The character of the electronic absorption spectra of the metal complexes with 1,4-dihydroxy-9,10-anthraquinone depends on the ligand state, namely, on the degree of its ionization and predominant contribution of the tautomeric 9,10-, 1,10-, and 1,4-anthraquinoid resonance structures. The known complexes are classified in accordance with the ligand structure. The maximal contribution of the 1,10-anthraquinoid structure of the ligand is observed for the majority of monometal complexes, while that of 9,10-anthraquinoid structure is typical of bimetal complexes. Differences in the composite electronic absorption spectra of the mixed-ligand complexes are explained in terms of contribution of different quinizarin tautomeric forms with different degree of ionization.