Monoanionic complexes of chromium(III) like [Cr(sq)2(cat)]-, where the ligands are anionic radicals such as 1,2-semiquinones and catecholates unsubstituted and 3,5-di-tert-butyl-substituted were detected after interaction of chromate anion (Cr(VI), natural isotope, I = 0 or53Cr-enriched isotope, I = 3/2) with 1,2-catechol or 3,5-di-tert-butyl-1,2-catechol in methanolic, ethanolic, or aqueous solution under both anaerobic and aerobic conditions. The very low 〈g〉 values (g = 1.972) obtained in methanol for both complexes, and the very strong metal hyperfine coupling, A53Cr= 25.71 G and A53Cr= 25.17 G for the complex with the ligands 1,2-catechol and 3,5-di-tert-butyl-1,2-catechol, respectively, suggest a ground state localized on the metal ion. The unpaired electron could be localized in a molecular orbital with a large metal character. The isotropic spectra obtained at room temperature as well as at low temperature suggest an essential octahedral arrangement around the chromium ion. The1H ENDOR spectra confirm the octahedral geometry of the complexes.
Branca, M., Fruianu, M., Sau, S., & Zoroddu, M. A. (1996). EPR and ENDOR studies on monoanionic complexes of 1,2-semiquinones with52Cr,53Cr(III). Journal of Inorganic Biochemistry, 62(3), 223–230. https://doi.org/10.1016/0162-0134(95)00156-5