NMR spectroscopy study of the complexation of l-mannonic acid with tungsten(VI) and molybdenum(VI)

By using multinuclear (1H, 13C, 17O, 95Mo, 183W) magnetic resonance spectroscopy (1D and 2D), l-mannonic acid was found to form nine complexes both with tungsten(VI) and molybdenum(VI), in aqueous solution, depending on pH and concentration conditions. Two isomeric 1:2 (metal:ligand) complexes involving the carboxylate and the adjacent OH group were present in the whole pH range covered (2–11). At high pH, three 2:1 complexes were detected, each one involving four OH groups of the ligand. At low and intermediate pH values, four additional complexes were found: two 4:2 isomers, more abundant with W(VI), involving the carboxylate group and all the hydroxyl groups, and two 2:1 isomers, more abundant with Mo(VI), involving the carboxylate group and three OH groups.