Abstract
The ortho-dihydroxyl group on the flavonoid B-ring of condensed tannins is the main group involved in metal complexation. Potentimetric and spectrophotometric studies have shown that iron(III) forms mono-di-, and trisdentate complexes with tannins and o-diphenol ligands as the pH is increased. These colored complexes have been found to owe their intense coloration to charge transfer processes. Similar types of complexes are formed with aluminum(III) ions although these are colorless. The germanium(IV)-o-diphenol complex is a tris-chelate, whereas, that with boric acid is a mono-chelated complex with tetrahedral symmetry. Formation constants of the above metal complexes are shown to be related to the ligand ionization constants and those metal ions that form soluble tannin complexes have been found to conform to the normal complexation behavior of o-diphenol ligands. A number of divalent metal complexes form well-defined complexes with o-diphenols although the tannin complexes with these metal ions are less well studied. The order of tannin complex stability has, however, been determined by the pH of incipient precipitation and some interesting orders of stability established. Titanium(IV), molybdenum(VI), and tungsten(VI) ions have also been found to form highly colored (charge transfer) complexes with condensed tannins.
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Slabbert, N. (1992). Complexation of Condensed Tannins with Metal Ions. In: Hemingway, R.W., Laks, P.E. (eds) Plant Polyphenols. Basic Life Sciences, vol 59. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-3476-1_23
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DOI: https://doi.org/10.1007/978-1-4615-3476-1_23
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