The structure and equilibria of a manganese(II) complex of fulvic acid studied by ion exchange and nuclear magnetic resonance

Abstract

The binding of Mn2+ to a well characterized fulvic acid sample is reported based on the competition with K+ in an ion exchange equilibration. The free energy of binding of [Formula: see text] to fulvic acid is only 1–2 kJ/equivalent more favourable than that for K+. This suggests an outer sphere electrostatic structure for the complex. The suggestion is confirmed by observation of minimal change in the presence of fulvic acid of the effect of paramagnetic Mn2+ on the nmr spectra of water. The interpretation of nmr spectra is supported by comparison with nmr measurements of Mn2+ complexes with simple ligands and contrast with nmr measurements on Fe3+ – fulvic acid complexing. The latter is confirmed as inner sphere.

Reactions Controlling Heavy Metal Solubility in Soils

“…This interpretation contrasts with the common assumption that bonding of metals to organic matter is a chelation or complexation reaction with more than one functional group. Spectroscopic evidence reveals that some metals (e.g., Mn 2 +) do not form inner-sphere complexes with organic matter (Alberts et al, 1976;Gamble et al, 1976;Deczky and Langford, 1978;McBride, 1978b). In fact, Cu 2 +-humic acid complexes at fairly high-Cu 2 + sorption levels are not particularly stable relative to Cu 2 + ch elates (see Figure 17) but comparable in stability to Cu-acetate.…”

Kinetic and spectrophotometric studies of binding of iron(III) by glutathione

“…The iron dependence is first power. binding of ligands at Fe(I1I) shortens the electronic relaxation times and alters nmr broadening (6). Nuclear magnetic resonance did not provide a probe of the number of water molecules remaining coordinated.…”

Manganese and Selenium

“…Manganese(II) forms complexes in soil solution with organic substances such as organic acids, amino acids, and humic acids. Nuclear magnetic resonance spectroscopy (NMRS) analysis showed that Mn 2þ bound to fulvic acid remains fully hydrated, indicating that the complex formed between humates and Mn 2þ can be described as an outer-sphere complex wherein the ligand does not interact directly with the Mn electrons [20]. Therefore, Mn-organic ligand complexes may be still quite labile and available to plants and microorganisms.…”