Kinetic Behavior of Mn(III) Complexes of Pyrophosphate, EDTA, and Citrate

Aqueous complexes of Mn3+ ion with the ligands pyrophosphate (P2O74-), EDTA (C10H12O8N24-, ethylenediaminotetraacetate4-), or citrate (CIT3-, C6H5O73-) were prepared rapidly in solution by the general reaction: Mn(VII) + 4Mn(II)L + L→ 5Mn(III)L, where L is the ligand (in known excess). Different initial pH values of the complex solutions were obtained by prior addition of required acid, base, or buffer. Disappearance of Mn(III)L complex with time was monitored by light absorbance at appropriate wavelengths. Rates of loss for each Mn(III)L complex are found to depend on pH and the ratio [ligand]T/[Mn]T. Relative chemical time scales for Mn(III)L disappearance at neutral pH and similar ligand-to-metal ratios were manganese(III) pyrophosphate > manganese(III) citrate >> manganese(III) EDTA. The kinetic observations are interpreted as reflecting ligand hydrolysis and disproportionation in the case of pyrophosphate (a nonredox active compound), inner-sphere electron transfer from citrate to Mn3+, and both outer-sphere and intramolecular electron transfer from EDTA to Mn3+. In the presence of O2, manganese(II) citrate complex is reoxidized to manganese(III) citrate. The results suggest a potential for formation of Mn(III) complexes with ligands in great excess and kinetic stabilization under certain natural water conditions (e.g., where pronounced redox gradients are present or where appreciable concentrations of superoxide, peroxide, and hydroxyl radical are produced). Similar Mn(III) behavior to that reported here might be anticipated for other ligands such as catechol and humic mattter in natural waters.