Aqueous complexes of Mn3+ ion with the ligands
pyrophosphate (P2O7
4-), EDTA (C10H12O8N2
4-,
ethylenediaminotetraacetate4-), or citrate (CIT3-, C6H5O7
3-)
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.