S. Fronaeus et al., IRON-MANGANESE REDOX PROCESSES AND SYNERGISM IN THE MECHANISM FOR MANGANESE-CATALYZED AUTOXIDATION OF HYDROGEN SULFITE, Inorganic chemistry, 37(19), 1998, pp. 4939-4944
The mechanism for manganese-catalyzed aqueous autoxidation of hydrogen
sulfite at pH 2.4 has been revised on the basis of previous comprehen
sive kinetic studies and thermodynamic data for iron-manganese redox p
rocesses and manganese(II) and -(III) protolysis equilibria. The catal
ytically active manganese species is concluded to be an oxo- (or hydro
xo-) bridged mixed-valence complex of composition (OH)(MnOMnII)-O-III(
aq) with a formation constant beta' of (3 +/- 1) x 10(4) M-1 from kine
tics or ca. 7 x 10(4) M-1 from thermodynamics. It is formed via rapid
reaction between Mn(H2O)(6)(2+) and hydrolyzed manganese(III) aqua hyd
roxo complexes, and it initiates the chain reaction via formation of a
precursor complex with HSO3-, within which fast bridged electron tran
sfer from S(IV) to Mn(III) takes place, resulting in formation of chai
n propagating sulfite radicals, SO3.-. The very high acidity of Mn3+(a
q). indicating a strong bond Mn-III-OH2 in hydrolyzed manganese(III),
makes an attack by HSO3- on substitution labile Mn(TI) in the bridged
complex more favorable than one directly on manganese(III). The synerg
istic effect observed in systems containing iron as well as manganese
and the chain initiation by trace concentrations of iron(III) of ca. 5
x 10(-8) M can also be rationalized in terms of formation of this bri
dged mixed-valence dimanganese(II,III) complex. The presence of iron(I
II) in a Mn(II)/HSO3- system results in rapid establishment of an iron
-manganese redox equilibrium, increasing the concentration of manganes
e(III) and of the catalytically active bridged complex. The bridged co
mplex oxidizes HSO3- several orders of magnitude faster than does iron
(III) itself. Comparison with some previous studies shows that the dif
ferent experimental rate laws reported do not necessarily indicate dif
ferent reaction mechanisms. Instead, they can be rationalized in terms
of different rate-determining steps within the same complex chain rea
ction mechanism, depending on the experimental conditions used.