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mechanism from the direct oxygen transfer to the Sc
3+
ion-coupled electron
transfer [14].
6.3.1.3 Ferrate(IV).
In the solid phase, metalferrates(IV), MFeO
3
(alkali
metals: Ba, Sr, Ag, and Pb), and orthoferrates(IV), M
2
FeO
4
(alkali metals: Sr
and Ba), have been synthesized by the thermal oxidation of two solid phases
with an M : Fe ratio, corresponding to that in ferrate(IV) [356]. The crystals of
Na
4
FeO
4
are unstable under humid conditions and dissociate to Fe
3+
and
Fe O
2−
in water (Eq. 6.102). The hydrolysis of Fe
3+
then gives amorphous
Fe(OH)
3
or FeO(OH) [357]:
VI
4
3
Na FeO
+
8
H O
→
12
Na
+
+
Fe O
VI
2
−
+
2
Fe
3
+
+
16
OH
−
.
(6.102)
4
4
2
4
Inorganic complexes of ferrate(IV) in basic solution have also been gener-
ated [358]. The Fe(IV) complexes with OH
−
and
P O
2
4−
in basic solution were
produced by the oxidation of corresponding parent complexes with the OH/
O
−
radical (Eqs. 6.103 and 6.104):
7
Fe OH
(
)
−
+
OH/O FeO OH
→
(
)
2
−
n
+
H O/OH
−
k
=
8 5 10
.
×
7
/M/s
4
n
2
113
(6.103)
[(
P O Fe OH
)
3
+
]
6
−
+
OH
→
[(
P O Fe O
)
IV
]
6
−
+
H O
k
=
7 8 10
.
×
7
/M/s
.
2
7 2
2
7 2
2
114
(6.104)
The spectrum of the ferrate(IV) complex is presented in Figure 6.24b. A
peak at λ
max
= 430 nm (ε = 1200/M/cm) was observed, similar to
FeO OH
2−
and [(P
2
O
7
)
2
Fe
IV
O]
6−
spectra. As the pH decreased, a blue shift in the 430-nm
peak was observed. The
Fe O( )
−
species decayed mainly by a first-order
process in 1 M NaOH (
k
decay
≈ 2 ± 1/s at 25°C), and the pyrophosphate complex
of iron(IV), formed at pH ≥ 10, was short lived (
t
1/2
= 100-600 ms) [358]. The
[(P
2
O
7
)
2
Fe
IV
O]
6−
complex decayed by a second-order process with the forma-
tion of an Fe(III) pyrophosphate complex and molecular oxygen (Eq. 6.105)
[358]:
n
(
)
n
2
[(
P O Fe O
)
IV
]
6
−
+
2
H O
→
2
[(
P O Fe OH
)
III
]
6
−
+
1 2
/ O
.
(6.105)
2
7 2
2
2
7 2
2
Reactions of Fe(IV)-pyrophosphate complexes with divalent transition
metal ions in pyrophosphate have been studied (reaction 6.116) (Table 6.4)
[358]:
Fe IV M II
(
)
+
(
)
→
Fe III M III
(
)
+
(
).
(6.106)
The Ni(II) and Cu(II) pyrophosphate complexes showed no reactivity,
and hence, the upper limits of the rate constants are presented in Table 6.4.
The rate constants are on the order of 10
6
/M/s. The mechanism involves a
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