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358 Electron Transfer Reactions

and the inertia of Cr(III) complexes makes it possible to [H 3 CrO , (Mo(CN) ] (outer sphere) [Cr(V) or (IV)
+
4− ∗
4
8
ascertain what is bound to Cr(III) when its precursor is re- N C Mo(IV)] . One or two Mo(CN) 4− are bound to the
∗
8
duced by electron transfer. The structures and properties Cr(III) product.
of known (and postulated) species of chromium in each
oxidation state are as follows.
2. Cr(VI)–Cr(IV)–Cr(III) (Induced Reactions)
1. Cr(VI) In acid solutions [HCrO reacts slowly or not at all with
−
4
−
2−
−
In CrO , esters (ROCrO ), and metal complexes I unless induced to do so by another reducing agent. If
4 3
+
[Fe OCrO ], tetrahedral coordination dominates. Octa- a two-electron agent is used, 0.5 mol I 2 is produced per
3
hedral activated states for reduction of Cr(VI) to Cr(IV) mole of Cr(VI). The powerful oxidant Cr(IV) is capable
−
or Cr(III) and containing cis-dioxo ligands are postulated of one-electron oxidation of I to I, which dimerizes to I 2 .
by analogy to stable complexes of cis-MoO 2+ (see Sec-
2
tion VI.A.1.b). Mechanisms for substitution reactions on
chromium(VI) postulate five-coordinate activated states.
The potential for Cr(VI) to accept electrons increases
as the number of electrons increases from one to three
(Fig. 2).
3. Cr(VI)–Cr(V)–Cr(III)
2. Cr(V)
If I is present when some one-electron reducers are add-
−
Tetrahedral CrO 3− and square pyramidal complexes of
−
4 ed to HCrO , 1 mol I 2 is produced per mole of Cr(VI)
Cr O 3+ have been characterized. Octahedral complexes 4
−
reduced. Chromium(V) has high potential as a 2e accep-
+
of cis-CrO have been postulated as activated complexes
2 tor. (Its reaction as a one-electron acceptor with Fe 2+ is
for reduction to Cr(IV) and Cr(III). Cr(V) has a much
the slow step in the sequence in Section VII.C.1 above.)
higher potential for accepting two electrons than one and
−
−
can disproportionate. e +Cr(VI) → Cr(V) + 2I → Cr(III) + I 2
[Cr(V) O I]‡ → Cr(III) + OI −
3. Cr(IV)
−
−
+
2H + I + OI → I 2 + H 2 O
Chromium(IV) species are not detected during reductions
of Cr(VI) to Cr(III). It can be inferred from kinetics and
product distributions that Cr(IV) has a very high potential 4. Cr(VI)–Cr(III) (Three-Electron
for accepting one electron and is very labile to substitution Transfer Reactions)
in octahedral complexes. It will disproportionate in the
The seemingly unlikely one-step three-electron reduction
absence of oxidizable material in aqueous acid.
of Cr(VI) to Cr(III) has been found to take place when
two ligands, one a two-electron donor and the other a
4. Cr(III)
one-electron donor, can both bind to cis-CrO 2+ to give
2
Chromium(III) forms inert octahedral complexes. It is a an octahedral activated state. Activated states for three-
2−
one-electron donor of low potential. electron reductions of Cr(VI) by two sulfite ions (SO ),
3
by two oxalate ions (C 2 O ), and by an oxalate and an
2−
4
C. Pathways from Cr(VI) to Cr(III) alcohol are shown below:
∗
and Activated States ( )
1. Cr(VI)–Cr(V)–Cr(IV)–Cr(III)
2+
a. Electrophilic donors—.Fe , VO , Ti , etc.
3+
2+
3Fe 2+ + HCrO + 7H → 3Fe 3+ + Cr 3+ + 4H 2 O
−
+
4
3+ ∗
[Fe O CrO 3 ] , [(FeO) 2 CrO 2 (H 2 O) ] , and [Cr(IV)
∗
2
∗
O Fe(II)] are all one-electron, inner sphere.
4−
4−
b. Nucleophilic donors—.Fe(CN) , Mo (CN) , etc.
6 8
4Mo(CN) 4− + HCrO + 7H → 3Mo(CN) 3−
+
−
8 4 8

+ Cr N C Mo(CN) − + 4H 2 O
7