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352 Electron Transfer Reactions

2−
to dithionate (S 2 O ) by one-electron acceptors. The lat-
6
−
ter oxidize SO 2− to · SO , which dimerizes. (The abil-
3 3
ity of hydrazine and sulfite ions to discriminate between
“mono-” and “di-delectronators” was observed in the
1920s by A. W. Browne. The concept of one-step two-
electron transfer then fell into disfavor and is still treated
with a fair amount of skepticism.)
FIGURE 2 Heme iron pocket in cytochrome c. The heme iron
redox center in cytochrome c exists in a hydrophobic pocket with a
small opening to the surface of the protein, which provides means
for alternative pathways for electrons to travel between heme iron V. INNER SPHERE AND OUTER
and external redox centers. (Left) Pocket open to the surface of
cytochrome c can be penetrated by a conducting ligand bound to SPHERE TRANSFERS
a redox center (M). (Right) NN is ethylenediamine, which cannot
conduct or penetrate the pocket. Many atom transfer reactions are also called inner sphere
when donor and acceptor atoms are joined by a bridg-
ing atom bound to each redox center. The bridging atom
may or may not be transferred in the opposite direction to
the electron(s) as in the oxygen and chlorine atom trans-
fers described above. IrCl 2− is reduced to IrCl 3− by Cr 2+
6 6
without transferring a Cl to the Cr 3+ product. By incor-
−
c has been isolated, its structure determined, and electron 2−
porating radioactive iridium into IrCl , one can follow
6
pathways to the heme iron center investigated by attaching
the electron exchange reaction:
metal ions to its surface or by attaching ligands to reducing
metal ions that can probe the hydrophobic pocket in which ∗ IrCl 2− + IrCl 3− → IrCl 2− + IrCl .
3−
∗
6 6 6 6
the heme iron atom is found (see Fig. 2).
Such a reaction, in which no change takes place in the co-
ordination spheres of reactants and products, is called an
outer sphere reaction, assuming no direct bridging atom
IV. ELECTRON TRANSFERS BETWEEN or atoms bind donor and acceptor together in the activated
PAIRS OF ATOMS: NUMBERS state. Exchange reactions have been intensively studied,
OF ELECTRONS
but only for one-electron processes. (Detailed theory con-
cerning such processes is based on the work of Rudolf
There have been arguments for many years over the possi-
Marcus and Noel Hush.)
bility of simultaneous multiple-electron transfer. The for-
Probing the surface of cytochrome c for electron path-
mation of electron pair bonds by donation from Lewis
ways to the heme redox center has been pioneered by
bases to Lewis acids suggests that a two-electron trans-
Harry Gray. Conducting ligands like CN − that can be
fer might occur by such a route. Several reagents have
inserted into the pocket cause deviations from the pre-
been found to discriminate between one- and two-electron
dictions of the Marcus relation for outer sphere electron
donors or acceptors in terms of products formed.
transfer reactions, whereas nonconductors or ligands that
−
+
Nitrate ion (NO ) is reduced to N 2 O, NH 3 OH ,or
3 cannot penetrate the pocket, like ethylenediamine com-
+
NH by two-electron donors, Zn, Sn(II), and so forth,
4 plexes, are “well behaved.” This indicates that various
and to NO 2 or NO by one-electron donors such as Cu,
pathways with different energy barriers to electron trans-
2+
Fe(II), Ti(III), and VO . Similarly, chlorate ion (ClO )
−
3 fer may be found for electrons entering and leaving redox
is reduced to ClO 2 (gas) by one-electron donors and to Cl −
centersinlargebiomoleculesbyouterspheremechanisms.
by two-electron donors. In each case it appears that two-
electron donors can bypass stable odd-electron molecules
by donating electrons in pairs to the acceptor, even when
VI. CLASSIFICATION OF ELECTRON
several successive donations are required to reach the final
TRANSFER STEPS
product. Hydrazine is oxidized by one-electron acceptors
by the following mechanism:
Several postulated combinations of numbers of electrons
− +
2H 2 NNH 2 → 2(e + H + H 2 NNH·) → and numbers of pathways have been proposed for acti-
vated states in which redox centers exchange electrons.
+
[H 2 NNHNHNH 2 ]‡ → N 2 + 2NH 3 + 2H ,
Structures of such states are deduced from experimen-
but two-electron acceptors produce only N 2 . Sulfite ion tal evidence such as rate laws, product analyses, isotopic
(SO ) is oxidized to SO 2− by two-electron acceptors and tracer experiments, and deuterium isotope effects. Let
2−
3 4