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Electron Transfer Reactions 351

Understanding of individual electron transfer processes The result is the same whether one chooses to call either
involves obtaining (to various degrees) knowledge of the of these processes electron transfer followed by transfer
−
following: of labilized ligand (O 2− or Cl ) or atom transfer result-
ing in a net electron transfer in the opposite direction. In
1. Potentials of half-reactions—“Is the reaction ther- each case, electron transfer would produce states in which
modynamically feasible?” the bond that breaks would have become labilized and the
2. Structures of reactants and products. These can give bond formed would have become inert. Electron transfer
clues to what must happen in the system to provide sensi- astheprimaryactionisconsistentwiththeFranck-Condon
ble pathways for the overall electron transfer to occur. principle that electron transitions in atomic systems are
3. Identification of intermediates—evidence for steps very rapid compared with nuclear motions, but one can-
along the way. notexperimentallydeterminethatitisprimaryunlessthere
4. Kinetics. Reaction rates can be analyzed to postulate is detection of intermediate complexes—in this case pre-
steps leading to an activated state, the formula, and the ceding and following formation of the oxygen or chlorine
structure of the activated state for electron transfer. Most bridge.
redox reactions involve very complex rearrangements of
atoms and ions as well as electron transfer, as witness the
−
reduction of MnO to Mn . Not only are five electrons
2+
4
accepted by Mn(VII), but eight protons are needed to con-
vert four coordinated oxide ions to water (see Table II).
This complex, in which electrons have been trans-
III. ATOM TRANSFER VERSUS ferred from S to Cl, could also hydrolyze to give 2H ,
+
ELECTRON TRANSFER SO , and ClO . In most cases the order of events
2−
−
4 2
following formation of the active state—electron transfer,
Chemists have characterized two classes of redox reac- hydrolysis, O–Cl bond scission, and so forth—cannot
tions: atom transfer and electron transfer. The classic ex- be determined. (This powerful isotopic tracer method
ample of atom transfer involves the aqueous oxidation of of observing atoms being transferred in redox reactions
sulfite ion by chlorate ion in which 18 O atoms initially from an inert reactant to form an inert product was
bound to chlorine are found bound to the product sulfate, pioneered by Henry Taube, who received the Nobel Prize
which thus precludes exchange of oxygen with water dur- in 1984.) For electron transfer to occur, there must be
ing the process: contact between electron donor and acceptor, adequate
18
18
−
−
3SO 2− + Cl O → 3 OSO 2− + Cl . potential to effect transfer and overcome any barriers, and
3
3
3
a mechanism for conduction of electron(s) from donor to
The rate of the reaction is given by rate = acceptor. Conduction may take place through delocalized
+ 2
2−
k[SO ][ClO ][H ] , consistent with the formation
−
3 3 molecular orbitals as in the conduction bands of metals or
of an activated complex:
π-bonding systems as in graphite. Reactions have been
observed to occur with a breakdown in Arrhenius plots
of the temperature coefficient of reaction rate. Electron
transfers involving no energy of activation are thought
to occur by quantum-mechanical tunneling. Electron
transfer reactions over long distances are of great current
The S and Cl atoms have identical electronic structures interest in the study of redox phenomena in biological sys-
as SO 2 , produced by two protons acting on SO , coordi- tems, where currents are found to be carried by electrons
2−
3
nates to an oxygen of ClO . A positive potential allows through connected orbital systems, by tunneling, and by
−
3
sulfur to transfer two electrons to chlorine. This causes series of coupled redox reactions as in the respiratory
scission of the O–Cl bond as water attacks the sulfur, form- chain below. In this overall process oxygen molecules
ing SO 2− containing an oxygen atom originally bound to accept electrons from NADH. The electrons are trans-
4
−
the chlorine in ClO . mitted by a series of electron transfer reactions between
3
In a similar way chlorine atoms are transferred in the metal centers embedded in enzymes and labile organic
reaction: redox couples in FAD and coenzyme Q. Cytochrome