122
Chemistry 1983
ing the rate of a virtual process such as (1), today made commonplace for many
systems by the introduction of new spectroscopic methods, seemed more gla-
morous than measuring the rate of oxidation of V
2+
(aq) by Fe
2+
(aq), for
example. But I also recall from informal discussions that it was felt that driving
force would affect the rate of reaction, and thus there would be special interest
in determining the rates for reactions for which
(except for the entropic
contributions to the driving force) is zero.
The interest in the measurement of the rates of self-exchange reactions which
I witnessed as a graduate student, is not reflected in the literature of the years
immediately following. Many of those who might have had plans to do the
experiments were engaged in war related activities. Post war, at least live
different studies on the rate of reaction (1), all carried out in non-complexing
media, were reported, with conflicting results, some indicating a half-life for
exchange on the order of days at concentration levels of 10
-2
M. The discrepan-
cies led to considerable controversy, and in informal discussions, strong opin-
ions were expressed on just what the true rate of self-exchange might be. The
basis for this kind of judgment, exercized in the absence of any body of
quantitative measurements, is worth thinking about. I believe it reflected an
intuitive feeling that there would be a relation between the rates of self-
exchange and of the related cross-reactions, and of course each of us had at
least some qualitative information on redox rates for the Fe
3+/2+
couple. The
definitive measurements on the rate of reaction (1) in non-complexing media
were made by Dodson (6). These measurements were soon extended (7) to
reveal the effect of [H
+
] and of complexing anions on the rate and yielded rate
functions such as [Fe
3+
] [X
-
] [Fe
2+
] (because substitution is rapid compared to
electron transfer, this is kinetically equivalent to [FeX
2 +
] [ F e
2 +
] and to
[ F e
3+
][FeX
+
]), in addition to [Fe
3+
][Fe
2+
], none specifying a unique struc-
ture for an activated complex. Particularly the terms involving the anions
provided scope for speculation about mechanism. The coefficient for the simple
second order function was found (7) to be 4
at
µ=0.5, and those
who had argued for “fast” exchange won out.
Another important experimental advance during the same period, important
for several reasons, was the measurement (8) of the rate of self-exchange for
Coen
3
5.2
at
µ = 0.98). This is, I believe, the first
quantitative measurement of a rate for a self-exchange reaction and it may also
be the first time that the oxidizing capability of a cobalt (III) ammine complex
was deliberately exploited. In the article of ref. 8, the rate of the reaction of
C o ( N H
3
)
6
3+
w i t h C o e n
3
2 +
is also reported; this is, I believe, the first deliberate
measurement of the rate of an electron transfer cross reaction. In contrast to the
F e
3+/2+
system, where both reactants are labile to substitution, Coen
3
3+
is very
slow to undergo substitution and thus an important feature of the structure of
the activated complex for the Coen
3+/2+
self exchange appeared to be settled.
In considering the observations, the tacit assumption was made that the
coordination sphere of Coen
3
3+
does not open up on the time scale of electron
transfer, so that it was concluded that the activated complex for the reaction
does not involve interpenetration of the coordination spheres of the two reac-
H. Taube
123
tants. We were thus obliged to think about a mechanism for electron transfer
through two separate coordination spheres (descriptor “outer-sphere” mecha-
nism) (9).
In 1951, an important symposium on Electron Transfer Processes was held
at the University of Notre Dame, and the proceedings are reported in J. Phys.
Chem., Vol. 56, (1952). Though the meeting was organized mainly for the
benefit of the chemists, the organizers had the perspicacity to include physicists
in the program. Thus, the gamut of interests was covered, ranging from
electron transfer in the gas phase in the simplest kind of systems, for example
Ne + Ne
+
, to the kind of system that the chemist ordinarily deals with. Much
of the program was devoted to experimental work, the chemistry segment of
which included reports on the rates of self-exchange reactions as well as of
reactions involving net chemical change - but none on cross reactions. Two
papers devoted to theory merit special mention: that by Holstein (10) whose
contributions to the basic physics are now being applied in the chemistry
community, and the paper by Libby, (11) in which he stressed the relevance of
the Franck-Condon restriction (12) to the electron transfer process, and ap-
plied the principle in a qualitative way to some observations. It is clear from
the discussion which several of the papers evoked that many of the participants
had already appreciated the point which Libby made in this talk. Thus, in the
course of the discussion, the slowness of the self-exchange in the cobaltammines
was attributed (13) to the large change in the Co-N distances with change in
oxidation state, then believed to be much larger than it actually is. During the
meeting too) the distinction between outer- and inner-sphere activated com-
plexes was drawn, and the suggestion was made that the role of Cl- in affecting
the rate of the Fe
3 + / 2 +
self-exchange might be that it bridges the two metal
centers. (14) But of course, because of the lability of the high spin Fe(II) and
Fe(III) complexes, no unique specification of the geometry of the activated
complex can be made on the basis of the rate laws alone. During the discussion
of Libby’s paper, a third kind of mechanism was proposed (15), involving
“hydrogen atom” transfer from reductant to oxidant.
PREPARATION
My own interest in basic aspects of electron transfer between metal complexes
became active only after I came to the University of Chicago in 1946. During
my time at Cornell University (1941 - 1946) I had been engaged in the study of
oxidation-reduction reactions, and I was attempting to develop criteria to
distinguish between 1e
-
and 2e
-
redox changes, and as an outgrowth of this
interest, using le
-
reducing agents to generate atomic halogen, X, and studying
the ensuing chain reactions of X
2
with organic molecules. My eventual involve-
ment in research on electron transfer between metal complexes owes much to
the fact that I knew many of the protagonists personally (A. C. Wahl, C. N.
Rice, C. D. Coryell, C. S. Garner; the first two were fellow graduate students at
Berkeley), and to the fact that I had W. F. Libby, with whom I had many
provocative discussions, and J. Franck as well as F. H. Westheimer as col-