126
Chemistry 1983
THE INNER SPHERE ACTIVATED COMPLEX
R. A. Plane undertook to measure the rate of self-exchange for Cr(H
2
O )
6
3+
-
C r
2+
(aq) by using Cr
2+
(aq) as a catalyst for the exchange of water between
C r ( H
2
O )
6
3 +
and solvent. The expected catalysis was found, but owing to our
inexperience in handling the air sensitive catalyst, the data were too irreprodu-
cible to lead to a value for the self-exchange rate. Catalysis on electron transfer
was expected because the aquo complex of Cr
2+
(aq) was known to be much
more labile than Cr(H
2
O )
6
3 +
- the lability is now known (24) to decrease by a
factor of at least 10
14
when Cr
2+
(aq) is oxidized to Cr(aq)
3+
(note that Cr
2+
,
but not Cr
3+
, has an anti- bonding electron). It occurred to me in the course of
Plane’s work that it would be worthwhile to test the potential of the Cr(III)/
Cr(II) couple for diagnosis of mechanism using a non-metal oxidant. Following
up on the idea, I did a simple test tube experiment, adding solid I
2
to a solution
of Cr
2+
(aq) which Plane had prepared for his own experiments. I observed that
reaction occurs on mixing, that the product solution is green, and that the
green color fades slowly, to produce a color characteristic of
The
fading is important because it demonstrates that
which is re-
sponsible for the green color, is unstable with respect to
+ I-, and
thus we could conclude that the Cr(II)-I bond is established before Cr(II) is
oxidized.
The principle having been demonstrated with a non-metal oxidant, I turned
to the problem of finding a suitable metal complex as oxidant. What was
needed was a reducible robust metal complex, having as ligand a potential
bridging group, and the idea of using (NH
3
)
5
CoCl
2+
surfaced during a discus-
sion of possibilities with another of my then graduate students, R. L. Rich.
Because at that time virtually nothing was known about the rates of reduction
of Co(III) ammines, and because they were not thought about as useful
oxidants, I was by no means sanguine about the outcome of the first experi-
ment, which again was done in a test tube. I was delighted by the outcome.
Reaction was observed to be rapid (the specific rate has since been measured
(25) as
at 25º) and the green color of the product solution
indicated that (H
2
O)
5
CrCl
2+
is formed. Further work (16,17) showed that this
species is formed quantitatively, and that in being formed it picks up almost no
radioactivity when labelled Cl- is present in the reaction solution, thus demon-
strating that transfer is direct, i. e., Cl- bridges the two metal centers, and this
occurs before Cr
2+
is oxidized.
These early results were presented at a Gordon Conference on inorganic
chemistry, which was held only a short time after they had been obtained, and
they were received with much interest. E. L. King was present at the meeting,
H. Taube
127
and together we planned the experiment in which self-exchange by an “atom”
transfer mechanism was first demonstrated (26).
Cr(III) center, but the auxiliary ligands about the original Cr(III) center will
be exchanged or replaced because of the high lability of Cr(II).
(Several years later, King and co-workers (27) extended the self-exchange
work to include other halides as bridging ligands, and still later (28) encoun-
tered the first example of “double bridging.”
There is a brief hiatus in my work after the early experiments on inner-
sphere mechanisms, caused by my taking leave from the University of Chicago
in 1956. But before leaving, I hurriedly did some experiments (29), which,
though semi-quantitative at best, showed that not only atoms but groups such
a s N
3
-
, NCS
-
, and carboxylates transfer to chromium when the corresponding
pentaamminecobalt(III) complexes are reduced by Cr(aq)
2+
, and that there
are large differences in rate for different dicarboxylate complexes (maleate
much more rapid than succinate). Moreover, it was shown that Cr(II) in being
oxidized can incorporate other ligands such as H
2
P
2
O
7
2-
which are present in
solution. In this paper the possibility of electron transfer through an extended
bond system of a bridging group was raised, but was by no means demonstrat-
ed by the results. While I was away, Ogard (30) began his studies on the rates
of aquation of (NH
3
)
5
C r X
2+
(X=Cl, Br, I) catalyzed by Cr
2+
. By following
the arguments made in connection with reaction (2), it can be seen that if an
inner sphere mechanism operates, (H
2
O )
5
C r X
2+
and NH
4
+
(acid solution)
will be products. The contrast with uncatalyzed aquation is worth noting,
where (NH
3
)
5
C r H
2
O
3+
and X
-
are the products.
GENERAL PROGRESS
The hiatus referred to above provides me with a suitable opportunity to outline
some of the advances that were being or were soon to be made on other fronts.
An important one is that the rates of numerous self-exchange reactions were
being measured. Here I want especially to acknowledge the contributions from
the laboratories of A. C. Wahl and C. S. Garner. Some of the experiments by
Wahl and co-workers made use of rapid mixing techniques - see for example,
the measurement (31) of the rate of self-exchange for MnO
4
1-/2-
. An impor-
tant experimental contribution was also made by N. Sutin who introduced the
rapid flow method (32), and later other rapid reaction techniques, into this
field of study, and who helped others, including myself, to get started with the
rapid flow technique. A spate of activity on the measurements of the rates of
cross reactions followed, motivated in large part by the desire to test the
validity of the cross reaction correlation (5).