132
Chemistry 1983
electron in Cr(II) is antibonding, thus accounting for the tetragonal distortion
in Cr(II) complexes, and their enormous lability compared to those of Cr(III).
By contrast, the complexes of both oxidation states of ruthenium undergo
substitution slowly, with the useful exception of water as a ligand, where the
residence time on Ru(II) is a fraction of a second.
The reducing agent Cr(II) shows preference for an inner sphere mechanism,
and this is especially marked if the acceptor orbital has symmetry. There is a
great economy of motion for electron transfer by an inner sphere path for the
donor -
σ acceptor cases which arises from the reciprocity, at the two centers, of
the events which are required in overcoming the inner sphere Franck-Condon
barrier. This point is illustrated for the
+
self
exchange reaction in Fig. 1 where the electronic levels are shown for the
precursor complex, for the activated complex, and for the successor complex.
Motion of the bridging Cl
-
from Cr(III) to Cr(II) lowers the energy of a
acceptor orbital on Cr(III), and at the same time raises that of the donor
orbital on Cr(II), and although other nuclear motions are also required, there
is some correlation of the events required for activation to electron transfer, a
correlation which is absent in the case of an outer sphere mechanism. The high
substitution lability of Cr(II) of course means that the precursor complex can
be formed rapidly.
The comparison of the rates of self exchange for
vs.
and how these respond when the
higher oxidation state for each couple is converted to the hydroxo complex
is quite instructive. The upper limit for the specific rate of self-exchange for
the Cr(III)/Cr(II) couple is 2 x 10
-5
M
-1
s
-1
(65, 55); although it has not
been directly measured for the ruthenium couple, the specific rate can
reasonably be taken to be close to that (67) for Ru(NH
3
)
63+/2+
, namely ~
Fig.
1. Electronic structure
and “atom” transfer in the self-exchange reaction:
( H
2
O )
5
C r C 1
2+
+
Cr(H
2
O)
6
2+
.
H. Taube
133
1
× 10
3
M
-1
s
-1
. Because the redox change for the ruthenium couple in-
volves a
π electron, it causes only a small change in the dimensions of the
complex (68). Thus the Franck-Condon barrier to electron transfer arising
from inner sphere reorganization is small, and facile transfer by an outer
sphere mechanism is observed. By contrast, because Cr(III) has no anti-
bonding electrons, and the electron added on reduction is anti-bonding,
there is a large change in dimensions and shape attending the reduction,
and the slowness of the selfexchange can be attributed in part at least to
the inner sphere barrier (69). Why the water molecule, which as a ligand
on the oxidizing agent still has available an electron pair for sharing with
the reducing agent, is such a poor bridging group, remains to be under-
stood.
On deprotonation of a water molecule in each oxidant, the inner sphere path
for the Cr(III)/Cr(II) system opens up, and a marked increase in rate is
observed (65) (k=0.66 M
- 1
s
-1
at 25º) - the increase may be as large as a
factor (66) of 10
9
. The self-exchange rate for (NH
3
)
5
R u O H
2+
+ ( N H
3
)
5-
RuOH
2
2+
has not been measured, but it can be asserted with confidence that
the rate by either an inner sphere path or an orthodox outer sphere path will be
much less than it is for the aquo couple. The rate by the inner sphere path will
be limited by the rate of bridge formation, and thus will be no greater than
1
×
1 0
-2
M
-1
s
-1
(neutral ligands in substituting on (NH
3
)
5
R u O H
2
2+
s h o w
specific rates (70) of the order of 0.1 M
-1
s
-1
). The orthodox outer sphere path
now has a composition barrier, as well as a Franck-Condon barrier to overcome
for the production of
a n d
f r o m
and (NH
3
)
5
R ~ O H z
2
+ is ca. 10
-9
). Reaction by “hydrogen
atom” transfer (15) is a reasonable possibility, that is, electron transfer con-
comitant with proton transfer from the Ru(II) complex to the Ru(III), and
some evidence in support of this kind of mechanism has been advanced to
explain observations (71)made in the oxidation by Fe
3+
a n d F e O H
2+
o f
Reaction by such a path might be quite facile and a specific
rate in excess of 1
would be strong evidence in its favor.
APPLICATIONS OF THE RU(III)/RU(II) COUPLE
Some unexpected benefits have accrued from introducing Ru(III)-Ru(II) am-
mine couples into this field of research. Ruthenium(II) engages in back
bonding interactions to a degree unprecedented among the dipositive ions
of the first transition series. A discovery (72) which forcefully brought this
message home, is that (NH3)
5
RuN
2
2
+ is formed in aqueous solution by the
direct reaction of
with N
2
. When the heteroligand in
is pyridine or a derivative, the complexes of both oxidation
states are slow to undergo substitution, and by changing the number of
π
acid ligands, a versatile series of outer-sphere redox couples is made
available, spanning a range in redox potential of over 1 volt. When
derivatives of the Os(III)-Os(II) ammines are included, the useful range in
aqueous solution is extended by approximately 0.5 V. These reagents are
finding wide application in research on redox processes.