Bioinorganic Chemistry
655
through the varying degrees of the in-
terchange route to the wholly associative
mechanism, there is a continuous spec-
trum of possibilities. Moreover, in aqueous
systems, virtually all such substitution re-
actions occur via an aquo intermediate
L
n
M(OH
2
). Thus, for the most general
case in which X and Y are not water, the
question just posed as to the coordination
number of the intermediates needs to be
answered twice: for both formation and
reaction of the aquo intermediate.
Detailed answers about the mechanism
of substitution reactions in metal ion com-
plexes have been sought for many years.
Some general conclusions now seem reli-
able, but universal agreement on all details
remains to be reached.
Two statements
sum up many of the results
: (1) since room
is available for approach of an axial lig-
and, most substitutions in linear, trigonal,
and planar complexes proceed by the as-
sociative mechanism with an increase in
coordination number and (2) since the en-
vironment about the metal ion is more
crowded, most substitutions in octahedral
complexes proceed in the interchange-
dissociative region of the continuum.
15.2
Ligand Exchange
Even if they do not appear as reactants
or products, the intermediacy of aquo
complexes in most substitution reactions,
whatever the detailed mechanism of sub-
stitution in any particular case, makes the
properties of the aquo complexes of pri-
mary importance.
The
characteristic
rate
constant
for
exchange of inner-sphere water (both X
and Y are H
2
Ointheabovereaction)gives
insight into the differences among metal
ions. Increasing water exchange rates on
wholly aquo metal ions follow the order
Cr
3
+
,
Ru
3
+
¿
Pt
2
+
¿
Ru
2
+
<
Co
3
+
<
Al
3
+
¿
V
2
+
,
Fe
3
+
<
VO
2
+
,
Ga
3
+
,
V
3
+
,
Pd
2
+
,
Be
2
+
<
Ni
2
+
<
Mg
2
+
<
Co
2
+
,
Fe
2
+
<
Mn
2
+
,
Zn
2
+
,
Sc
3
+
<
Ca
2
+
,
Cd
2
+
,
Gd
3
+
<
alkali metal ions, Cr
2
+
,
Cu
2
+
,
Hg
2
+
,
Pb
2
+
with each inequality sign indicating an
approximate
10-fold
increase
in
rate.
Water exchanges slowly with the ±rst 6
aqua cations through Al
3
+
,r
ap
id
l
yw
i
th
the next 14 through Sc
3
+
, and virtually
instantaneously with the last 8 entries.
The series spans a phenomenal 15 powers
of 10 from a lifetime at 25
Co
fabou
t
5d
a
y
sf
o
rt
h
em
e
a
nt
im
eo
faw
a
t
e
r
molecule on aqueous Cr
3
+
to 10
9
sfo
r
the metal ions at the end of the list.
Indeed, exchange lifetimes are so short
for the cations in the last line that some
values are not known with certainty: some
of the lifetimes may be 10
10
so
reven
shorter.
The above series is useful in several
ways. The contrast between the presence of
Zn
2
+
in numerous mammalian enzymes
and of Ni
2
+
in only a few plant enzymes
has proved puzzling. Ni
2
+
differs by
neither size nor most complex stabilities
from Zn
2
+
. The above exchange rate
series explains the difference since Zn
2
+
undergoes ligand exchange about 10
3
times faster
than
Ni
2
+
,as
i
g
n
i
±
c
a
n
t
factor for a metal ion bound at enzyme
active sites.
In four cases, reduction of the metal
ion substantially increases the rate. An
approximate 10
4
-fold rate increase occurs
for the d
5
d
6
reduction in both Ru
3
+
and Fe
3
+
.Ane
veng
rea
te
r10
7
-fold rate
increase takes place upon the reduction of
Co
3
+
Co
2
+
.F
ina
l
l
y
,the10
15
span of
theent
ireseriesisgaineduponreduct
ion
of Cr
3
+
Cr
2
+
.
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