Chemistry Reference
In-Depth Information
Na
+
K
+
Ba
2+
Hg
2+
Li
+
Rb
+
Cs
+
Mg
2+
Be
2+
Ca
2+
Cu
2+
Sr
2+
Ni
2+
Co
2+
Fe
2+
Mn
2+
Zn
2+
Cd
2+
Al
3+
In
3+
Tl
3+
Ga
3+
Sc
3+
La
3+
Y
3+
10
0
10
-2
10
-4
10
-6
10
-8
10
-10
t
−
(slow)
(sec)
(fast)
Figure 5.8
Half-lives for water exchange of aquated metal ions.
conditions when the system has reached equilibrium. The rate of formation of a species
leading to equilibrium
is a measure of what we can call
kinetic stability
. Reactions occur
with a vast spread of half-lives (the time by which half the precursor molecules have
reacted) or reaction rates. Complexes of metal ions which undergo reactions
rapidly
are
termed
labile
. Complexes of metal ions which undergo reactions
slowly
are termed
inert
.
Henry Taube suggested a suitable definition was for a reaction with a half-life of less
than a minute to equate with a labile complex. Thermodynamic stability does not imply
kinetic inertness, nor does thermodynamic instability imply lability. For example, in acidic
solution Co(III) amine complexes are thermodynamically unstable, but are inert towards
dissociation, and as a result most can be stored in solution for extensive periods of time
(even decades).
A key basic reaction in aqueous solution is
water exchange
, which is the process whereby
a metal ion changes its coordination sphere of water molecules for other solvent water
molecules. This process defines aqua metal ions as dynamic species that undergo a series
of exchange reactions continuously (5.28):
[M(OH
2
)
x
]
n
+
+
[M(
O
H
2
)
x
]
n
+
+
xH
2
O
xH
2
O
(5.28)
The half-life for water exchange spans a broad range, and follows the pattern shown in
Figure 5.8.
A wide variation in rates is seen. Factors influencing how fast ligands are exchanged
are metal ion size, metal ion charge and (to some extent for transition elements) electronic
configuration. For example, as the ionic radius increases from Mg
2
+
to Ca
2
+
, the exchange
rate increases from
10
8
s
−
1
. The left-hand side of Figure 5.8 equates with inert
compounds, and ions here have a large charge/radius ratio. The right-hand side equates with
labile compounds. There is obviously a 'grey' area, as will always be the case where we
partition into two classes. However, it is clear that complexes need not be static species, and
coordinated water molecules undergo exchange with their solvent environment, measurable
where practicable by employing isotopically distinctive
18
OH
2
or
17
OH
2
as solvent and
following its introduction into the metal coordination sphere in place of normal
16
OH
2
(such
as by using
17
O NMR spectroscopy). While we can measure these processes, understanding
how they occur is another thing altogether.
10
5
∼
to
∼
