Chemistry Reference
In-Depth Information
From the perspective of the central metal, ligand exchange can vary with metal ion from
extremely fast (what we refer to as '
labile
' complexes) to extremely slow (termed '
inert
'
complexes). Whereas direct exchange of one ligand by another of exactly the same type
is the simplest process inherently, the fact that such exchanges can occur suggests that, if
other potential ligands of a different type are present, they may intercept the process and be
inserted in the place of the original type - ligand exchange has become
ligand substitution
.
3.5.3.1
Lability - Party Animals
Chemistry is full of 'opposites' - resulting from often setting two extremes as the limits for
defining behaviour (the 'black or white' approach). Of course, these limits are not isolated
options, but are usually the extremes of a continuum of behaviour (the chemical equivalent
of saying that something is rarely ever black or white, but more a shade of grey). How
rapidly metal ions take up or lose ligands is a good example of this characteristic. There
are two extreme positions; very fast reactions of labile compounds or very slow reactions
of inert compounds. Labile systems are the party animals of metal complexes - making
and breaking relationships rapidly. We can measure the rate at which ligand exchange with
the same type of ligand occurs, even though it appears that nothing changes, by using
radioactive isotopes to allow the rate of the process to be monitored, provided the process
is not too rapid. It's a bit like painting a white house with fluorescent white paint - nothing
appears to have changed, until you see it at night. At the molecular level, we simply measure
the uptake of radioactive ligand into the coordination sphere of the metal over time as it
replaces nonradioactive ligand, which allows us to define the rate of ligand exchange.
3.5.3.2
Inertness - Lasting Relationships
An inert system is like modern marriage - maybe not joined for ever, but willing to give it a
good try. These are complexes which, once formed, undergo any subsequent reaction very
slowly. Inertness can be so great that it overcomes thermodynamic instability. This means
that a complex may be pre-disposed towards decomposition, but this will happen so very,
very slowly that to all intents and purposes the complex is unreactive, or inert. Cobalt(III)
amine complexes are the classic example of this; thermodynamically they are unstable in
aqueous solution, but they are so inert to ligand substitution reactions that they can exist in
solution with negligible decomposition for years.
Even for a metal ion regarded as inert, however, the rate of substitution or replace-
ment of particular ligands will differ, and may differ significantly. For example, the
half-life for replacement of the coordinated perchlorate ion from the cobalt(III) complex
[Co(NH
3
)
5
(OClO
3
)]
2
+
by a water molecule in acidic aqueous solution is about seven sec-
onds, whereas the half-life for replacement of an ammonia from the related [Co(NH
3
)
6
]
3
+
is about 3 800 years! Chemistry is astounding in its diversity, if nothing else.
Note one important aspect of the above discussion - lability and inertness are
kinetic
terms, and all about the rate at which something reacts. A species that is inert is kinetically
stable. This does not require that it be thermodynamically stable, however (although it
may be). Thermodynamic stability is about being in a form which has no other readily
accessible species lower in energy. The reason we can have kinetic stability in a system
that is thermodynamically unstable is that the two differ. Kinetics is about transition
to
equilibrium; thermodynamics is about the situation
at
equilibrium. For a molecule to convert
from one form to another, by any process, it is considered necessary for it to overcome
an activation energy barrier, whereby it must proceed through a higher energy transition
