Chemistry Reference
In-Depth Information
detect directly and cannot isolate, so our identification of it must rely on inference from
secondary observations. For example, the variation of rate constants with concentrations
of various reagents, the measured activation parameters, the range and various amounts of
products, and incorporation of isotopically labelled components of the reaction mixture in
products are devices that can assist the chemist in predicting the way a reaction proceeds.
5.3.1
A New Partner - Substitution
We shall examine in some detail here one of the most important, simplest and best under-
stood classes of reactions in coordination chemistry, namely
substitution
. This involves one
(or several) ligand(s) departing the coordination sphere to be replaced by one (or several)
others, without a change in coordination number and basic shape between the reactant com-
plex and its product. It is usually a clearly observable reaction, at least where the nature of
the ligands involved in the substitution are distinctively different, and can often be followed
by simple approaches such as observing the change of colour as reaction proceeds.
In considering how a substitution can occur, there are really a very limited number of
sensible options, which are:
(a) the departing ligand can leave first (leaving a temporary 'vacancy' in the coordination
sphere), and then the incoming ligand enters in its place;
(b) the entering ligand can add first (causing a short-lived increase in the number of ligands
around the metal ion), and then the departing ligand leaves; or
(c) one ligand can leave while the other enters, in a concerted manner (where a 'swapping'
of these ligands occurs with neither favoured in the transition state).
In all cases, regeneration of the preferred coordination number and geometry in forming
the stable product occurs.
In the above options, (a) involves ligand loss (or
dissociation
) as the key initiation step,
whereas (b) involves ligand gain (or
association
) as the key initiation step. The third option
(c) involves concerted replacement (or
interchange
) as the key step. As a consequence, they
tend to be referred to as
dissociative
(abbreviated as D),
associative
(A) and
interchange
(I) mechanisms in turn.
The concepts rely also on our understanding of preferred coordination numbers and
geometries. For example, for many metal ions six-coordination is common, but some stable
examples of both five- and seven-coordination are known. Therefore, it isn't too much of
a jump to consider a short-lived five- or seven-coordinate species existing for a species
stable only in six-coordination - in other words, a transition state of lower or higher
coordination number. As an example, six-coordinate octahedral is the overwhelmingly
dominant coordination mode for stable cobalt(III) complexes. Yet, in recent years, rare
examples of isolable but usually very reactive compounds with five-coordination and seven-
coordination have been prepared. It doesn't take too much of an act of faith to assert that
such geometries form as short-lived transition state species in substitution reactions of this
whole family of complexes.
5.3.1.1
Octahedral Substitution Mechanisms
Since octahedral complexes dominate coordination chemistry of particularly lighter ele-
ments, the mechanisms of these reactions are both important and well studied. There are
two limiting mechanisms,
dissociative
and
associative
; they are distinguished by the order
