Chemistry Reference
In-Depth Information
Because we can initiate and observe reactions, we know that we start with certain
reagents and end with others. We can usually separate and quantify the products of a
reaction. However, we cannot really understand the reaction unless we can answer the
question of how the reactants have been converted into the products. This process is the
mechanism
of the reaction, and it is difficult to tease out because the stages along the way
involve only transient non-isolable species - at best, we can infer their nature from a range
of experiments and by analogy to known stable species. At the core of reaction mechanisms
is the concept of the transition or activated state, the assembly present at the peak of
the activation barrier prior to relaxation to form the products. In coordination chemistry,
the predicted activated state species are considered to be based on known geometries,
but ones that are not inherently stable for the particular system under investigation;
there is good supporting evidence based on reactivity and products that support this
approach.
5.2
Complexation - Will It Last?
You've probably heard the doomsayers, in commenting on a new partnership, predict that
'it will never last'. Of course, given the fragility of life, this is hardly a prophetic statement,
but more of a comment on longevity. At the molecular level, we are also dealing with
finite rather than infinite time periods, and the longevity of a molecular partnership is not
constant for all assemblies. Coordination complexes are not uniform in their behaviour - in
fact, each complex has unique physical properties, which include unique thermodynamic
and kinetic stabilities. The fate of a complex relates to its environment; in solution in
particular, it will need to deal with the presence of other compounds, which include the
solvent itself. How it conducts itself, and whether it retains its integrity, depends on both its
thermodynamic and kinetic stability. Let's look at a human-scale example: we recognize
a bank (usually, at least) as a stable place that coordinates money, but if it parts with its
money in too undisciplined a way it can become, over time, insolvent and cease to exist. The
money has not disappeared, merely moved elsewhere. Coordination compounds follow the
same path; they may appear to have a certain form and stability, yet may eventually under
external influences undergo further change that leads to their demise - all a bit like life,
really.
5.2.1
Thermodynamic and Kinetic Stability
If a complex has a large thermodynamic stability that means the complex is a greatly
favoured product, resulting from a particular reaction, and possibly even the only sig-
nificant product, although one shouldn't immediately assume that this is the case. This
behaviour says very little about the manner by which and pace at which it undergoes its
formation and any following reactions, however. Having a high stability constant may
mean that a particular product forms essentially exclusively, but its accommodation of
subsequent changes in its environment relates in part to the rate at which it can undergo
reactions. There are, nevertheless, relationships between thermodynamic and kinetic pa-
rameters. Consider a simple equilibrium involving M, L and ML. Let's define reactions that
involve formation of ML (the 'forward reaction') and the reverse decomposition of ML (the
'back reaction') with rate constants for the forward (
k
f
) and back (
k
b
) reactions as follows
