Undergoing Change– Kinetic Stability
Reactions have a start and an end; thermodynamics is concerned with the end whereas kinetics is concerned with the processes leading from the start to that end. Thermodynamic stability is concerned with the extent of formation of a species under certain specified
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):
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 Mg2+ to Ca2+ the exchange rate increases from ~105 to ~108 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 18OH2 or 17OH2 as solvent and following its introduction into the metal coordination sphere in place of normal 16OH2 (such as by using 170 NMR spectroscopy). While we can measure these processes understanding how they occur is another thing altogether.
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.
