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 significant 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 (kf) and back (kb) reactions as follows
We can then consider combining Equations 5.29 and 5.30 into one equation (5.31) namely:
This expression as written also corresponds to the one we use to define the equilibrium constant, K. It can be shown that there is a simple relationship between these terms (5.32) namely:
This holds provided that no stable intermediate is involved in the reaction. If the rate at which ML dissociates into M and L (defined by the rate constant kb) is slower than the rate at which M and L assemble into ML (defined by the rate constant kf) then there will always at equilibrium be a larger amount of ML present than of M and L, which translates into a large thermodynamic stability constant K, which equates with the ratio of these two kinetic terms. In effect, for such simple steps there is a thermodynamic–kinetic link. At this level however we shall not dwell on this too deeply.
