The Stability Constant, Kstab, is the equilibrium constant for the equilibrium that exists between a transition metal ion surrounded by water molecule ligands and the same transition metal ion surrounded by ligands of another kind in a ligand displacement reaction.
The individual ligands are displaced stepwise, and an equilibrium expression could be written for each step, but it is more common to write an overall expression for the overall ligand displacement reaction. Like all equilibrium constants, stability constants vary with temperature. For example,
[Cu(H2O)6]2+(aq) + 4NH3(aq) [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)
The value is often expressed as its logarithm, log Kstab, making the numbers easier to handle. The larger the value of log Kstab, the more powerfully stabilising are the ligands of the transition metal ion. Since for a given transition metal ion, the different types of ligands always displace water molecule ligands in the equilibrium reaction, the numerical values can be used to decide upon the order in which the ligands are stabilising of a particular transition metal ion. The higher the value of the stability constant, the more stable the complex.
See also Equilibrium Constant.