Effect of Ligand Exchange on Stability Constant
- The stability constants (Kstab) of ligands are often given on a log10 scale so that it becomes easier to compare them with each other
- Ligand exchange in a complex occurs to form a more stable complex with a larger Kstab
- The stability constants can be used to explain the substitution of ligands in a copper complex
Ligand substitution in a Co(II) complex
- When excess ammonia is added to the [CoCl4]2- complex a brown solution is obtained
Ligand exchange of the [CoCl4]2- complex by ammonia
The chloride ligands are substituted by the ammonia ligands to form the more stable ammonia complex
- The formation of the ammonia complex could be explained by comparing the stability of the chloride and ammonia ligands
Stability of chloride and ammonia ligands table
| Ligand | Stability (log10 Kstab) |
| Cl– | 5.6 |
| NH3 | 13.1 |
- The stability constant of the ammonia ligand is greater than that of the chloride ligands
- The brown ammonia complex is therefore more stable
- As a result, the position of the equilibrium is shifted to the right
Worked example
The numerical values for the stability constants, Kstab, of three silver(I) complexes are given.
| Silver(I) complex | Numerical value of Kstab |
| [Ag(S2O3)2]3– | 2.9 x 1013 |
| [Ag(CN)2]– | 5.3 x 1018 |
| [Ag(NH3)2]+ | 1.6 x 107 |
An aqueous solution of Ag+ is added to a solution containing equal concentrations of S2O32– (aq), CN– (aq) and NH3 (aq). The mixture is left to reach equilibrium.
Deduce the relative concentrations of [Ag(S2O3)2]3–, [Ag(CN)2]– and [Ag(NH3)2]+ present in the equilibrium mixture. Explain your answer.
Answer
- The highest concentration will be [Ag(CN)2]–
- This is because the Kstab value for [Ag(CN)2]– is the largest value
OR - [Ag(CN)2]– is the most stable
- This is because the Kstab value for [Ag(CN)2]– is the largest value
- The lowest concentration will be [Ag(NH3)2]+
- This is because the Kstab value for [Ag(NH3)2]+ is the smallest value
OR - [Ag(NH3)2]+ is the least stable
- This is because the Kstab value for [Ag(NH3)2]+ is the smallest value
- An alternative explanation could be to state that higher Kstab values form a more stable complex
