Complex ions

Stability constants
The animation above shows ammonia molecules replacing the water ligands on the Cu(H2O)62+ complex ion (coordination complex)

Some complex ions are more stable than others. Certain ligands can replace existing ligands, as shown on the left. For example, the water molecules from the Cu(H2O)62+ complex ion can be replaced by NH3 molecules. The Cu(NH3)42+ is more stable and produces a deep blue colour.

Cu2+(aq) + 4NH3(aq) <=> Cu(NH3)4+2(aq)

The equilibrium expression for the above reaction can be written as

Kst = [Cu(NH3)4+2] / ([Cu2+][NH3]4) = 1012 at 25oC
Kst is also known as the stability constant. The equilibrium constant, at 1012 , is very high and this equilibrium will take place in preference to the formation of the Cu(H2O)62+ complex ion.

How to use stability constants.

SIlver ions(Ag+) are placed in a solution containing 0.1M NaCl and 0.1M NH3 . Assuming we know the stability constants of the following equilibria at some unknown temperature, what will be the most abundant complex ion in the mixture?
Ag+(aq) + 2NH3+(aq) <=> Ag(NH3)2+(aq) Kst = 107

Ag+(aq) + 2Cl-(aq) <=> Ag(Cl)2-(aq) Kst = 10

Looking at the stability constants we see that the formation of Ag(NH3)2+ is favored. The dominant complex will be Ag(NH3)2+.

Home