When a free metal ion is in aqueous medium, it is surrounded by (coordinated with) water molecules. It is represented as [MS6]. If ligands which are stronger than water are added to this metal salt solution, coordinated water molecules are replaced by strong ligands. Let us consider the formation of a metal complex ML6 in aqueous medium.(Charge on the metal ion is ignored) complex formation may occur in single step or step by step.
If ligands added to the metal ion in single step, then [MS6]+6L ☰ [ML6]+6S
βoverall is called as overall stability constant. As solvent is present in large excess, its concentration in the above equation can be ignored.
If these six ligands are added to the metal ion one by one, then the formation of complex [ML6] can be supposed to take place through six different steps as shown below. Generally step wise stability constants are represented by the symbol k.
