The stability of a complex in solution refers to the degree of associationbetween the two species involved in the state of equilibrium. The magnitudeof the (stability or formation) equilibrium constant for the association,quantitatively expresses the stability. Thus, if we have a reaction of thetype : M L + 4 ⇄ ML‌4
then, larger the stability constant, the higher is the proportion of ML‌4that exists in solution. Free metal ions rarely exist in the solution so thatM will usually be surrounded by solvent molecules which will competewith the ligand molecules, L and be successively replaced by them. Forsimplicity, we generally ignore these solvent molecules and write fourstability constants as follows :
M+ L ⇄ ML‌‌‌‌‌ K ‌1 [ ML]/ [M][L]
M+ L ⇄ ML2‌‌‌‌‌ K ‌2 [ ML‌2]/ [ML][L]
ML‌2+ L ⇌ ML3‌‌ K ‌3 [ ML‌3]/ [ML‌2][L]
ML‌3+ L ⇌ ML4‌‌ K ‌4 [ ML‌4]/ [ML‌3][L]
Where K‌1, K‌2, etc., are referred to as stepwise stability constants.Alternatively, we can write the overall stability constant as :
M + 4L ⇄ ML4‌‌ β ‌4 [ ML‌4]/ [M ][L] ‌4
The stepwise and overall stability constant are therefore related asfollows:
β4=K1×K2×K3×K4 or more generally,
βn=K1×K2×K3×K4...........Kn
(i) Smaller the size of cation, greater will be the stability of complex e.g.,Fe‌3+ forms more stable complex than Fe‌2+.
(ii) Greater the charge on central metal ion, more stable will be thecomplex e.g., Pt‌4+ forms more stable complex than Pt‌2+.
(iii) Stronger the ligand, more stable will be the complex formed e.g., CN‌–forms more stable complex then NH‌3.