`color{green}("Definition" )` : The stability of a complex in solution refers to the degree of association between the two species involved in the state of equilibrium.

● The magnitude of the (stability or formation) equilibrium constant for the association, quantitatively expresses the stability.

● Thus, if we have a reaction of the type :

`color{red}(M +4L ⇆ ML_4)`

● Then the larger the stability constant, the higher the proportion of `color{red}(ML_4)` that exists in solution.

`=>` Free metal ions rarely exist in the solution so that `color{red}(M)` will usually be surrounded by solvent molecules which will compete with the ligand molecules, `color{red}(L)`, and be successively replaced by them. For simplicity, we generally ignore these solvent molecules and write four stability constants as follows :

`color{red}(tt (( M , + , L , ⇆ , ML , K_1 = [ML]//[M][L] ) , ( ML , + , L , ⇆ , ML_2 , K_2 = [ML_2]//[ML][L] ) , ( ML_2, +, L, ⇆, ML_3 ,K_3 = [ML_3]//[ML_2][L] ), ( ML_3 , + , L ,⇆ , ML_4 ,K_4 = [ML_4]//[ML_3][L]) ))`

where `color{red}(K_1)`, `color{red}(K_2)`, etc., are referred to as `color{green}("stepwise stability constants")`. Alternatively, we can write the `color{green}("overall stability constant")` thus :

` color{red}(M + 4L \ \ \ ML_4 \ \ \ \ \ \ \ β_4 = [ML_4]//[M][L]^4)`

`=>` The stepwise and overall stability constant are therefore related as follows :

`color{red}(β_4 = K_1 xx K_2 xx K_3 xx K_4)` or more generally,

`color{red}(β_n = K_1 xx K_2 xx K_3 xx K_4.............k_n)`

`=>` If we take as an example, the steps involved in the formation of the cuprammonium ion, we have the following :


`color{red}(Cu^(2+) + NH_3 \ \ \ Cu(NH_3)^(2+) \ \ \ K_1 = [Cu (NH_3)^(2+)]//[Cu^(2+) ] [NH_3])`

`color{red}(Cu (NH_3)^(2+) + NH_3 ⇆ Cu(NH_3)_(2)^(2+) \ \ \ K_2 = [Cu ( NH_3)_(2)^(2+) ]//[Cu (NH_3)^(2+)] [NH_3])` etc.


● where `color{red}(K_1)`, `color{red}(K_2)` are the stepwise stability constants and overall stability constant.

● Also `color{red}(β_4 = [Cu (NH_3)_(4)^(2+)]//[Cu^(2+)] [NH_3 ]^4)`

`=>` The addition of the four amine groups to copper shows a pattern found for most formation constants, in that the successive stability constants decrease. In this case, the four constants are :

`color{red}(logK_1 = 4.0, logK_2 = 3.2, logK_3 = 2.7, logK_4 = 2.0)` or `color{red}(log β_4 = 11.9)`

`color{green}("Instability Constant ")` : The instability constant or the dissociation constant of coordination compounds is defined as the reciprocal of the formation constant.

`=>` The coordination compounds are of great importance.

`=>` These compounds are widely present in the mineral, plant and animal worlds and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine.

`=>` These are described below :

`=>` Coordination compounds find use in many qualitative and quantitative chemical analysis. The familiar colour reactions given by metal ions with a number of ligands (especially chelating ligands), as a result of formation of coordination entities, form the basis for their detection and estimation by classical and instrumental methods of analysis. Examples of such reagents include EDTA, DMG (dimethylglyoxime), `color{red}(α)`–nitroso–`color{red}(β)`–naphthol, cupron, etc.


`=>` Hardness of water is estimated by simple titration with `color{red}(Na_(2)EDTA)`. The `color{red}(Ca^(2+))` and `color{red}(Mg^(2+))` ions form stable complexes with EDTA. The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes.

`=>` Some important extraction processes of metals, like those of silver and gold, make use of complex formation.

● Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity `color{red}([Au(CN)_2]^–)` in aqueous solution.

● Gold can be separated in metallic form from this solution by the addition of zinc.

`=>` Similarly, purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds. For example, impure nickel is converted to `color{red}([Ni(CO)_4])`, which is decomposed to yield pure nickel.

`=>` Coordination compounds are of great importance in biological systems.

● The pigment responsible for photosynthesis, chlorophyll, is a coordination compound of magnesium.

● Haemoglobin, the red pigment of blood which acts as oxygen carrier is a coordination compound of iron.

● Vitamin `color{red}(B_12)`, cyanocobalamine, the anti– pernicious anaemia factor, is a coordination compound of cobalt.

●Among the other compounds of biological importance with coordinated metal ions are the enzymes like, carboxypeptidase A and carbonic anhydrase (catalysts of biological systems).

`=>` Coordination compounds are used as catalysts for many industrial processes. Examples include rhodium complex, `color{red}([(Ph_3P)_3RhCl])`, a Wilkinson catalyst, is used for the hydrogenation of alkenes.

`=>` Articles can be electroplated with silver and gold much more smoothly and evenly from solutions of the complexes, `color{red}([Ag(CN)_2]^–)` and `color{red}([Au(CN)_2]^–)` than from a solution of simple metal ions.

`=>` In black and white photography, the developed film is fixed by washing with hypo solution which dissolves the undecomposed `color{red}(AgBr)` to form a complex ion, `color{red}([Ag(S_2O_3)_2]^(3–))`.

`=>` There is growing interest in the use of chelate therapy in medicinal chemistry.

● An example is the treatment of problems caused by the presence of metals in toxic proportions in plant/animal systems.

● Thus, excess of copper and iron are removed by the chelating ligands D–penicillamine and desferrioxime B via the formation of coordination compounds.

● EDTA is used in the treatment of lead poisoning.

● Some coordination compounds of platinum effectively inhibit the growth of tumours. Examples are : cis–platin and related compounds.