What is meant by stability of a coordination compound in solution? State the factors which govern

Explanation

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 :
`M + 4L harr ML_4`
then the larger the stability constant, the higher the proportion of `ML_4` that exists in solution. Free metal ions rarely exist in the solution so that M will usually be surrounded by solvent molecules which will compete with the ligand molecules, L and be successively-replaced by them. For simplicity, we generally ignore these solvent molecules and write four stability constants as follows:
`M+L harr ML" "K_(1)=[ML]//[ML][L]`
`ML+L harr ML_2" "K_2=[ML_2]//[ML][L]`
`ML_2+L harr ML_3" "K_3=[ML_3]//[ML_2][L]`
`ML_3 +L harr ML_4" "K_4=[ML_4]//[ML_3][L]`
where `K_(1), K_2`, etc., are referred to as stepwise stability constants. Alternatively, we can write the overall stability constant thus :
`M +4L harr ML_4" "beta_4 = [ML_4]//[M][L]^4`
The stepwise and stability constant are therefore related as follows:
`beta_4=K_(1) xx K_2 xx K_3 xx K_4` or more generally
`B_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:
`Cu^(2+)+NH_3 harr [Cu(NH_3)]^(2+), K_(1)= [Cu(NH_3)^(2+)]//[Cu^(2+)][NH_3]`
`[Cu(NH_3)]^(2+) + NH_3 harr [Cu(NH_3)_2]^(2+), K_2 = [Cu(NH_3)_2^(2+)]//[Cu(NH_3)^(2+)][NH_3)` etc.
Where `K_(1), K_2` are the stepwise stability constants and overall stability constant.
Also `beta_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 constant i.e.,the successive stability constant decrease. In this case, the four constant are :
`log K_(1) gt log K_2 gt log_3 gt log K_4`
The instability constant or the dissociation constant of coordination compounds is defined as the reciprocal of the formation constant.