# Question 4fe75

Mar 15, 2015

The stability constant of metal complexes is actually an equilibrium constant that describes the overall process of the formation of a metal complex in a solution.

This formation process is usually a substitution reaction in which one ligand replaces another ligand. In your case, EDTA, which is a hexadentate ligand (it has 6 lone pairs of electrons), replaces water, a simple ligand.

The overall reaction looks like this - all the species are in aquesous solution, except water, of course

[Ni[(H_2O)_6]^(2+) + EDTA^(4-) rightleftharpoons [Ni(EDTA)]^(2-) + 6H_2O#

To write the stability constant for this reaction you can use the same rules as you would for writing classic equilibrium constants - the concentrations of the products (water is excluded) divided by the concentration of the reactants

${K}_{\text{stab}} = \frac{\left[{\left[N i \left(E D T A\right)\right]}^{2 -}\right]}{\left[N i {\left[{\left({H}_{2} O\right)}_{6}\right]}^{2 +}\right] \cdot \left[E D T {A}^{4 -}\right]}$