Ladder Diagrams for Complexation Equilibria
The same principles used in constructing and interpreting ladder diagrams for acid–base equilibria can be applied to equilibria involving metal–ligand com- plexes. For complexation reactions the ladder diagram’s scale is defined by the concentration of uncomplexed, or free ligand, pL. Using the formation of Cd(NH3)2+ as an example
we can easily show that the dividing line between the predominance regions for Cd2+ and Cd(NH3)2+ is log(K1).
Since K1 for Cd(NH3)2+ is 3.55 ´ 102, log(K1) is 2.55. Thus, for a pNH3 greater than 2.55 (concentrations of NH3 less than 2.8 x 10–3 M), Cd2+ is the predominate species. A complete ladder diagram for the metal–ligand complexes of Cd2+ and NH3 is shown in Figure 6.6.
We can also construct ladder diagrams using cumulative formation constants in place of stepwise formation constants. The first three stepwise formation con- stants for the reaction of Zn2+ with NH3
show that the formation of Zn(NH3)32+ is more favorable than the formation of Zn(NH3)2+ or Zn(NH3)22+. The equilibrium, therefore, is best represented by the cumulative formation reaction
A complete ladder diagram for the Zn2+–NH3 system is shown in Figure 6.8.
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