CC2. How Tightly Do Ligands Bind?
Show that the overall equilibrium constant for formation of Ni(NH3)62+ is a product of the equilibrium constants for individual ammonia binding steps, K1 x K2 x ... x K6.
Determining an equilibrium constant requires that the concentrations of the individual species can be measured, such as [Ni2+], [NH3] and [Ni(NH3)62+]. This measurement might be accomplished using spectroscopic or electrochemical techniques. This measurement has been done for this example at 25 ° C in water, and the value of K = 1.0 x 108 M-6.
The numerical values for individual binding constants for ammonia with nickel at 25 ° C in water are as follows:
K1: 4.7 x 102 K2: 1.3 x 102 K3: 41 K4: 12 K5: 4.2 K6: 0.81
- Use these values to confirm the value of the complex formation constant for [ Ni(NH3)62+].
- What explanations can you offer for the trend in individual K values?
The reality of the experimental determination of the binding constant is more complex than illustrated in these figures. For example, a Ni2+ ion does not really exist on its own. First of all, there must be counterions involved. If the counterions are not selected carefully, they might affect the measurements. For example, if the counterions are chlorides, maybe they could form NiCl2 or other chloride species. Chloride is a Lewis base and it could compete with ammonia as an alternative nucleophile or ligand. Other counterions are called "non-coordinating" because they do not generally bind with Lewis acids; these include BF4-, PF6- and ClO4-.
In addition, the fact that the experiment was done in water means that Ni2+ ion was not really involved. Water is a nucleophile, too. That means that the real starting species was [Ni(OH2)62+]. Instead of just binding to a bare nickel ion, the ammonia was replacing the water molecules, one at a time.
Exactly how one nucleophile substitutes for another in a coordination complex is the subject of another chapter.
Explain the differences between formation constants for the following of complexes at 25 ° C:
- [Zn(NH3)42+]: 7.8 x 108 and [Zn(CN)42-]: 4.7 x 1019
- [Fe(CN)64-]: 1.0 x 1024 and [Fe(CN)63-]: 1.0 x 1031