Chapter 21, Problem 99CP

Chelating ligands often form more stable complex ions than the corresponding monodentate ligands with the same donor atoms. For example,

$\begin{array}{llll}{\text{Ni}}^{2+}\left(aq\right)+6{\text{NH}}_3\left(aq\right)\hfill & \rightleftharpoons \text{Ni}{\left({\text{NH}}_3\right)}_6{}^{2+}\left(aq\right)\hfill & K\hfill & =3.2\times {10}^8\hfill \\ {\text{Ni}}^{2+}\left(aq\right)+3\text{en}\left(aq\right)\hfill & \rightleftharpoons \text{Ni}{\left(en\right)}_3{}^{2+}\left(aq\right)\hfill & K\hfill & =1.6\times {10}^{18}\hfill \\ {\text{Ni}}^{2+}\left(aq\right)+\text{penten}\left(aq\right)\hfill & \rightleftharpoons \text{Ni}{\left(\text{penten}\right)}^{2+}\left(aq\right)\hfill & K\hfill & =2.0\times {10}^{19}\hfill \end{array}$

where en is ethylenediamine and penten is

This increased stability is called the chelate effect. Based on bond energies, would you expect the enthalpy changes for the above reactions to be very different? What is the order (from least favorable to most favorable) of the entropy changes for the above reactions? How do the values of the formation constants correlate with ∆S°? How can this be used to explain the chelate effect?