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

where en is ethylenediamine and penten is

The increased stability that results is called the chelate effect. From the bond energies, would you expect the enthalpy changes for the preceding reactions to be very different? What is the order (from least favorable to most favorable) of the entropy changes for the preceding reactions? How do the values of the formation constants correlate with ˆ†S0? How can this be used to explain the chelate effect?

K 3.2 x 108 Kf= 1.6 1018 K 2.0 x 1019 entena)Nipenten)2+aq NH2CH CH2 CH2CH,NHb N-CH2-CH2-N NH2CH2CH2 H,CH.NH,