Question

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

Where en is ethylene diamine 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 different? What is the order (from least favourable to most favourable) of the entropy changes for the preceding reactions? How do the values of the formation constants correlate with . How can this be used to explain the chelate effect?

Short answer

• · The enthalpies are almost same for the above reactions
• · Most favourable is the penten complex and least one is the amine complex
• · Formation constants increase with entropy change

Step-by-step solution

All the complexes formed have same bond energies between metal and nitrogen atoms

Therefore, the enthalpy changes for the above reactions giving nickel complexes are almost similar.

The most favourable complex has high value of ∆S°. 

The penten complex formation has relatively high entropy change value compared to ethylene diamine and ammonia complexes of nickel.

Change in entropy increases with the formation constants with high value for penten complex

The entropy change$∆\mathrm{S}°$ is relatively high for penten complex which in turn has high chelate effect compared to other nickel complexes