Q8(b). Chelate effect, template effect and entropy in complex stability

Chelate effect

1. A ligand binding through two or more donor atoms forms a ring with the metal (chelate). Its complex is more stable than that of monodentate ligands with the same donor atoms: [Ni(en)₃]²+ is stable in dilute solution, while [Ni(CH₃NH₂)₆]²+ loses methylamine and Ni(OH)₂ precipitates.
2. Data (Cd²+): 2pt] [Cd(H₂O)₆]²+ + 4CH₃NH₂ → [Cd(CH₃NH₂)₄(H₂O)₂]²+ + 4H₂O: Δ H°=-57.3 kJ mol⁻¹, Δ S°=-67.3 J K⁻¹ mol⁻¹, K=3.3×10⁶ 2pt] [Cd(H₂O)₆]²+ + 2en → [Cd(en)₂(H₂O)₂]²+ + 4H₂O: Δ H°=-56.5 kJ mol⁻¹, Δ S°=+14.1 J K⁻¹ mol⁻¹, K=4.0×10¹⁰
3. The enthalpies are almost the same (four Cd–N bonds each), yet K differs by a factor of about 1.2×10⁴. The difference is in Δ S°.
4. Ring size: five-membered rings (en, oxalate, glycinate) are best; six-membered suit conjugated ligands (acac⁻); smaller rings are strained, larger ones place the second donor too far away.
5. Kinetic side: once one end is bound, the other is held close and re-binds quickly, so a chelate dissociates slowly (kinetic chelate effect).

Entropy effect

1. With methylamine the number of particles is unchanged (5 → 5); with en it rises (3 → 5). The extra translational entropy of the released water makes Δ S° positive: -TΔ S° is +20.1 kJ mol⁻¹ for CH₃NH₂ but -4.2 kJ mol⁻¹ for en, so Δ G°=-37.2 against -60.7 kJ mol⁻¹.
2. The same idea favours EDTA⁴⁻ (one ligand replaces six water molecules) and the complexation of highly charged ions, where release of ordered hydration water dominates.

Template effect

1. A metal ion holds reactive ligand groups in the right positions (it acts as a template or “collector”) so that they close into a ring that forms poorly or not at all without the metal.
2. Examples: Cu²+ + 1,2-ethanediamine + formaldehyde + nitroethane in methanol give, in one step, a copper complex of a 14-membered tetraaza macrocycle; metal phthalocyanines form from phthalonitrile around Cu²+; crown ethers form in better yield in the presence of K⁺.
3. The product enjoys the macrocyclic effect: a macrocycle binds more strongly than the open-chain chelate with the same donors, because it is pre-organised and needs little reorganisation or solvent loss on binding.

Application. the chelate effect is the basis of EDTA titrations (water hardness), chelation therapy (EDTA for lead, desferrioxamine for iron) and of stable gadolinium chelates used as MRI contrast agents.