The Chemistry of d- and f- Block: The Chelate Effect

In this section, we will build up one of the concept of the trend in stability constant where changing the ligands into chelating agent has effect on the stability constant. A chelating agent or a chelate can be defined as polydentate ligand where it has more than 1 attachment point of ligand-metal ion. In this section, we will also see the origin and the physical interpretation of the chelate effect.

[M(EDTA)]^(n-4)+ complex, the chelate effect

When we have a complex with polydentate ligands, we will see different value of stability constant even the donor atom is the same. For example in the reaction below:

It is shown that [Ni(en)₃]²⁺ is 10¹⁰ times more stable than [Ni(NH₃)₆]²⁺ although the donor atom is the same and this is caused by the chelate effect. The chelate [chelos (greek) = claw] effect is defined as the enhanced stability of a complex containing chelating (i.e. polydentate) ligands over one containing similar monodentate ligands. On the table below shows the chelate effect at trien ligands.

The thermodynamics view of this effect can be seen from the ΔG of the reaction by relating these two equations:

When β is bigger so ΔG is getting more negative. 

For reactions that involve the same metal ion and the same donor atom (such as in our first example), the bond strength should be very similar so ΔH should be not change much. Therefore, the difference must arise from the TΔS term. If we calculate the enthalpy for [Ni(en)₃]²⁺ complex, we can get the ΔH is -12.1 kJ mol^-1 and TΔS is 55.1 kJ mol^-1 at 25^o C; hence it justify our point that the main driving force of this reaction is from TΔS term.

Another unique case is in this reaction:

In this case, ΔH is positive (+13 kJ mol^-1) because of steric strain in tren ligand and weaker bond between tertiary amine and Ni, but we are now making more chelate rings. Hence, the term of TΔS gives the driving force of this reaction (23.7 kJ mol^-1), so ΔG is -10.7 kJ mol^-1. The term of TΔS is related closely to the increasing entropy of the system. When we see our first example again as shown below,

for the first reaction there is 7 species in solution on both sides of the equation, so the entropy change is relatively small whereas for the second reaction there are 7 species in solution on the right hand side and 4 species in solution on the left hand side of the equation. Hence, the second reaction has more increased in disorderedness (entropy) rather than the first reaction. However, entropy does not necessarily means the disorderedness but the how the energy can be distributed within the system.

Another interpretation of this effect is at monodentate ligands the ligand substitution is independent from each other so the substitution does not affect the ligands changes. However, in polydentate ligands once one end is attached, the other end is not independent any more because it is already in the same molecule. Hence, an intramolecular reaction is happened and more likely to join on. In reverse reaction, if one end dissociates, the other end has more chance to reattach, so this makes the polydentate complex is more stable than monodentate one.

Another factor that determines the stability of polydentate ligands complex is the bite angle which is the angle between the two donor atoms of a chelate ring. The ligands with sp³ carbons in the backbone prefer 5 and 6-membered rings.

Furthermore, the 5-membered rings are not planar but puckered.

Meanwhile, sp² carbons are quite stable in 4-membered rings which means smaller bite angle.