Chelate effect 

The chelate effect can be seen by comparing the reaction of a chelating ligand and a metal ion with the corresponding reaction involving comparable monodentate ligands. For example, comparison of the binding of 2,2'-bipyridine with pyridine or 1,2-diaminoethane (ethylenediamine=en) with ammonia.

Consider the following equilibria: 

For first reaction log K₁ = 10.6 ΔH°=-54 kJ mol^-1 ΔS°=+23 J K ^-1 mol^-1 while for second reaction log β₂ = 7.7 ΔH°=-46 kJ mol^-1 ΔS°=-8.4 J K ^-1 mol^-1

Although essentially the same copper-nitrogen bonds are formed in the two complexes, the formation of the en complex is distinctly favoured. Note in particular that the first reaction involves an increase in entropy. This is because two water molecules are released into solution when one en coordinates. The preference of chelate coordination is called the chelate effect and, as derived, is an entropic effect.

Thermodynamic origin of Chelate Effect:

It has been known for many years that a comparison of this type always shows that the complex resulting from coordination with the chelating ligand is much more thermodynamically stable. This can be seen by looking at the values for adding two monodentates compared with adding one bidentate, or adding four monodentates compared to two bidentates, or adding six monodentates compared to three bidentates.

A number of points should be highlighted from the formation constants in Table E4. In the first table, it can be seen that the ΔH° values for the formation steps are almost identical, that is, heat is evolved to about the same extent whether forming a complex involving monodentate ligands or bidentate ligands. What is seen to vary significantly is the ΔS° term which changes from negative (unfavorable) to positive (favorable). Note as well that there is a dramatic increase in the size of the ΔS° term for adding two compared to adding four monodentate ligands. (-5 to -35 JK^-1mol^-1). What does this imply, if we consider ΔS° to give a measure of disorder?

In the case of complex formation of Ni²⁺ with ammonia or 1,2-diaminoethane, by rewriting the equilibria, the following equations are produced.

Using the equilibrium constant for the reaction (3 above) where the three bidentate ligands replace the six monodentateligands, we find that at a temperature of 25° C: 

  ΔG∘=−2.303RTlog10K

=−2.303RT(18.28−8.61)

=−54 kJ mol−1

Based on measurements made over a range of temperatures, it is possible to break down the ΔG∘ term into the enthalpy and entropy components.

ΔG∘=ΔH∘−TΔS∘

The result is that: ΔH∘=−29kJmol−1

- TΔS° = -25 kJ mol^-1
     and at 25C (298K)
ΔS° = +88 J K^-1 mol^-1

Note that for many years, these numbers have been incorrectly recorded in textbooks. For example, the third edition of "Basic Inorganic Chemistry" by F.A. Cotton, G. Wilkinson and P.L. Gaus, John Wiley & Sons, Inc, 1995, on page 186 gives the values as:

ΔG° = -67 kJ mol-1
ΔH° = -12 kJ mol-1
-TΔS° = -55 kJ mol-1

The conclusion they drew from these incorrect numbers was that the chelate effect was essentially an entropy effect, since the TΔS° contribution was nearly 5 times bigger than ΔH°.

In fact, the breakdown of the ΔG° into ΔH° and TΔS° shows that the two terms are nearly equal (-29 cf. -25 kJ mol^-1) with the ΔH° term a little bigger! The entropy term found is still much larger than for reactions involving a non-chelating ligand substitution at a metal ion. How can we explain this enhanced contribution from entropy? One explanation is to count the number of species on the left and right hand side of the equation above.

It will be seen that on the left-hand-side there are 4 species, whereas on the right-hand-side there are 7 species that is a net gain of 3 species occurs as the reaction proceeds. This can account for the increase in entropy since it represents an increase in the disorder of the system.

An alternative view comes from trying to understand how the reactions might proceed. To form a complex with 6 monodentates requires 6 separate favorable collisions between the metal ion and the ligand molecules. To form the tris-bidentate metal complex requires an initial collision for the first ligand to attach by one arm but remember that the other arm is always going to be nearby and only requires a rotation of the other end to enable the ligand to form the chelate ring.

If you consider dissociation steps, then when a monodentate group is displaced, it is lost into the bulk of the solution. On the other hand, if one end of a bidentate group is displaced the other arm is still attached and it is only a matter of the arm rotating around and it can be reattached again.

Both sets of conditions favor the formation of the complex with bidentate groups rather than monodentate groups.  =>See Previous Post Also