Chelating ligands often form more stable complex ions than...

Chelating ligands often form more stable complex ions than the corresponding monodentate ligands with the same donor atoms. For example, This increased stability is called the chelate effect. Based on bond energies, would you expect the enthalpy changes for the above reactions to be very different? What is the order (from least favorable to most favorable) of the entropy changes for the above reactions? How do the values of the formation constants correlate with $\Delta S^{\circ}$ ? How can this be used to explain the chelate effect?

Chelating ligands often form more stable complex ions than the corresponding monodentate ligands with the same donor atoms. For example,
This increased stability is called the chelate effect. Based on bond energies, would you expect the enthalpy changes for the above reactions to be very different? What is the order (from least favorable to most favorable) of the entropy changes for the above reactions? How do the values of the formation constants correlate
with $\Delta S^{\circ}$ ? How can this be used to explain the chelate effect?

Key Concepts

Formation Constants and Thermodynamic Correlation

Formation constants (Kf) quantitatively reflect the overall stability of metal complexes. A larger Kf indicates a more stable complex, which, in the case of chelating ligands, is largely due to the favorable entropy change accompanying complex formation. Since the free energy change ?G is related to both the enthalpy (?H) and entropy (?S) by ?G = ?H - T?S, the large formation constants observed for chelate complexes can be correlated with their more positive (or less negative) entropy contributions, thereby explaining the chelate effect.

Entropy Changes

Entropy plays a crucial role in the chelate effect. Complex formation with chelating ligands typically results in a greater increase in the system's entropy compared to formation with monodentate ligands. This is because the chelate reaction often results in the release of more particles (such as solvent molecules or counterions) per metal ion, which leads to a favorable ?S. The order of entropy changes from least favorable to most favorable generally follows the trend of monodentate complexes showing lower entropy gains and chelate complexes showing higher entropy gains.

Chelate Effect

The chelate effect describes the enhanced stability of complexes formed with multidentate (chelating) ligands compared to similar complexes with equivalent monodentate ligands. This increased stability is not primarily due to stronger individual bonds but arises from a combination of entropic and, to a lesser extent, enthalpic factors. When a chelating ligand binds to a metal ion, it forms ring structures that reduce the number of particles in solution less dramatically compared to equivalent substitutions with monodentate ligands, leading to a favorable change in entropy.

Enthalpy and Bond Energies

Bond energies provide insight into the strength of individual metal-ligand bonds; however, when comparing chelating and monodentate ligands with the same donor atoms, their bond energies are often very similar. Thus, the magnitude of the enthalpy changes for complex formation is not drastically different between these two types of ligands. This similarity in ?H suggests that the large difference in stability (as seen in formation constants) is due to other factors.

Transcript

00:01 This is another giant problem and we're talking about curating ligands and they often form more stable complex ions than the corresponding monodentate ligands with the same donor atoms.

00:11 So i've written the three equations that we're given here.

00:15 We've specified, you already know this, that in is eithelene diamine and penton.

00:19 I've written the structure right here.

00:22 And this increased stability is called the chelate effect.

00:26 Then we're going to answer some questions.

00:28 Based on the bond energies, would we expect the enthalpy changes to be different? okay, problem at 79.

00:44 Okay, so let's go with this.

00:51 No, we do not expect enthalpy changes to be very different.

01:30 All three, six bonds are formed between the nickel, 2 plus, and nitrogen.

01:54 So delta h will be similar.

02:03 That's the first part.

02:04 Our next one says for the delta s order.