Question

1.In the context of chelation, what does binding strength mean? What happens at the molecular level that cuases chelation to be so favorable?

2.In this experiment, you measured ligand field strength (NH3<H2O<EDTA). What interactions affect ligand field strength at the molecular level?

3.These two effects, chelation and ligand field strength, were confounded in this experiment because of the choice of ligands we made. Design a ligand molecule that would allow you to measure just the effect of chelation without modifying ligand field strength in comparison to ammonia.

Answer 1

1. The binding strength means that the strength that a multi-dentate ligand provides to the metal ion by binding it through metal-ligand cooperation, where metal behaves as an electrophile (acidic) and the ligand behaves as a nucleophile (basic).

Through the chelation of a ligand to the metal ion, the ligand transfers all its steric electronic properties to the metal center (or metal ion). Hence, chelation is favorable.

2. In general, the ligand field strength depends on the ligating atoms.

Decreasing order of field strength for various ligands: C-donor ligands > N-donor ligands > O-donor ligands > X-donor ligands.

Where 'X' is the halogen atom.

Hard(metal ion)-hard (ligating atom)and soft(metal ion)-soft(ligating atom) interactions are always favorable.

For example, Fe-O and Cr-N interactions are more favorable interactions in the complex formation.

3. Ammonia (NH₃) is a monodentate ligand. If you make it bidentate, for example, ethylenediamine (H₂N-CH₂-CH₂-NH₂) makes the metal ions more stable by chelating through both of its nitrogen donor atoms. Hence, it provides more strength to the metal center or metal ion, to which it coordinates or chelates.