Question

10. EDTA (C10H16N208) is an aminopolycarboxylic acid, widely used for scavenging metal ions. EDTA has high denticity, and can displace the H2O ligands that bind to hydrated metal cations. For example: [Fe(H2O).]3+ (aq) + EDTA (aq) → Fe-EDTA'(aq) + 6H20 (1) Explain conceptually, what you think drives the aforementioned reaction (enthalpy or entropy), and why.

Answer 1

Answer:

Monodentate ligands bind through only one donor atom. Monodentate means "one-toothed." The halides, phosphines, ammonia and amines seen previously are monodentate ligands. Bidentate ligands bind through two donor sites. Bidentate means "two-toothed." An example of a bidentate ligand is ethylenediamine. It can bind to a metal via two donor atoms at once.

Bidentate binding allows a ligand to bind more tightly. Tridentate ligands, which bind through three donors, can bind even more tightly, and so on. This phenomenon is generally called the "chelate effect."

Multidentate ligands bind more tightly because of the chelate effect.

H₂O is a monodentate ligand and EDTA is a hexadentate ligand.

The chemical reasons for the chelate effect involve relative enthalpy and entropy changes upon binding a multidentate ligand. In terms of enthalpy, in order to completely remove a hexadentate ligand, six coordinate bonds must be broken. That costs more energy than breaking one coordinate bond for a monodentate ligand.

In terms of entropy, which deals with the distribution of energy within a system, it is generally thought that bringing two molecules together (a hexadentate ligand and a metal complex) costs less than bringing seven molecules together (six monodentate ligands and a metal complex). That's because individual molecules are free to move around, tumble and vibrate independently. Once they come together, they have to do all these things together. Since these different types of motion represent different ways of distributing energy, if the system becomes more restricted, energy can't be distributed in as many states.

• Energy is lowered even more by six bonding interactions
• Compared to six separate donors, hexadentate donation is entropically favoured