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Chelating Ligand

Chelate ligands or chelates in short are by definition ligands where the ligating atoms and the coordination center forms at least one closed loop, when the connectivity of atoms are considered.
Chelating ligands must therefore have at least two ligating atoms.
The total number of ligand atoms that can bind simultaneously to one metal center is the density of the ligand.

When a multi-dentate ligand simultaneously coordinate to metal ion by more than one donor site, then a ring-like structure is obtained.
It is called a chelate ring and the ligand is known as chelating ligand. (From Greek word chele means claws)

The category of coordination compounds which is formed by chelating ligands is known as chelates. The chelates find important application in micro detection and determination of metal ions in analytical chemistry.


Chelating Ligand Stability

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A chelating ligand is symmetrical if the two coordinating atoms are the same. In case the coordinating atoms are different it is unsymmetrical. All types of bi-dentate, tri-dentate and poly-dentate ligands can act as chelating ligands.

The stability of complexes is greatly increased by chelating ligand. This is known as chelate effect. The more the rings present in a complex the more is the stability it acquires. Chelating ligands forms more stable complexes than does non-chelating ligands. This can be readily explained in terms of thermodynamic factors. Large value of stability constant K are favored by large negative enthalpy and positive entropy changes.

For example, in the reaction

[M(H2O)6]2+ + en [M(H2O)4en]2+ + 2H2O

Two H2O are replaced by bi-dentate en ligand. This process increases the number of particles in the system and hence its disorder and entropy. Thus, [M(H2O)4en]2+ is more stable than [M(H2O)6]2+ .

This supports the fact that the stability of complexes increases with the increase of entropy.
  • Larger the number of the chelate rings in a complex, the greater is its stability.
  • Chelates with 5 membered rings including the metal atom are more stable if these do not contain double bonds, if the ligands are saturated. On the other hand, chelates with six membered rings are more stable if the ligands contain conjugate double bonds. eg:- acetylacetanato complexes.
  • Ligands with more than six donor group or larger groups form unstable rings. This is due to steric hindrance.

Factors Affecting Stability

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Stability constants are influenced by many factors. These include
  1. The size of the chelate ring.
  2. The number of rings bounded to the metal ion.
  3. The Lewis base strength of the chelating ligand.
  4. The size and charge of the ligand.
  5. Whether the ligand is multi-dentate or bi-dentate.
  6. Modifications of the p bonding strength.
  7. The nature of the donor ligand atom.
  8. The resonance effect within the chelate ligand and.
  9. The steric effect of the ligands.

Even though chelating a mineral does not guarantee its absorption or metabolism, the way a mineral is chelated and which ligand is employed will influence its ultimate absorption or metabolism. When stability conditions favor the organism, the chelate can be considered nutritionally functional.

Chelating Ligand Copper and Acetylacetonate

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Cu(acac)2 is unique among the metal complexes of acetylacetanato. The bivalent metal complexes of M = Ca, Mn, Fe, Co, Ni and Zn are polymeric with general formula [M(dik)2]n except square planar monomeric copper(II) complex.

In case of complex [{Cu(acac)(phen)(ClO4)}{Cu(phen)(μ1,3-N3)2}]2, the Cu(II) ions are ferromagnetic ally coupled, as expected for the bridging mode of azido ligands. The Cu(II) ion is in square pyramidal coordination achieved with chelating ligands and the azido bridge to the other part of the molecule. Significant π-π staking interaction occurs between these two units and the presence of extended systems like acac, phen or bipy favors this interaction.

The strongest interaction takes place between acac ligands. The crystal structure can be described as a 3D supra molecular structure consisting of parallel layers, which are formed by a π-π stacking interaction between [Cu(acac)(bipy)] fragments of the trans-isomers and the [Cu(acac)(bipy)] fragments from one head of the cis-isomers. Basically each trans-isomer interacts with two cis-isomers through an acac ligand and each cis-isomer interacts with another cis-isomer through the bipy ligand and one trans-isomer.

Acac Ligands

Chelating Ligand EDTA

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On of the most important chelating ligands is ethylene-diaminetetraacetate ion, abbreviated [EDTA]4- or simply EDTA. EDTA is a hexa-dentate ligand because it has six donor atoms, all of which can bond to a single metal ion.

Ethylene Diaminetetraacetate Ion

EDTA titration are used for the analytical determination of metal ion concentration. This is possible because [EDTA]4- binds so strongly to most metal ions that essentially 100% of the ion is bound to the ligand.
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