Chelating ligands have higher affinity for a metal ion than analogous monodentate ligands. The chelate effect is the enhanced affinity of a chelating ligand for a metal ion compared to its monodentate ligand counterpart(s). This term comes from the Greek chelos, meaning "crab". A crab does not have any teeth at all, but it does have two claws for tightly holding onto something. A very simple analogy is that, if you are holding something with two hands rather than one, you are not as likely to drop it. For example, ethylenediamine (en, H2NCH2CH2NH2) is a bidentate ligand that binds metal ions more strongly than monodentate amine ligands like ammonia (NH3) and methylamine (CH3NH2). Tridentate ligands, which bind through three donors, can bind even more tightly than bidentate, and so on.
Chelate Effect And Its Thermodynamic Origin Pdf Downloadl
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The chelate effect can be explained using principles of thermodynamics. Recall that reactions are spontaneous when the Gibbs Free Energy change is negative \(-\Delta G\); this is true when change in enthalpy is negative (\(-\Delta H\)) and the change in entropy is positive (disorder increases, \(+\Delta S\). (From the equation \(\Delta G = \Delta H - T\Delta S\).)
In each the reactant Cu complex and product Cu-complex in Figure \(\PageIndex4\), there are two N-Cu bonds. Electronically, the ammonia and en ligands are very similar, since both bind through N and since the Lewis base strengths of their nitrogen atoms are similar. The enthalpy change due to breaking two H3N-Cu bonds and replacing them with two new N(en)-C bonds is almost zero. Thus, enthalpy is not a major driving factor in the chelate effect.
The example above gives a case when just one bidentate ligand is involved. When multiple bidentate ligands are involved, or when denticity increases, the chelate effect is enhanced further. Consider the two complexation equilibria in aqueous solution, between the cobalt (II) ion, Co2+(aq) and ethylenediamine (en) on the one hand and ammonia, NH3, on the other.
The bottom line is that the chelate effect is entropy-driven. It follows that the more binding groups a ligand contains, the more positive the ΔS and the higher the Kf will be for complex formation. In this regard, the hexadentate ligand ethylenediamine tetraacetic acid (EDTA) is an optimal ligand for making octahedral complexes because it has six binding groups. In basic solutions where all four of the COOH groups are deprotonated, the chelate effect of the EDTA4- ligand is approximately 1015. This means, for a given metal ion, Kf is 1015 times larger for EDTA4- than it would be for the relevant monodentate ligands at the same concentration. EDTA4- tightly binds essentially any 2+, 3+, or 4+ ion in the periodic table and is a very useful ligand for both analytical applications and separations.
The macrocyclic effect follows the same principle as the chelate effect, but the effect is further enhanced by the cyclic conformation of the ligand. Macrocyclic ligands are not only multi-dentate, but because they are covalently constrained to their cyclic form, they also allow less conformational freedom. The ligand is said to be "pre-organized" for binding, and there is little entropy penalty for the ligand to wrap around the metal ion. For example heme b is a tetradentate cyclic ligand which strongly complexes transition metal ions, including (in biological systems) Fe+2. 2ff7e9595c
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