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I'm getting a little confused about the definition of chelation and its implications (I should probably point out I'm only an A2 Chemistry student). The IUPAC defines chelation as the following:

The formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom

Does this mean that any ligand that is not monodentate (e.g. even ethanedioate) is an example of chelation? I thought chelation was only for complexes with a large coordination number (e.g. $\ce{EDTA^4-}$), it would seem odd to call a single metal ion with 1 ethanedioate ligand chelated?

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    $\begingroup$ You dare doubt in IUPAC definitions? ;) It's just like it says. $\endgroup$ – Mithoron May 31 '15 at 16:08
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I think you should not focus on the ligand to define chelation. You should instead focus on the complex that is formed. In fact, the very same multidentate ligand may participate in binding in more than one way:

  • by forming more than one bond with the same central atom, therefore forming a chelated complex;
  • by forming only one bond with that central ion, even though it could form two; this may be the case when a vast excess of this ligand is present with respect to the metal ion;
  • by bridging between two adjacent central ions.

Only the first of these possibility is referred to as chelation. In other words, forming a chelated complex doesn't depend solely on the fact that the ligand is multidentate. Being multidentate is a necessary, but not sufficient condition for a ligand to form chelated complexes.

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  • $\begingroup$ Since this is the accepted answer, I'll comment here. There are some single molecules such dimethylglyoxime which ligate fairly specifically with one cation ($\ce{Ni^{2+}}$ in this case) because the size of the "hole" in the molecule into which the metallic ion must fit has a very specific size. $\endgroup$ – MaxW Nov 6 '15 at 16:12
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This is what my textbook says:

All polydentate ligands are the example of chelating ligands.

However, it must be noted that $\ce{NH2NH2}$ and $\ce{N(CH2CH2)N}$ cannot act as chelating ligands due to the formation of a three membered ring and locked structure respectively.

(Also, if you're looking for a little extra information - it isn't essential that a polydentate ligand will always give a chelating compound. If it is a flexidentate ligand (i.e. it can show variable denticities) and can hence be monodentate as well a polydentate, then it will obviously not display chelation in its monodentate state.)

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    $\begingroup$ Could you also mention which textbook you're talking about? $\endgroup$ – M.A.R. Oct 25 '15 at 17:52
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    $\begingroup$ This answer seems contradictory. You state that All polydentate ligands are the example of chelating ligands and then go on to say that $\ce{H2NCH2CH2NH2}$, a bidentate ligand, cannot act as a chelating ligand. Please can you clarify your answer. $\endgroup$ – bon Oct 25 '15 at 17:53
  • $\begingroup$ @bon i think the bidentate ligands are exceptions.. $\endgroup$ – TanMath Oct 25 '15 at 23:10
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    $\begingroup$ So the statement All polydentate ligands are the example of chelating ligands is wrong. $\endgroup$ – bon Oct 26 '15 at 10:04
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Chelation is simply foratiom of a complex by a ligand which has two or more then two sites within a molecule,the word is derived from crabs claw as it grips the central metal atom and all polydentate ligands can act as chelating agents...

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