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You can tell if a ligand is polydentate if it has 2 or more binding sites. A great example of this is ethylenediamine which has two binding sites at the Nitrogens, it is considered to be bidentate! Another example is diethylenetriamine which has three binding sites as the molecule has three nitrogen binding sites! Attached below is images of Ethylenediamine and Diethylenetriamine (imagine the lone pairs on diethylenetriamine as the image did not include them). I hope this helps!
- Diethylenetriamine.png (1.28 KiB) Viewed 30 times
- Ethylenediamine.jpg (16.72 KiB) Viewed 30 times
A great way to find out if it is polydentate is by drawing the lewis structure of the molecule and then filling in the bonds and the free pairs. Once you have drawn them correctly you can easily find where it can bond due to the free pair electrons and if it is more than 2 then it is polydentate.
Since a ligand bonds in a chelating complex by donating a lone pair, look for several atoms with one pairs within the same molecule; two lone pairs on the same atom can't bond to the metal atom (lone pair e- repulsion) but otherwise two different Nitrogens with lone pairs, for example, in the same molecule could show bidentate nature, or two places to bond as ligands, in that compound. It could help to draw the Lewis structures out, and/or calculate formal charges on atoms like nitrogen to determine is there is a lone pair to be donated.
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