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Glycinate as far as I can guess from the structure is a monoanion with negative charge on oxygen atom (though that negative charge is resonating). But when I used this logic in my exam it costed me 4 marks. I don't understand why.

Is it because it is derived from glycine in strongly basic medium which is not a monoanion but a zwitterion? Still, it doesn't seem to be making sense.

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  • $\begingroup$ Solid glycine is in zwitterionic form. That's why it is hard to dissolve. That's probably what the question asked. Can you elaborate a bit? $\endgroup$ Mar 25, 2021 at 14:42

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,Assuming glycinate is in its anionic form ($\ce{H2N-CH2-CO2^-}$), one thing to consider is it may not act just as a simple anionic ligand like chloride or hydroxide. You can count off four atoms from the nitrogen to either carboxyl-ate oxygen and those atoms both have available electron pairs. So you add the metal atom and you get a five-membered ring similar to that formed with ethylenediamine. Thus, glycinate anion may act as a chelating agent.

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In solutions more basic than pH 9.6 glycine mainly exists as the carboxylate anion you describe. Between 9.6 and 2.34 it exists mainly in the zwitterion form.

titration curve

titration curve from here

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    $\begingroup$ So, what exactly do I consider glycinate ligand to be-mono anion or zwitterion, from an exam point of view ? $\endgroup$ Mar 25, 2021 at 11:54
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    $\begingroup$ It entirely depends on the pH of the solution it is in. What exactly does the question say? $\endgroup$
    – Waylander
    Mar 25, 2021 at 12:22
  • $\begingroup$ You may find more info here. $\endgroup$ Mar 25, 2021 at 14:31

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