Why is the acetate ion considered a monodentate ligand as opposed to a bidentate ligand considering both oxygen atoms can donate their respective lone pair of electrons?

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    $\begingroup$ There is a delocalization of the negative charge over the two Oxygen atoms of the carboxylate group $\ce{-COO^-}$. These Oxygen atoms are not independent from one another. They behave together as a mono dentate ligand. $\endgroup$ – Maurice May 25 at 9:32
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    $\begingroup$ Acetate ion is considered a monodentate ligand because it is a monodentate ligand (except when it isn't). Then come the explanations. $\endgroup$ – Ivan Neretin May 25 at 10:43

This depends a lot on the compound in question.

In general, a bidentate acetate would lead to a four-membered ring with the central metal; since the $\ce{O-C-O}$ angle is relatively fixed at approximately $120^\circ$ and since we expect the resulting compound to be at least somewhat symmetric, it would typically lead to unusually small bond angles or a $\ce{C\bond{...}M}$ distance that is too short. If, for example, we imagine $\angle(\ce{O-M-O})=90^\circ$, that means each $\ce{M-O-C}$ angle would be $75^\circ$ which is not what we usually observe in such compounds without a significant force holding it together. Coordinate bonds are usually not very strong, do not liberate enough energy and it would thus be more favourable for one of the two $\ce{O-M}$ bonds to be broken leading to acetate as a monodentate ligand.

There are, however, exceptions. My favourite is probably $\ce{[Cr2(OAc)4(H2O)2]}$ or chromium(II) acetate monohydrate. In this compound, four acetate ions arrange around two chromium atoms in a bidentate fashion; however, each individual acetate bonds to two different chromium ions via its two oxygens. In addition, a water molecule sits on each end of the $\ce{Cr-Cr}$ axis and it turns out the bond between the two chromium ions is, in fact, a quadruple bond. The quadruple bond permits the chromium atoms to come close enough to each other so that the acetates can arrange in a preferable configuration.

chromium(II) acetate monohydrate
(Image taken from Wikimedia, where a full list of authors is available)

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    $\begingroup$ My favorite is uranium. With it, acetate works as a bidentate ligand to one atom. $\endgroup$ – Ivan Neretin May 25 at 10:40
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    $\begingroup$ @IvanNeretin Ah, but then it can’t compete with a quadruple bond, can it? ;) $\endgroup$ – Jan May 25 at 10:41
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    $\begingroup$ Yeah, that's a rare gem indeed. $\endgroup$ – Ivan Neretin May 25 at 10:41
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    $\begingroup$ Not the same thing, but fun to see, are basic acetates of zinc and beryllium. Six acetate ligands bond to all the edges of a metal tetrahedron with a four-coordinate oxygen in the center. $\endgroup$ – Oscar Lanzi May 25 at 13:33
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    $\begingroup$ @BuckThorn You’re raising a good point, namely whether denticity refers to bonds made to a single atom or bonds made to any dative bond acceptor. I understood it to be the latter, but the definitions I can find in two seconds of Googling are not clear enough to exclude either meaning. That said, I prefer chromium(II) over the other metals because it features a 3d metal in a quadruple bond which never ceases to fascinate me. $\endgroup$ – Jan May 27 at 3:22

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