4
$\begingroup$

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?

$\endgroup$
2
  • 1
    $\begingroup$ 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 '20 at 9:32
  • 1
    $\begingroup$ Acetate ion is considered a monodentate ligand because it is a monodentate ligand (except when it isn't). Then come the explanations. $\endgroup$ May 25 '20 at 10:43
8
$\begingroup$

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)

$\endgroup$
6
  • 2
    $\begingroup$ My favorite is uranium. With it, acetate works as a bidentate ligand to one atom. $\endgroup$ May 25 '20 at 10:40
  • 1
    $\begingroup$ @IvanNeretin Ah, but then it can’t compete with a quadruple bond, can it? ;) $\endgroup$
    – Jan
    May 25 '20 at 10:41
  • 1
    $\begingroup$ Yeah, that's a rare gem indeed. $\endgroup$ May 25 '20 at 10:41
  • 3
    $\begingroup$ 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. $\endgroup$ May 25 '20 at 13:33
  • 1
    $\begingroup$ @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 '20 at 3:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.