How to Determine if a Ligand is polydentate?

[Arieanne De Guzman 2J]

Hi, I was wondering how would you know if a ligand is polydentate through looking at the formula of the coordination compound?

[AnjikaFriedman-Jha2D]

I believe there should be a prefix before the name of the ligand that would indicate the number of ligands present. If there is a bis-, tris etc prefix, this means the ligand is polydentate or the name of the ligand already includes a greek prefix

[Thomas Vu 1A]

When given a formula, you would have to draw out the structure of the formula of the ligands or recall from memory some of the polydendates on Dr Lavelles ligand sheet such as (en). If you draw out the structure of the polydendate ligand, you would find multiple regions of lone electron pairs on different atoms which allows this ligand to make multiple bonds to the central metal atom.

[tamara masri_3D]

Hello! Going off of what Thomas said, when you draw the lewis structure of the compound and see that there are multiple atoms that can potentially donate their lone pairs to be ligands, you need to remember that e-e repulsion still plays a huge role. For example, if you had an O atom that has 2 lone pairs of electrons, the O cannot be bidentate for the same reason why molecules with 4 regions of electron density are tetrahedral-shaped— the electrons (especially lone pairs) want to be as far away from each other as possible. That is why Professor Lavelle spent some time talking about the "formula" of a polydentate ligand where there is an electron donor, spacer, spacer, electron donor. The bond angles in many cases cannot physically be small enough without the spacers to attach to the metal, even if all the atoms involved have lone pairs they want to donate. Hope this helps!

[Arieanne De Guzman 2J]

Hi! Thank you for your further clarification on ligands. Going off of H2O, because the two lone pairs are on the same atom (O), electron-electron repulsion causes H2O to be monodentate instead of bidentate because it can technically only have one binding site? Does this also imply that for a ligand to be bidentate there would have to be two donor atoms that each donate their electron pair?

Hello! Going off of what Thomas said, when you draw the lewis structure of the compound and see that there are multiple atoms that can potentially donate their lone pairs to be ligands, you need to remember that e-e repulsion still plays a huge role. For example, if you had an O atom that has 2 lone pairs of electrons, the O cannot be bidentate for the same reason why molecules with 4 regions of electron density are tetrahedral-shaped— the electrons (especially lone pairs) want to be as far away from each other as possible. That is why Professor Lavelle spent some time talking about the "formula" of a polydentate ligand where there is an electron donor, spacer, spacer, electron donor. The bond angles in many cases cannot physically be small enough without the spacers to attach to the metal, even if all the atoms involved have lone pairs they want to donate. Hope this helps!

[tamara masri_3D]

Hey! Sorry for the late reply but from what I understood, yes, though I think there are some specific exceptions mentioned in lecture that I think Dr. Lavelle said we don't have to have memorized.

Hi! Thank you for your further clarification on ligands. Going off of H2O, because the two lone pairs are on the same atom (O), electron-electron repulsion causes H2O to be monodentate instead of bidentate because it can technically only have one binding site? Does this also imply that for a ligand to be bidentate there would have to be two donor atoms that each donate their electron pair?

Hello! Going off of what Thomas said, when you draw the lewis structure of the compound and see that there are multiple atoms that can potentially donate their lone pairs to be ligands, you need to remember that e-e repulsion still plays a huge role. For example, if you had an O atom that has 2 lone pairs of electrons, the O cannot be bidentate for the same reason why molecules with 4 regions of electron density are tetrahedral-shaped— the electrons (especially lone pairs) want to be as far away from each other as possible. That is why Professor Lavelle spent some time talking about the "formula" of a polydentate ligand where there is an electron donor, spacer, spacer, electron donor. The bond angles in many cases cannot physically be small enough without the spacers to attach to the metal, even if all the atoms involved have lone pairs they want to donate. Hope this helps!

[Arnav Saud 2C]

I believe that you can recognize it by finding how many lewis base sites are present in the molecule (having multiple lone pair donating sites that are used to bond to a central atom). An example is EDTA, a hexadentate ligand, is an example of a polydentate ligand. EDTA has six donor atoms with electron pairs that can be used to bond to a central metal atom or ion.Another example is EN, which is bidentate and has 2 donor atoms.