mono, bi, tridentate

[isha dis3d]

Is whether a ligand is mono, bi, or tri dentate dependent on how many lone pairs it has?

[Sophia Hu 1A]

It is dependent on the number of lone pairs a ligand has as well as how many of the lone pairs it has that will be used in binding.

For instance, Cl- has 4 lone pairs, but only one of the lone pairs participates in bonding with a transition metal cation. The reason for this is because the four regions of electron density have a tetrahedral arrangement, so they are about 109.5 degrees apart. Therefore, only one of the lone pairs will participate in bonding to a single transition metal cation, so it is monodentate.

Another example is, CN- which has 2 lone pairs: one on the C and one on the N. However, the molecule is linear so the lone pairs are 180 degrees apart. Therefore, a single transition metal cation can only form one coordinate covalent bond with CN-, so it is monodentate.

Ethylenediamine (en) has 2 lone pairs on the Nitrogen, and because it has the geometry of sigma bonds as well as lone pair-spacer-spacer-lone pair, the molecule is able to rotate and the 2 lone pairs on the Nitrogen can bond to a single transition metal cation, and therefore it is bidentate.

So overall, it does depend on the number of lone pairs, but also the number of lone pairs that actually form the coordinate covalent bond with the transition metal cation.

I hope this helps!

[Brittney Nguyen 2L]

To add onto what Sophia said, kind of.

If a ligand has multiple lone pairs, it could donate these pairs to a TM as long as the ligand has single bonds that allow the atoms to orient their lone pairs towards the ligand, or as long as the ligand's lone pairs are naturally close enough together to create multiple bonds with the TM.

For example, CN- has 2 lone pairs available for bonding, but it is only monodentate because the lone pairs on the C and the N point away from each other. The triple bond connecting the C and N does not allow the lone pairs to orient themselves towards a shared TM

On the other hand, EDTA 4- has 6 lone pairs available for bonding, and it is hexadentate because the single bonds that connect the molecule allow the O and N with the lone pairs to rotate towards the TM.

Hope this helps!

Edit: Typo