Classification of Ligands - Online Tutor, Practice Problems & Exam Prep

• Ligands are classified based on the number of donor atoms they possess: monodentate (1 donor atom), bidentate (2 donor atoms), and polydentate (more than 2 donor atoms).
• Donor atoms in ligands must be separated by at least two atoms, and atoms with a negative charge are considered donor atoms.
• Examples of monodentate ligands include hydroxide, halogens, ammonia, water, cyanide, and carbon monoxide.
• Bidentate ligands, such as catecholate and oxalate, have two donor atoms separated by two atoms.
• Polydentate ligands, like diethyl diethylenetriamine and EDTA, have more than two donor atoms; EDTA has six donor atoms and is commonly used as a food preservative.
• Bidentate and polydentate ligands are known as chelating agents because they form ring structures with the central metal in a complex ion, enhancing stability.
• Chelating agents help form more stable complex ions compared to monodentate ligands, with 5 or 6 membered rings being ideal for stability.

Ligand Classification

In this video we're going to take a more in-depth look at different types of ligands or ligands. Now we're calling to represent Lewis bases because they have at least one lone pair that can be donated, and when it comes to their classification, they're classified by the number of donor atoms that can donate 1 lone pair to the central metal. When we talk about ligands or ligand classifications, we have monodentate, bidentate and polydentate.

In a monodentate dentate ligand, we can donate only one lone pair from 1 donor atom. In a bidentate, it's two donor atoms and Poly, it's going to be more than two. Now before we take a look at the list, let's just understand how do we find the number of donor atoms. Now we're going to say two or more atoms with lone pairs. OK, to find the number of donor atoms, 2 or more atoms with lone pairs, rule one is donor atoms must be separated by two or more atoms, And rule 2 atoms with a negative charge equal the donor atom.

All right, so let's come up here and see for the monodentate ligands or ligands. It's pretty easy to see them because there's only one structure that can donate a lone pair. For hydroxide. O has a negative chart, so it represents our donor atom. X represents A halogen. It's negatively charged. So here it's the donor atom and ammonia only Nitrogen has a lone pair, so it's a donor atom. In water, it donates one of these lone pairs for Sinai, the negative charges on the carbon in terms of formal charge, so it is the donor atom.

Then we're going to say carbon monoxide here. Remember, carbon ideally wants to make four bonds, and carbon monoxide, it's only making three bonds, so it would love to share this lone pair in order to make that 4th bond. So it is the donor atom. Next we have here catecholate. In it, the oxygens are just making one bond, so they're formal charges are -1 we would say here. Remember, donor atoms must be separated by two or more atoms. These two negatively charged oxygens are separated from each other by 1-2 atoms, so both of them can act as donor atoms.

We have two donor atoms here, oxalate here. This oxygen and this oxygen are negative in terms of formal charge. So they're the two donor atoms and they're separated by two atoms from each other. Next we have ethylene diamine, it's abbreviation as EN here. It's the nitrogens that have lone pairs. They're the ones that can act as the donor atoms. They're also separated from each other by two carbons, 2 atoms now polydentate.

We usually don't see very much of this in Gen. Cam. For those of you who take higher advanced more chemistry such as organic chemistry or analytical chemistry or even inorganic chemistry, you'll come into more of a all view of these types of ligands for here. Just remember they're the ones that have more than two donor atoms. A good example here is diethyl diethylene triamine. So we'd have one 2-3 donor atoms and then finally we have EDTA, which is ethylene diamine Tetra acetate.

Now here, this one actually has six donor atoms within it. You're not responsible in knowing its structure. Just realize that it's the most famous of all the polydentate ligands, or ligands that exist. If you look on different types of food packaging, you'll see EDTA as part of the ingredients. It works as a preservation agent for a lot of types of foods. It's pretty common in a lot of things. OK, so just remember, EDTA is just a common type of polydentate. You don't need to know the structure. It's polydentated because it has more than two donor atoms, in this case 6. Alright, so keep this in mind. We're looking at different types of ligands.

Classification of Ligands Example

Here it says classify the following anionic ligands as monodentate, bidentate and polydentate. For the first one here, this is called our armide ion. It's nitrogen. Here it's only making two bonds but it's negatively charged. That happens when nitrogen has 2 lone pairs. It is the only structure with lone pairs, so there's only one donor atom here, so this would be monodentate.

For the next one here. This structure we have an oxygen that's double bonded, so it would have two. It would have two lone pairs on it. Remember when oxygen is single bonded, it has three lone pairs, one of which could be a donor element. And remember oxygen? When it's single bonded, it has a -, 1 formal charge. That's what gives us the overall -1 charge here.

So this oxygen at single bonded is one of the donor atoms, but it's not the only one. Nitrogen here is making three bonds. It's making one to the carbon and two more to the two hydrogen. So that's three bonds. So it has a lone pair that we don't see. It also serves as a donor atom, so that's two donor atoms.

Now what about this oxygen down here that's double bonded? Well, for us to have it act as a donor atom, it needs to be two or more atoms away from the single bonded oxygen. It's not. It's only one carbon away, so it can't be counted as a donor atom. So in this structure, we'd say that we have two donor atoms. So this would be a bidentate ligand, right. So this will be our answer for each of them. So we'd have monodentate and bidentate as our answers.

Chelating Agents

Here we're going to talk about chelating agents. Now bidentate and polydentate ligands, or ligands are considered chelating agents. Now a chelating agent, they create ring structures in a complex ion when donor atoms bond to the central atom, a metal which we're going to call M.

So here what do we have? We have this is monodentate, this is monodentate and this here is our bidentate ligand or ligand attached. Those two nitrogens each had a lone pair which they used to connect to the metal and as we can see we created a ring on the left side. This here would represent its overall complex ion structure formula.

Now here we're going to say complex formed with chelating agents are more stable than with monodentate ligands. And when we talk about stability of these rings, ideally we're trying to make five or six membered rings if we were to count this out with C 123455. And six membered rings are pretty common when it comes to these bidentate and polydentated ligands or ligands, right. So just keep that in mind when we're looking at different types of attachments that occur when they connect to our central metal.

Classification of Ligands Example

Chelating agents can be used to remove toxic heavy metals from the human body by forming stable complex ions with the metal. Identify a ligand or ligand that is capable of removing excess lead. So remember we learned earlier that bidentate and polydentate ligands, or ligands are the best type of ligands to use as chelating agents.

If we take a look here, C is out because C is monodentated because only sulfa here can donate its lone pairs at that time. If we take a look at the others, we're going to say here that this oxygen is negative and this oxygen here is negative. But we can't count both of them as helping to be bidentated because they have to be separated by two or more atoms from one another. They're only separated by 1 carbon, so only one of them would actually act as a donor atom at a time. So this is out.

A is the answer because an A we have this oxygen single bonded, so its charge is -1. Same with this sulfur and same with this oxygen and same with this sulfur. Each of these negatively charged or -1 formal charged atoms is at least two atoms away from the next one, so they each connect as a donor atom. In this case we have 4 donor atoms within this structure so it'd be polydentate so it could form or act as a chelating agent.

So out of all the options, only option A is the correct choice.

How many donor atoms are in each ligand?

• A. a) 2
  b) 2
  c) 3
• B. a) 1
  b) 1
  c) 2
• C. a) 2
  b) 1
  c) 3
• D. a) 1
  b) 2
  c) 4