Metal Chelate Complexes - Online Tutor, Practice Problems & Exam Prep

Metal Chelate Complexes

So here we're going to talk about the different complexes that a metal and a ligand can form once they are combined. First, we're going to say that a ligand can be thought of as a Lewis base because it bonds to a Central Metal Cation in a Complex Ion using its lone pair. When you talked about General Lewis Acid-Base Theories in General Chemistry, remember the Lewis Base was the Electron Pair Donor and the Lewis Acid was the electron pair acceptor. A common example that we would see is, for example, a metal ion reacting with a compound or element with lone pairs.

Here the wall would serve as the Lewis base or the ligand and attach itself to the Lewis acid, which is the calcium ion in this example. Together, you would add them. And when you add them to create a product, we call that product an adduct because you added them together. Now typically, these Lewis acids or metal ions form 6 connections with ligands. Now we're going to say here, ligands can be characterized by the number of elements in the molecule that can donate a lone pair.

There's a different number of ligand atoms within any given compound that can donate a lone pair. We're going to say that they are referred to as chelating compounds or agents because they use their lone pairs to grab onto metal cations. And in fact, chela just refers to as Greek for crab claw because they kind of form a vice grip around the metal ion itself. Now, first, we can talk about ligands that have only one ligand atom within them to donate a lone pair. We're going to say ligands that possess only one element able to donate a lone pair referred to as monodentate ligands.

Monodentate just means one tooth. Common examples here we have are water, X- here just represents a typical halogen like your chloride ion or your fluoride ion or your bromide ion. Next, we have our cyanide ion. Now, your cyanide ion, technically, there's a lone pair on the carbon and on the nitrogen.

But it is the carbon itself that acts as the ligand atom. It is the one using its lone pair to connect to a metal ion. It is the one with the negative charge that gives the whole compound a negative charge overall. Next, we have our hydroxide ion where the oxygen serves at donating a lone pair to connect to a metal ion. We have ammonia here.

Next, we have our thiocyanate ion. We're going to say here technically, it can be either sulfur or nitrogen that donates a lone pair to form a connection with a metal ion, but it's only one or the other, not both at the same time. That's why it's still characterized as a monodentate ligand. Then finally here we have our nitrite ion. And in example, it's the oxygen making a single bond that can donate a lone pair to make that connection.

Here with our nitrite ion, it has resonance involved. We could also draw it this way. In this case, it would be this oxygen here that's making the single bond now that can form a connection with the metal ion. Just remember, we have our ligand which serves as a Lewis base where it donates a lone pair to a Lewis acid which in this case is a metal ion. Now, click on to the next video and see as we go deeper into the different types of ligands where we can donate more than just one ligand atom.

Metal Chelate Complexes

Now we take a look at ligands that possess more than one atom that can donate a lone pair. Here, we say ligands that possess two elements able to donate a lone pair are referred to as bidentate ligands. By bidentate, we mean "two-tooth". It has two atoms within the structure that can each donate a lone pair to a metal ion. Here, we are going to have the two most common examples of bidentate ligands as our oxalate ion.

And then here, this one goes by two different names. We can call it ethylenediamine. By ethylene, ethylene just refers to two CH₂ groups connected to one another. Amine just refers to the fact that each one is connected to a nitrogen. Nitrogen is in group 5A, so traditionally wants to make three bonds.

Each nitrogen right now is making a bond to one carbon. They both still need two more bonds to get to that sweet spot of three bonds. To get to three bonds, they each have to connect to two hydrogens. That's why here in this example, our amines are NH₂s. We're going to say that bidentate and polydentate ligands usually form more stable complexes with metal ions.

That's because when bidentate ligands and polydentate ligands connect with the metal ion, they form cyclic structures. This increase in stability is called our chelate effect. Now, for example here, if we had a metal ion, commonly shown as cadmium here, we could have cadmium forming a connection with ethylenediamine. Here, we'd have this nitrogen using a lone pair to connect to the cadmium. And it's still connected to its two hydrogens.

And then that's connected to a CH₂ which is connected to another CH₂. And then the other nitrogen forms a connection. Again, the nitrogens are using their lone pairs to connect to the cadmium. Typically, we're going to say that metal ions want to form six connections with ligands. We could have three of these ethylenediamines forming cyclic structures around this cadmium ion.

Here, we'd have another nitrogen forming a connection. And then, finally, this last one here. So here we'd have one of the Ethylenediamine ligands making a cyclic structure.

Then we'd have the second one here making a cyclic structure. And then finally this last one here which makes another cyclic structure. Again, bidentate ligands and polydentate ligands do the chelating effect or chelate effect for more stable complex ion structures like this because they can make cyclic structures as a result. Monodentate ligands can't do this because there's only one atom involved in donating a lone pair so they cannot make these cyclic structures. Also remember, when it comes to the metal ions, they typically want to form six bonds when connecting to ligands.

That helps to make a stable structure for them. Now that we've seen this, continue onward to the last video where we take a look at polydentate ligands.

Metal Chelate Complexes

Here we're going to say that ligands that possess more than 2 elements that are able to donate a lone pair referred to as our polydentate ligands. Here we have the most common of the polydentate ligands. Remember, polydentate in this case means many tooth. Here we have first our triphosphate ion. In this one, although there are 5 negative charges, it is these 3 oxygens here that are able to form the ligand connection with our metal ion.

Next, we have diethylenetriamine. Diethylene refers to our 2 CH₂ groups and then triamine refers to our 3 amine groups. Remember, nitrogen wants to make 3 bonds ideally, so the ones on the ends are making one bond apiece to carbons. To complete their 3 bonds needed, they each form 2 more bonds to hydrogens. The nitrogen in the center though is making 2 bonds to carbon, so it only needs one more so that's why it's NH.

Out of the polydentate ligands, EDTA or Ethylenediaminetetraacetate ion is the most common one that we're going to refer to. This one has numerous forms and it's all based around the fact of the oxygens either having hydrogen or not. When they don't possess a hydrogen, they'll be negatively charged and when they do, they'll be neutral. It's also based on the nitrogens possessing hydrogens or not. There are numerous forms of EDTA that we'll be talking about.

This is just one of the forms that we're seeing here. And then we have these lesser known ones here, DCTA, NTA, and EGTA. For the most part, we're going to say that the ratio of ligand to metal is 1 to 1. The only one that is not a 1 to 1 ratio with the ligand and the metal is NTA. NTA actually does a 2 to 1 ratio.

So that means that we'd have 2 ligands for every one metal. And the effect that this has is in terms of stoichiometry. So you would say that you have 2 NaNTA for every one metal. For example, we could talk about calcium ion. For all the other ones, we'd have a ratio of 1 to 1.

For EDTA, we could say that we have 1 EDTA reacting with 1 metal ion. Again, remember, normally the ligand to metal ratio is 1 to 1. The only exception is NTA. NTA has a 2 to 1 ratio instead. Just remember, ligands are just really Lewis bases.

They use their lone pairs to form a connection with a Lewis acid which is in these examples typically a metal ion. Keep this in mind as we delve deeper and deeper into different types of complexes that can form, the different types of titrations as well as calculations dealing with concentration of ions.