Complex Ions, Ligands, & Coordination Compounds, Basic Introduction Chemistry In this video, we're going to talk about complex ions. But first, what is a complex ions? A complex ion is the combination of a transition metal cation with a ligand molecule or a ligand ion. So let's use an example. Here we have the silver cation. It has a charge of plus 1. And we're going to react to it with two ligand molecules, in this case two ammonia molecules. In this case, the ligand is not an ion. It's a neutral molecule. And it's going to form this particular complex ion-- the diamine silver cation. Now, what is the coordination number of that complex ion? The coordination number is 2, because it's attached to two ligand molecules. So this is the ion or the metal ion, this is the ligand, and here we have the complex ion. Now, for complex ions that have a coordination number of 2, the geometry will be linear, as you can see from this particular drawing. And so, that's a basic introduction into complex ions. So now, let's look at another example. the hexacyanoferrate ion. Identify the ligand and also determine the coordination number and the oxidation state of Fe. The ligand in this example, it's not a molecule, but it's a polyatomic ion. In this case, it's cyanide, which has a Lewis structure that looks like this. The carbon bears the negative charge or the negative formal charge. Now, the coordination number is basically the number of ligand ions attached to the transition metal cation. So the coordination number in this example is 6. Now, how can we determine the oxidation state of Fe. Would you say it's positive 3, negative 3, positive 4, negative 2? What is it? Well, let's write a reaction. So we know that Fe probably has some charge, which we'll call x. And it's going to react with 6 cyanide ions to produce the hexacyanoferrate ion. Almost lost the word there. So let's write an equation, where the total charge will remain balanced. So Fe has a charge of x. Cyanide has a charge of negative 1, but there's 6 of them. And the product complex or the complex ion has a net charge of negative 3. So x minus 6 is equal to negative 3. So if we add 6 to both sides, we can see that x is negative 3 plus 6 or plus 3. So this is the oxidation state of Fe. So Fe he has a positive 3 charge. Now, there was a guy called Alfred Werner, and he came up with a theory for coordination compounds and transition metal ions. And he basically stated that transition metal ions have two types of valence, and that is that the way that these metal cations can combine with other things like ligands and anions. And the two types of valence that these metal ions can have, the first one is the primary valence, and the second one is the secondary valence. Perhaps you've heard of these terms. And in the case of the Fe 3+ ion as it relates to this particular complex ion, what do you think the primary valence and the secondary valence of this transition metal cation is? The primary violence has to do with its oxidation state and thus the way it forms ionic bonds. The secondary valence has to do with the coordination number, And. So, it relates to how it interacts with ligands. And those ligands could be ions or molecules. So in this case, the primary violence is positive 3, because that's the oxidation state of Fe. And the secondary valence is 6, because the coordination number is 6. It could be attached to six ligands. Now what, type of geometry does the Fe CN 3 minus island have? If the coordination number is 6, the geometry will be octahedral. And so, it's going to look like this. And so, that's how you can draw this particular complex ion. And if you want to show the octahedron structure, you could. You can Can make it look like this. So that's the octahedral structure of this particular complex ion. Now, if the coordination number is 4, then there's two types of geometries that can occur. One example is the tetrahedral geometry, another one is the square planar geometry. So the tetrahedral geometry looks something like this. Or it can have a square planar of geometry, which looks like this. So if you have a coordination number 4, the geometry could be any one of those two. So keep that in mind. Now, let's talk about the coordination numbers for certain transition metal ions. Let's see if we have a transition metal ion with a charge of plus 1, such as copper plus or Ag plus. The coordination of copper plus 1 is 2 and 5. For silver, it's only 2. And so, if the charge is low, the coordination number is relatively low. But notice what happens if we increase the charge, let's say, to plus 2. So cobalt, for example-- the cobalt 2+ ion-- can have a coordination number of 4 or 6. Nickel 2+ is the same. Fe 2+ can be up to 6. And zinc 2+ can be 4 or 6. Now, let's look at the plus 3 ion. One example is gold 3+, which has a coordination of 4. Chromium 3+ has a coordination number of 6. And cobalt 3+ has a coordination number of 6. So notice the general trend. As the charge on the transition metal ion increases, the coordination number generally increases. And it makes sense. Because if a metal ion has a very high charge, it makes sense that you can bind to more ligands. Now, you need to be familiar with the different types of ligands. The first type is called a unidentate, also known as a monodentate ligand. So uni and motto is associated with the number 1. So this tells us that the ligand can only make 1 interaction with a metal ion. Some good examples of these types of ligands are water, ammonia, chloride, cyanide, thiocyanate, just to name a few. So in the case of ammonia, it has 1 lump here. And so, it can only form one interaction per molecule with a transition metal ion. A bidentate ligand can have two interactions per molecule. Bi is associated with the number 2. So one example is the oxalate ion. It has two oxygen atoms with a negative charge. And let's use a zinc metal kkyne. So this is one interaction, and that's another. So there's two way interactions per molecule, which makes it a bidentate ligand. Another example is ethylenediamine, which you can abbreviate as en. So this particular ligand can also make two interactions. Each nitrogen atom has one lump here which cand bind with a transition metal kyne. So this is one interaction, and this is the second interaction. So that makes ethylenediamine a bidentate ligand. So those are known as chelating ligands. A chelating ligand can interact with a metal ion in more than one way. So bidentate ligands are chelating ligands. Now, let's talk about coordination compounds. And here is an example. A coordination compound is neutral overall, but it consists of a complex ion and a counter ion. What do you think is the complex ion in this example? Well, you can clearly see it. It's in brackets in this particular example. And so it's Cr NH3 6. Now, what do you think the charge or the oxidation state of chromium is in this example? Now, we know that cyanide has to be the counter ion. It's there to balance the charge on chromium. And there's three of them, which indicates that chromium has to have a plus 3 charge. So this particular complex island has a positive 3 charge. So this is the complex ion, and in this example this is known as the counter ion. The counter ion that we have in this example is negatively charged. So it's called an anion. But it doesn't always have to be an anion. It can be a cation. In this example, the complex is the cation, and so the counter ion has to be an anion. But sometimes the roles can be reversed, like in this example-- K3 Fe CN 6. So this is the cation, and this has to be the anion. So this time, the anion is the complex ion, Fe CN 6 with a negative 3 charge. And the cation is the counter ion, which is used to neutralize the negative 3 charge of the complex ion. And so we have three of them. And so, the coordination compound is always neutral. The net charge has to be 0. So that's it for this video. Hopefully it gave you a basic introduction into complex ions, ligands, and coordination compounds. Thanks for watching.