 In the previous video, we talked about coordination entity, central metal atom or iron and the different types of ligands. So let's continue discussing a few more terms related to coordination compounds in this video. Let's begin by talking about coordination number. Coordination number of a metal iron can be defined as the number of ligand donor atoms to which the metal is directly bonded. It is enclosed within the square brackets. The coordination number of nickel in this particular complex is 4. Nickel is bonded to 4 ammonia molecules. Similarly, the coordination number of platinum in this particular complex is 6. Platinum is bonded to 6 chloride ions, right? And in complexes that have bidentate ligands such as oxalate or ethylene diamond ligands, the coordination number of iron and cobalt in these complexes are 6 and 6. We know that both oxalate and ethylene diamond are bidentate ligands, right? That means they bind to the central metal atom through two donor atoms. Now remember that the coordination number of a central metal is determined only by the number of sigma bonds. Yes, pi bonds between the ligand and the metal are not considered or included in the coordination number. Let's now talk about counterions. Counterions are nothing but the ionizable groups that are written outside the square brackets which essentially neutralize or balance the charge of the entire complex. For example, if you have this particular coordination complex, what is the counterion here? Well, the counterion in this case would be potassium that is K plus. The charge on this complex is Fe Cn 6 4 minus. So in order to neutralize this charge, you will need four potassium ions. Now it's fairly easy to identify counterions because they're always present outside the square brackets and when you dissolve this complex in water, you will see that the counter ions or in this particular case the potassium ions will be ionized. So what you will have is essentially Fe Cn 6 4 minus which is a non ionizable group and 4 K plus. Next we are going to discuss a very important term which is coordination polyhedra. Coordination polyhedron is nothing but the spatial arrangement of the ligand atoms that are directly attached to the central metal atom or ion. Now the most common coordination polyhedra are octahedral, square planar and tetrahedral in structure. For example, Co Ns 3 6 3 plus. Here we have cobalt bonded with six ammonia molecules and here the coordination polyhedra would be octahedral. Now PtCl 4 and NiCl 4. This can be confusing because both of them have coordination number 4, right? Platinum is associated or bonded to 4 chloride ions and nickel is bonded to 4 Co groups. But here platinum or PtCl 4 2 minus acquires a square planar structure but NiCl 4 is tetrahedral in structure. But the coordination polyhedra of NiCl 4 is tetrahedral. Now why does platinum take up a square planar structure and nickel take up a tetrahedral structure? Well you will get the answer to that when we talk about bonding in coordination compounds. But remember these are the most common polyhedra. We also have shapes corresponding to other coordination polyhedra like trigonal bipyramidal and square pyramidal. Let's now talk about oxidation number. Oxidation number of the central metal atom in a complex can be defined as the charge it would carry if all the ligands are removed along with their electron pairs that are shared with the central metal atom. For example, the oxidation number of copper in this particular complex would be let's assume X is the oxidation state of copper so cyanide has minus 1 charge so that would be X plus 4 into minus 1 is equal to what is the overall charge of the complex minus 3 and this would give us X is equal to plus 1. So we would write the oxidation state of copper as copper 1. Similarly the oxidation number of chromium in this particular complex would be again let's assume X or let's say in this case Y as the oxidation state of chromium then Y plus 6 into 0 because water is a neutral molecule would be what is the overall charge of the complex plus 3. So Y is nothing but plus 3. And here we can write the oxidation state as chromium 3. Now can you find out the oxidation state of cobalt in this particular complex pause the video for a moment and give it a try. So let's again assume that the oxidation state of cobalt is X. We can see that this has no charge this is a neutral coordination compound. So X plus chlorine has minus 1 charge so that is 2 into minus 1 plus NS3 is neutral and you have one more chlorine here so plus minus 1. And what is the overall charge of the compound which is 0 so on solving this you will get X minus 2 minus 1 is 0 which is nothing but X is equal to plus 3. So the oxidation state of cobalt can be written as CO3. Let's now look at the last important topic but before that tell me what is the difference between these two coordination compounds. We have the same central atom which is cobalt we have ligand ammonia in both of them. The only difference is that in the first complex cobalt is bonded to 6 ammonia molecules that is you have only one type of ligand here but in the second complex cobalt is bonded to ammonia as well as chloradans right so we have two types of ligands here. Now the complexes in which a metal is bound to only one kind of donor groups are known as homo-leptic complexes. On the other hand complexes in which metal is bound to more than one type of donor groups are referred to as heteroleptic complexes. Alright so that's pretty much the end of our discussion on the important terminology related to coordination compounds. So to summarize if this is a coordination complex then this is what we refer to as a coordination entity ML6 and M can be any transition metal atom or iron while A refers to the counter ions. L here refers to the ligands which can be anionic or neutral molecules and these ligands can also be monodentate, bidentate, polydentate or even ambidentate ligands. And if the central metal atom is attached to only one type of ligand then what we have is a homo-leptic complex and if the central atom is bound to more than one type of ligand then we have a heteroleptic complex. Now depending on the number of ligands bound to the central metal atom we get the coordination number and depending on this we also can predict the coordination polyhedral. The most common ones being octahedral when we have six ligand groups and square planar or tetrahedral when there are four ligands attached to the central metal atom.