 We know that transition elements form a number of co-ordination compounds and in this video we will take a brief introduction to co-ordination compounds and see how the idea about them were first found. Coordination compounds are found everywhere in nature. Hemoglobin in our blood for instance is a co-ordination compound where metal iron is bonded to a large protein molecule as you can see here. Similarly, vitamin B12 is another co-ordination compound that contains a transition metal cubalt. Coordination compounds also find a number of applications in our everyday life like in metallurgical processes as catalysts in electroplating, medicinal chemistry and so on. Now when we talk about its inception it was a scientist named Alfred Werner who first recognized the concept of co-ordination compounds and tried to explain the bonding in these compounds. He prepared and studied a number of such compounds and compared their physical and chemical properties to finally propose the famous Werner's theory of co-ordination compounds and based on his experiments he explained that the metals showed two different types of valencies in a co-ordination compound which are primary valencies and secondary valencies. So what are these valencies and how did he arrive at it? Let's see. So what he did was that he took a series of compounds of cobalt chloride with ammonia and reacted it with cold silver nitrate solution and he observed that in some cases agi-sil was precipitated but in some other cases when the compound was reacted with cold silver nitrate solution agi-sil was not precipitated. Now the importance of this is that agi-sil can be formed or agi-sil can be precipitated only if a free chloride ion is there or if the chloride ion is actually ionizable so that it can react with silver nitrate solution and precipitate in the form of silver chloride which means some of the compounds of cobalt chloride with ammonia had ionizable chloradions which precipitated in the form of agi-sil but some of them did not have that. For example 1 mole of COCl3.6 NH3 which is yellow in color when it was reacted with excess cold silver nitrate solution 3 moles of agi-sil were precipitated. On the other hand when 1 mole of COCl3.5 NH3 was treated with the same excess cold of silver nitrate solution only 2 moles of agi-sil were precipitated and just like that when 1 mole of COCl3.4 NH3 which is green in color was reacted with silver nitrate solution only 1 mole of agi-sil was precipitated. In addition to this the conductivity studies that were performed on these compounds also showed similar results. For instance based on the conductivity studies Werner proposed the following formula for these cobalt complexes. CONH363 plus plus 3Cl minus so this corresponds to 1 is to 3 electrolyte. Similarly conductivity studies on COCl3.5 NH3 and COCl3.4 NH3 showed that the corresponding formula for these should be CONH35Cl and 2Cl minus and CONH34Cl2 and Cl minus whose solution conductivity corresponded to 1 is to 2 and 1 is to 1 electrolyte. From these experiments he proposed that the metal in a coordination compound had two different types of valencies. Secondary valency or the one where the atom within the square brackets form a single entity and do not dissociate under experimental conditions and primary valency which was ionizable. So all of those chloride ions that precipitated as agCl were the ones that were formed by primary bonding or the one that was ionizable and if a particular chloride ion was within the square brackets or coordinated to the metal atom through secondary valency then that was non ionizable and did not precipitate as agCl. For example if you look at this particular coordination compound you can see that the reason it precipitates 3 mole of agCl is because there are 3 ionizable chloride ions right it is outside the square brackets and NH3 is bound to the cobalt metal ion through secondary bonding that is none of the ammonia molecules are ionizable. On the other hand if you look at the second compound here you can see that 1 chloride ion is within the square bracket that is one of them is coordinated to the central metal through secondary linkages and if this is reacted with silver nitrate solution you can see that 2 moles of agCl are precipitated because you have 2 ionizable chloride ions that are present outside the square bracket in other words which are linked through primary valency and if you look at the last example here you have 2 chloride ions that are linked to the metal atom through secondary bonding and only 1 chloride ion that is ionizable or linked via primary bonding and this is why only 1 mole of agCl gets precipitated in the last case. Now in each of these cobalt compounds the cobalt metal has a fixed secondary valency of 6 here the cobalt could be coordinated with 6 ammonia ligands or 5 ammonia ligands and 1 chloride ion or 4 ammonia ligands and 2 chloride ions essentially it had a fixed secondary valency of 6. We will discuss more about what ligands are and the different types of ligands in the upcoming videos but for now let's quickly look at the main postulates of Werner's theory that he formulated based on these experiments. The first one is that coordination compounds show 2 types of linkages or valencies which as we said before are the primary and the secondary valencies. Now primary valencies are ionizable and basically refer to the charge on the central metal for example binary compounds like CRCl3 has a primary valency of 3 as we can see from here 3 also refers to the charge on the central metal atom. Similarly the complex CONH36Cl3 exists as CONH36 3 plus and 3Cl minus. Here again the oxidation state or the primary valency of the central metal is 3. Since this coordination compound has 3 ionizable chloradans or the primary valency is 3 when we react it with excess silver nitrate solution we would get 3 moles of AGCl. Now the second postulate is about the secondary valencies. Now secondary valencies are non ionizable and refer to the number of ligand atoms that are coordinated with the central metal atom. It also gives us the coordination number. Primary valence gives us the oxidation number or oxidation state. Secondary valency gives us the coordination number. For example in the same coordination compound CONH36Cl3 we said that the primary valency is 3 corresponding to 3 chloradans whereas the 6 ammonia ligands are held together by secondary linkages or secondary valencies. Now these ligands that are bonded directly to the central metal atom through the secondary linkages are always enclosed within the square brackets. So here you can see that the coordination number is 6 that corresponds to 6 ammonia ligands. And lastly the groups that are bound to the central metal atom through the secondary linkages have a distinct geometrical and spatial arrangement that corresponds to different coordination numbers. And these spatial arrangements are called coordination polyhedra. We will learn more about coordination polyhedra in the upcoming videos. But to explain briefly if you look at the previous example where cobalt is bonded to 6 ammonia molecules you can see that here we have a distinct octahedral geometry right. Now if you have coordination number 4 where the metal atom is bonded to only 4 ligands in that case you can have 2 different polyhedra like tetrahedral or square planar arrangement. As I mentioned before we will look at coordination polyhedra in more details especially when we talk about bonding. In the next video we will discuss some important terminologies that are related to coordination compounds like what exactly is a ligand, how many different types of ligands are there, what is a coordination entity and so on.