 Hello everyone, once again I welcome you all to MSP lecture series on Transmittal Chemistry. In the last few lectures we have been discussing on the classification of ligands by donor atoms. In my previous lecture I was talking about nitrogen donor ligands and I did mention before concluding about another important class of ligands common to nitrogen donor atoms are skiff bases and a variety of skiff bases have been made and they explored their coordination chemistry and applications. Let me give the preparation and some important components of skiff bases. As you know the general method of preparation involves the reaction between ketone and primary amine. It is a typical condensation reaction. We take a specific example here. Let us take two equivalents of salicyloldehyde and treat this one with ethylene diamine. It gives two equivalents of water to form this condensate bis-salicyloldehyde ethylene diamine ligand. It leads to the formation of bis-salicyloldehyde ethylene diamine ligand. So if you remove this hydrogen it becomes a di anionic tetradentate ligand. This is bis amine. We know the general method of preparation. We can make a variety of bis amine complexes or skiff bases. One can also generate those things starting from for example oxides like this when we treat this one with RNCO. It can give a compound like this and this can readily eliminate carbon dioxide to form a compound like this. This on further treatment with another RNCO can give a compound like this. So these are very important class of ligands called ureto and carbameto complexes. So that means we have numerous examples of nitrogen donor ligands that we saw in last couple of lectures. And there is important class of macrocyclic I did mention having porphyrin as a base unit. Apart from that one we also have another interesting cage type compounds they are called sepul crates. They are essentially a cage ligands with various sized cavities. So let me show one example here. So we have here a typical sepul claret is given here and you can see all of them have Hc on the middle nitrogen atoms. So this is another important class of and also we have pyrozole borates also have, this pyrozole borates also we come across. So of course we can keep on discussing about nitrogen donor ligands in length. So at some point of time we have to stop it and proceed to look for further donor atoms such as oxygen and phosphorus because of time constraints I do not think I should be able to deal with almost all other nitrogen atoms. Nevertheless I can show you representative examples. And another class of ligands are nitriles that already I showed you while giving the preparation of important coordination compounds like acetonitrile and benzonitrile complexes. For example from synthetic point of view these are all very important nitrile compounds and another important compound of group 6 metal carbonyls is this one. This is a very useful compound. So one can prepare this compound for example you take metal carbonyl where M can be chromium, molybdenum or tungsten reflects for 24 hours in acetonitrile. So it gives a compound of this type. So this is a very useful ligand. Now you can ask me why only 3 acetonitriles have been substituted, why not 4 or 5? Yes you should remember metal is in zero valence state and once this compound is formulates in 80 electron species. Since metal is in zero valence state and more electron density is coming on to the metal always although many complexes do satisfy 80 electron rule and also another effect comes is repulsion inter-electronic repulsion that can always try to destabilize. In that context CO being by acceptors they can release little bit of electron density present on metal through back bonding. As a result these back bonding capable ligands stabilizes metals in their lower end state and here if you consider acetonitrile, acetonitrile is only a sigma donor and it is not a pi acceptor. As a result what happens when metal retains its zero axis state and now what would happen is electron density does not change but on the other hand number of back bonding ligands are decreasing. So that means when we have only pure sigma donor ligands then minimum of 3 carbon monoxide are needed to minimize inter-electron repulsion as a result no matter how long you reflects metal hexacarbonyls with astronitrile you cannot get more than 3 substituted and whether you take astronitrile or whether you take ammonia or whether you take any other alkyl amines you can make a mixed carbonyl and amine complexes or nitrile complexes but you have to have minimum of 3 carbonyl that is an advantage also. So you can make this one for example now if you take this one and reflects with toluene or something it can eliminate all astronitrile to form eta 6 are in MCO3 compound. In that context this is a very useful compound for example you take this one here and add in dichloromethane after formation of this compound you can remove all the solvent under vacuum to get dry stuff for this one at 0 degree temperature add dichloromethane and iodine one can generate a compound of this type. Now it will be oxidative addition metal will be in plus 2 state. So now let us say if we add any bisphosphine this bisphosphine is essentially substitute for this astronitrile to form a compound like this. So now you can see 1, 2, 3, 4, 5, 6, 7 are there. So this is a 7 coordinated compound and of course here depending upon what kind of phosphines we are using this can have a capped octahedral geometry or pentagonal by pyramidal. So now another important class of compounds among macrocyclic ligands are porphyrin this is the basic unit and it is very easy to prepare this one. One should remember here we have 2 N without with only lone pairs whereas other two have NH bonds so opposite side. One should remember this compound can be made by simply condensing aldehydes various aldehydes with 4 equivalents of pyrrole molecules. For example if we take 4 straight with 4 aldehydes or CHO it can give through the elimination of 4 equivalents of water molecules and of course it is very easy to write this structure. I shall show you how to write this structure there is no need to remember the structure of this one. You just write porphyrin units without adding double bonds and one here one here you for this do not add any hydrogen and then connect them with a methylene bridge. So now since this one is NH is there in this one what you should do is you should start putting double bond here when you put double bond here conjugation is there so one can go here and then one can go here and then only lone pair is there here. Now here next here it comes and then of course double bond is there now double bond should come here and then this only lone pair is there ok there should be something you have here ok and then this is ok. So this is how one can write porphyrin unit and of course here if you take simple acetaldehyde it will be H whereas here it can be anything depending upon what you are choosing here. So this is how the basic porphyrin can be prepared by simply condensing an aldehyde with pyrrole and of course we also come across another important cage compound used to trap cations alkali metal and alkaline earth metal cations that is cryptand when I talk about croneithers I shall give the preparation of that cryptand as well. With this let me conclude nitrogen donor ligands as I said nitrogen donor ligands are very wide in coordination chemistry and starting from as simple as ammonia or dinitrogen and two we have macrocyclic ligands having as many as 10 to 12 nitrogen atoms in the ring or cages very nicely they encapsulate. So this is the typical bis solicilaldehyde ethylene diamine complex preparation here of course here you can also take any alkyl one you can also get it instead of OH you can also have PPH2 then you have skip base with P donors or even S donors and a typical example of a metal complex I have shown here urethanium is in place two state these two are anionic and then of course one can also make very interesting compounds like this here you can see two cobalt are bridged by two bis, two skip bases in this fashion and also they are called compartmental ligands you can see here we have NH, NH is there and oxygen donors are there and then this can also become anionic once you deprotonate these two nitrogen atoms and also you can have homo metallic or heterometallic complexes can also be made here you can see here and then once the electrons can also delocalize and then you have two four negative charges also can be seen here if this also undergo tautomerism. So very interesting ligand system of course they come up with lot of applications so with this let me stop and start discussion about oxygen donor ligands when we talk about oxygen donor ligand the first ligand that comes to our mind is water and then oxygen itself molecular oxygen itself of course molecular oxygen one can reduce it to make O2 minus then is that is called super oxo and then you can also have peroxo and then of course hydroxyl ligands are known like O2 minus I mentioned O2 2 minus and O2 2 minus is also there and all alcohols aryl alcohols alkyl alcohols are all one way oxygen donors and then you should remember we have two more electron pairs are there once a covalent bond is established after elimination of hatch it can also triply bridge it can also bridge three metro centers one through covalent as either two other two through coordinate bonds and of course ethers and phosphine oxides sulfates phosphates nitrates and several other ligands. So when we look into the donor abilities and coordination modes we come across we can see in case of hydroxyl it can bridge in this fashion two metro centers also there can be terminal one also simple MOH bond and also two or three hydroxyl groups can bridge two metro centers and then this OH moiety can also bridge three metro centers or one can also have a cubane structure like this where alternate corners are occupied by metro centers and hydroxyl groups and also one can have something like this also these are important coordination modes are binding modes of hydroxyl ligands and then let us look into the molecular diagram of water here of course water when you consider we are taking the ligand group orbit is here two electrons are coming from two hydrogen atoms and then we are getting six electrons here and okay and of course you can see here this and this one. So this is mainly OH bond and this is the second OH bond and then this is the lone pair on oxygen and this is the lone pair on oxygen another one. So this lone pair it is non-bonding in nature is responsible for making water as a two electron donor and we normally do not come across water bridging two metro centers acting as a bident technique and as a four electron donor because this is little deeply buried because of this one what happens utilization of second lone pair for coordination is not possible when attempts are made what happens it loses one of the hatch and it forms a hydroxyl group and then hydroxyl group can bridge two metro centers as long as it remains as water molecule normally it does not donate two pairs of electrons to perform as a bridging ligand this for the same reason that the difference in the energy between these two is quite enormous and this is little deeply buried as a result taking out this one for donation to a metal is not that easy. So these are the binding modes of oxygen one can see oxo bridges are quite common and two oxo bridging is also quite common and this is symmetric oxo bridging this is unsymmetric oxo bridging so here what you have is O is there and then this is a coordinate bond and then one can also have a linear bond like this and also mu 3 bridging is also known or in this fashion or in this fashion pyramidal fashion or it can also stabilize four metals arranged in a tetrahedral fashion or something like this or something like this is also known or in some cases we also have examples of oxygen bridging as many as five metal centers having these five metal centers in trigonal bipyramidal arrangement some more example even it is not just five it can also you know bridge six metal atoms in an octahedral fashion you can see here six metals arranged in an octahedral fashion directed towards six vertices having central oxygen atom here and this is the common one and of course here another one where you have two oxygen and also you can have something like this or one can have something like this oxo or it can be like this so these are some of most commonly seen coordination modes of various oxygen and hydroxyl group and with different oxo states. So when oxygen is made to react with a metal center generally electron density is transferred from metal to oxygen with formal oxidation state of the metal and reduction of O2. For example if we look into metals where we can see only one electron donor properties in that case we will end up with super oxo complexes where we come across metals capable of donating two electrons or they have a stable ox state of plus two in that case we can also come across oxo complexes. A typical reaction I shall show you take this one and put oxygen so we come across this one here acetyl estronate complex plus if we add O2 so oxygen state is changed from 2 to 3 so you can now you know what is the nature of oxygen here this is about one electron donor let us look into two electron donor transfer. Again I consider a complex that you are very familiar with Vasca's compound with iridium is in plus one state this transchlorocarbonyl bis triphenyl phosphine complex on treating this one it forms a iridium three complex so we have a few more examples of oxygen donor ligands. I do not want to elaborate more about oxygen donor complexes for example you consider phosphate, sulfate, carboxylate, acetate all are ACAC most of the ligands we talk about all oxygen donor compounds and of course all alko oxides or iloxides are also oxygen donor ligands even simple ethers ketones aldehydes also some to an extent they are all oxygen donor ligands. Let me tell you about couple of more examples of oxygen donor ligands in my next class and I shall move on to discuss another important class of ligands very very important class of ligands that is phosphines which play a major role in organometallic chemistry and also utility of organometallic compounds and coordination compounds in homogeneous catalysis for a variety of organic transformation. Let me stop this lecture and continue in my next lecture about other ligand system.