 Hello and welcome to this next segment of CD spectroscopy and MOSBA spectroscopy for chemist. My name is Arnab Dutta and I am an associate professor in the department of chemistry at IIT Bombay. So today we are going to discuss about the multiple applications of MOSBA spectroscopy. In this particular segment we will take different examples and try to rationalize how we can use MOSBA spectroscopy to understand the basic structure associated with this particular complex. So let us start. So over here we are going to talk about MOSBA spectroscopy to differentiate different complexes and obviously most of the complex will be iron based complex because we are going to use that fact that iron 57 is a MOSBA active isotope and over here the factors we are going to use is the delta value which is the isomer shift which is directly coordinated with the S electron density present inside the nuclei and also delta E Q value which is the quadrupole splitting which says about the electric field gradient around the molecule is nonzero which says the asymmetry around the molecule. So both this factor is going to be very important when you are discussing this system. So let us start with an example. The first example we are going to discuss about Prussian blue and Tarnbull's blue. So these are well known complexes coordination complexes prepared well back like 78 years back. And there is a lot of controversy about this structure. So we will find it out today how MOSBA spectroscopy helped us to understand what is the actual structure and resolve the absolute structure and move out any controversy remaining there. So let us look into it. So first we start with Prussian blue. So what is Prussian blue? So Prussian blue is a complex where we add iron 3 plus salt into a solution of FeCN6 4 minus that means iron is in plus 2 state. So it is a ferrocyanide system and in that ferrocyanide solution we add the iron plus 3 salt and it produces a very bright blue color complex in which we call the Prussian blue. And it is expected that the structure of this molecule is the following Fe 3 plus go outside and bind to it and 4 of this iron centers bind whereas inside this particular system act as a ligand and 3 of them are actually binds to complete the overall coordination geometry. 4 of the iron plus 3 and 3 of this particular ligand which has a charge of minus 4 so you can say it is charge balance. And then I look into it and then I found that there are two different iron centers present over here one is iron 3 and the other is iron 2 and iron 2 is present in that ligand system and iron plus 3 is present on the outside system. Then there is another compound called Tarn bulls blue which is prepared a little bit on the different way. So where iron plus 2 salt was added instead of iron 3 plus you say is iron 2 plus salt added into iron 3 CN 6 3 minus solution. So it is a very cyanide solution. So that is the different previously in Prussian blue iron plus 2 is a part of that complex anion and iron plus 3 salt is added from outside over here it is just swapped iron plus 2 salt added from outside and iron plus 3 now is a part of this complex anion. So then we try to find out what is actually happening on the structure of this Tarn bulls blue. So that is a little bit of ambiguity remain what is the structure of the Tarn bulls blue and that we will try to resolve by using our MOSBA spectroscopy. So let us start with Prussian blue one more time and as we just discussed the complex structure is iron 3 iron CN 6 3 this is iron plus 2 state. So that is the structure of Prussian blue and over here when we try to figure it out how the MOSBA spectra will look like for this complex. So let us first assume and predict what will be the MOSBA spectra for this particular complex. So that I am drawing MOSBA spectra for this complex over here there are 2 different system one is iron plus 3 and iron plus 2 let us look into the iron plus 2 system first is a iron plus 2 with 6 cyanide system 6 cyanide system bound to it. And over here you can see this is a iron plus 2 system by the 6 cyanides. So iron plus 2 will be a low spin complex because cyanides are strong pi acceptor ligand. Now if it is iron plus 2 in a low spin system how do you expect is d orbital splitting will be that will be T2G and EG iron plus 2 means a DC system because it is low spin all the electrons will be like that. So it is a T2G 3 EG 0 system. Over here you can see the electrons are very symmetrically present in this molecule all the T2G are filled up no electrons in the EG. So the valence contribution towards the electric field gradient will be 0 because it is quite symmetrically stable. And on the other hand coordination wise lattice contribution we expect no because it is a perfect octahedral geometry with 6 cyanide ligands. So lattice contribution for EFG is also 0. So this iron plus 2 sample because it is actually having valence contribution 0 lattice contribution 0 so that means it is not going to have any delta E Q value that is also going to be 0. So no splitting we will see only one band representing the isomer shift of this iron plus 2. So let us say we draw it that is we are saying iron plus 2 low spin system. Then comes what is the other system? Other system is an iron 3 which is coordinated from the outside and even it is a octahedral geometry it is going to follow up that is going to figure it out little bit differently because it is going to be a high spin system. Not only that the coordination geometry because it is going to be coordinated with the cyanide in a different way compared to the iron plus 2 over there. So it will be also coordinatively asymmetric. So all those systems says that now because it is co-ordinate asymmetric I am going to see actually a high spin iron 2 system whose electronic configuration will be as following iron plus m is d5. So you can see valence contribution will be again 0 whereas the lattice contribution will be present that means E fg will be not equal to 0 for that. So that is going to come over here. So what I expect my iron plus 3 system will be splitted up I will see a delta eq value and it is coming because of the lattice contribution. So that I am expecting for the system over here iron plus 3 high spin system. The question is where does the iron plus 3 should come the isotopic pattern. So over here the question is where do I expect the delta value the isomer shift should be there for this iron plus 3 system and over here there are two different factors one is it is iron plus 3 and as you know iron plus 3 should go the value to be honest on the left hand side. But over here there is another difference it is a high spin versus low spin and over here I say the low spin systems of iron they generally go to the more negative side of d electron density compared to iron in high spin state that goes to the more positive side this is a delta value. Now the question is why why does it happen so that we will cover a little bit later but for now we found that this high spin system will be on the positive side and the low spin system will be on the negative side. So what I expect this band will come something like that over here and this will be the iron 2 sorry iron 3 high spin system that is we are going to expect for this full system again iron 2 is low spin no contribution from the lattice or balance contribution so single line iron plus 3 outside high spin system coordinatively asymmetric there is a delta aq value but the question is why the delta value of iron plus 2 low spin is shifted towards the more negative side compared to the iron 3 high spin that we will cover a little bit later but this is what we are going to see and that clearly says that yes I have two different iron centers this is the iron 2 and this is the iron 3. Now we look into the turned bulls blue the turned bulls blue if you remember we actually put iron 2 from outside into a solution of iron 3 Cn6 3 minus and then we try to find out what will be the oxygen state of this system. So then we recorded the MOSBUS spectra of this turned bulls blue and what we see is the following large peak like this and signal like this which is particularly same as Prussian blue so they have the similar MOSBUS spectra so if it is having a similar MOSBUS spectra this is what turned bulls blue and the same as Prussian blue so it is saying that I still have a iron 2 low spin system I still have a iron 3 high spin system so how that is possible. So after looking into this MOSBUS spectra 3 scientists their name was Flak, Karlar and Neworth they actually look into the details and try to figure it out what is actually happening over there and they figure it out that this complex is actually started with iron 2 in the outside and Fe3 Cn6 in the 3 minus in the inside and then there is a electron exchange. One electron is getting transferred from the outer iron to the inner complex iron iron and they produce iron 3 on the outside and Fe2 Cn6 4 minus on the inside which further coordinate and give us and that is why we are getting the same MOSBUS spectra. So it is showing that all the turn bulls blue is reacted in a different way at the end it is going to showcase the similar structure because there is an electron exchange step happening and MOSBUS spectra is actually gave us the first hint that it is actually happening in this particular molecule. Now this work was further explored where we try to find out is it really what is actually happening. To find it out that this electron exchange actually happening in the turn bulls blue case to understand that there is another experiment what was done. So an experiment was done in the following way a 57 iron enriched sample was used because typically the iron 57 is quite low in concentration however if I can enrich the iron 57 sample I will see the signals much better and over there they took iron 57 plus 3 from outside and added up in a iron 2 Cn6 salt which is present inside and this is typical one so no enrichment there. So now when it forms the complex in the case this is the Prussian blue I am talking about because adding 3 plus outside 2 plus is inside so over here what do you expect because I am going to look into the enriched sample data right now because over here what happens enriched sample is almost 100 percent whereas the iron 2 sample how much 57 iron it has less than 1 percent or so so that means this is not going to show up very strongly compared to the 100 percent enriched sample. So I am going to see only where I have the enriched iron plus C sample. So as we know we are going to get a split structure so this single structure that we expected earlier over here this signal we are not seeing over here because it is not enriched so it is there but it is so one person that with respect to this it is almost invisible. So this is actually showing it is an iron 3 outside system in high spin and that is expected when we added. Now did the turn blue blue case and then they add iron 3 again to a solution of sorry this started with iron 2 sample and then added to iron 3 C and 6 3 minus sample and over here again the outside iron is 57 enriched sample and what they found in the Mosba spectroscopy that this signature looks exactly the same as Mosba spectroscopy they are identical what that means is over here I am again seeing the 57 iron nothing else and this signature is for iron 3 high spin but when I added it is iron 2 but during the reaction it actually exchange the electron and it becomes iron 3 in the outside and the complex ion becomes iron 2 and they created the complex they require and over here because of this electron exchange happening I am getting iron 57 as a result so that is why the signature is exactly same as Prussian group if it is an iron 2 which is not reacting with the sample then I would have got different signature replicable to iron 2 high spin system but over here I am getting iron 3 high spin system which is again proving the concept that yes the reaction is exchanging an electron in between so Mosba spectroscopy again come into the rescue to find out what is actually happening in the reaction media and this particular reaction with is done by Meyer and co-workers who actually rationally use an iron 57 in this sample and figure it out I am going to get the similar kind of result which showcase the electron exchange in between. So, now the next example I am going to follow so the previous example this is the first example of application now we go to the second example it is about an iron carbonyl complex Fe 3 CO 12. So, it is quite a popular complex tetra-iron dodeca carbonyl complex where 12 carbonyls are there and then there was a question what is the structure of this iron complex. So, people have figured it out there different possibilities so one thing was quite sure that the iron is actually forming a triangular interaction between them first and each of them can take 4 carbonyls. So, that is how it is actually looking into and over there in this complex you can say all the irons are identical carbonyls are all bridging. So, now if I do a Mosba spectroscopy of that I should get only one type of signal which shows that all the irons are actually identical in nature. Then there is another possibility all the irons are always counted on each other that each of them has only 2 carbonyls bound to it in the terminal position and the rest of them is actually bridging in nature. So, over here sorry I put a wrong system all the carbonyls are terminal over here in the previous case and over here carbonyls are 50% bridging and 50% terminal 6 of them are bridging 6 of them are terminal. And all the irons are still in a similar position. So, they are expected to be identical because you can see all of them have been 6 coordination 2 of them terminal and 4 of them are bridging each of them. So, in that is the case I should also get only one type of signal because all the irons are same and I am talking about Mosba spectra. So, that was the thought that we are going to look into and can I differentiate whether all of them are terminal and some of them bridging some of them are terminal yes by looking into the delta value I can predict that. So, with that in this mind people go and try to look how that looks like in the Mosba spectroscopy and what we found is the following it is one portion of the signal which is actually say I am saying a signal is actually having a large quadrupole splitting and there is another signature in between which you call the B signal which has another delta. So, there are 2 signatures over here A and B and the ratio of A and B we are looking into the area they have covered that is 2 is to 1. So, I get a signature of 2 different irons present for Fe3CO12 in Mosba spectroscopy. So, which says I have 2 different irons now how it can be properly rationalized. So, when you look into the complex structure one more time we find that one of the iron has all carbonyls in its terminal position whereas the rest of this 2 have only 3 of them and what is happening to the rest of this carbonyls. So, they are actually binding as bridging ligand. So, over here you can see the total 12 carbonyls over here this is going to the 4, 3, 3, 10, 11 and 12. So, these 2 iron are obviously going to be different and these are 2 of them in. So, these are the A centers which is shown over here in the Mosba spectroscopy. Whereas one iron is there which is obviously different because it has all terminal iron, terminal carbonyl and this terminal carbonyls ensures this iron is behaving differently compared to these 2 and that is the B type of iron which is shown over here. So, that is why we get 2 different iron centers and over here the ratio is 2 is to 1 for obvious reason and the A iron because it is having both terminal and bridging ligand it will be more asymmetric. So, A iron are more asymmetric because it has both bridging and terminal COs compared to B iron which is actually having only terminal CO. So, that is why for A electric field gradient will be nonzero, but that will be more asymmetric and more asymmetry in the delta E of G at the electric field gradient is actually reflected into high value of quadrupole splitting. Whereas on this thing we have terminal carbonyl but it is not perfect octahedral because it has 2 iron bound to it. So, it has asymmetric but asymmetric extent is actually low compared to this other 2 irons. So, that is why this also shows quadrupole splitting but at a lower extent compared to the A irons. So, that is how we can use a example of iron carbonyl complexes organometallic complex and use it with respect to Mosba spectroscopy to find out what is the absolute structure and we found in this particular complex it is neither this particular structure nor this particular structure it is having somewhere between this structure and Mosba spectroscopy showcase that how it is actually can be achieved. So, with that we will like to stop over here for this particular segment and from tomorrow we will try to provide further examples on Mosba applications. Thank you. Thank you very much.