 Hello and welcome to this new episode of CD spectroscopy and MOSBIRS spectroscopy for chemist. My name is Arnab Dutta and I am an associate professor in the department of chemistry IIT Mumbai. So, in the previous segments we are discussing about MOSBIRS spectroscopy. We have gone through the basics of it and now we are looking through the applications of CD spectroscopy by which we can distinguish between the different oxidation state, spin state and more importantly the coordination atmosphere around an iron via this particular spectroscopy. So, let us start. So, just to give you the brief of this system in MOSBIRS spectroscopy we are primarily focusing on the excitation of a ground state nuclear state. So, which is generally i equal to half state for a 57 iron sample which is going to excited to i equal to 3 by 2 state and these states are actually well far away by 14.4 kilo electron volt energy which is a quite a high energy. So, we need a proper source of energy in the form of gamma ray which is the energy that is going to excite ground state to the excites state and this gamma ray of this particular energy has to match over there. So, that we can achieve the resonating condition and this is actually coming this particular electromagnetic radiation is actually coming from an already existing excited state i equal to 3 by 2 to i equal to half of a another 57 iron which we call them the source which is typically developed from a cobalt material radioactive cobalt material which gives this metastable iron 57 at 3 by 2 state which actually slowly comes down to i equal to half state and it release the gamma ray which is actually going to give you that energy and typically what happened the source and the sample actually spread forward. So, this is say the source this is say the sample are typically well separated. However, they are typically put in a solid matrix or lattice so that it has minimal change through translation during this gamma ray exchange because when the gamma ray comes out because gamma is a very high energy. So, it would feel some momentum to go backwards on the other hand this sample when we is getting this gamma ray it has a tendency to go forward to stop those changes which will disrupt the resonating condition we put them in solid matrix or lattice and then this particular system over here it is put on a wheel which we can move forward or backward whereas this particular system kept static and then we move forward or far away from the sample to ensure that they are actually a very good resonating condition which we typically follow by following the percentage of transmittance of the gamma ray where we have a detector over here which actually look into the gamma ray it is passing through the sample. So, if the energy is not matched we will see 100 percent transmitters and if it is matched we will see some band and this is looks like typically like this and this is in the percentage of transmitters which varies from 0 to 100 whereas the x axis is given in a velocity over here which is taking care of the Doppler effect ensuring a matching of energy over here. So, that is what it is actually happening over here over here when you are talking about this particular system there are two different parameters one is the delta value or isomer shift which ensures that what is the position of this maxima over here is coming over here. So, that is the isomer shift and then there is another parameter delta eq where quadrupole are splitting that is defined by the asymmetry around the center and isomer shift typically follows what is the s electron density on the nucleus because only s electron has finite possibility to be inside the nucleus and because we are talking about the energy state of nucleus over here and this will be only affected by the s electron density. So, that is we are covering and this delta value give us an idea how much is the s electron density over there and delta eq is the quadrupole splitting which shows up the asymmetry around it. Now, with those things in our mind let us go ahead with some other examples by which we can follow what is actually happening with this particular MOSBAR spectroscopy. We have already covered two examples. So, this is the number three examples we are going to cover today what is the effect of ligands on MOSBAR spectra. So, over here we are going to give you some particular ligands on a particular metal. So, there is an iron complex which is connected to this ligand x and this x ligand we are going to change and we will try to find out what is the isomer shift. So, let us put out some of the examples. The first example is NO plus ligand which actually shows for this particular iron complex is coming close to 0. So, that means the source I am using and the sample is having the same s electron density on this iron. So, it is showing a 0 unit isomer shift. Then comes carbon monoxide which is showing a shift of plus 1, 5 millimeter per second. So, now it is moving to the positive side there is nitrate ion, nitrite ion is a value of 0.26 then comes the ammonia 7.31 and at the end it is the H2O it has the value of plus 0.31 and ammonia put it wrong actually it is 0.28. So, these are the values I get for this iron complex and I want to mention the iron is in plus 2 oxidation state for all these complexes and I have to understand why I see a shift in the positive direction for the delta values. So, that we are going to cover and for that we are going to take a look into the properties of the ligand itself. So, the ligand can be distributed in three different forms depending on its property of sigma and pi donation because a ligand can interact with the metal in different formats. One is the sigma interaction and there is a metal there is orbital on the metal which gives the electron to a ligand orbital. So, over here what happens the ligand actually give electron to the metal orbital and this is the sigma interaction the electron moves from the ligand to metal why it is sigma interaction because if you take the inter nuclear axis over here you can see the electron density is present the inter nuclear axis whereas if I do a C2 rotation over here and there is no change in symmetry and hence these two factor defines it is a sigma interaction. So, sigma interaction definition is there should be electron density in the inter nuclear axis and if you do a C2 rotation there should be no change in the orbital symmetry or orbital lobe symmetry. So, it is a sigma interaction and over here electron density moves from ligand to the metal. Then comes the other possibility a metal have not only a head on interaction but it can also has a side on interaction and over here you can see there is interaction possible between these two lobes. Typically over here you can see there is no electron density in the inter nuclear axis for this particular point and secondly if I do a C2 rotation around the inter nuclear axis there will be a change in symmetry. So, for an example if you rotate that the shaded area will come to the bottom the unshaded area will go to the top similar thing happens over there. So, that is why there is a change in the symmetry or the phase of the orbital lobes. So, that is why it is called the pi symmetry or pi interaction. So, this is what is actually happening and over here typically it is found metal actually gives electron back to the ligand. So, over here electron density moves from metal to the ligand and when it is happening it happens that the ligand is mostly used there vacant pi star or p symmetric orbitals for this interaction. So, there should be a vacancy present at least for one electron on the ligand side. So, that the metal can put their electron over there through this pi interaction. So, these are the two interaction we can have sigma donation and pi interaction. Sigma interaction again it is a head on interaction where electron density is present in the inter nuclear axis and with the C2 rotation there is no change in the symmetry. Whereas, if you look into the pi interaction there is actually no electron density in between and if you do a C2 rotation there will be change in the symmetry. Now, when there is an interaction between metal and ligand the first bond it will form the sigma interaction and if the other orbitals are available with the current symmetry then there is a possibility of having a pi interacting orbitals. So, sigma is always there pi it can come up on the later stages depending on the availability of the orbitals and the correct symmetry orientation. So, with that now we know there are sigma interaction and pi interaction happening I am just drawing just the simple bonding interaction. So, there is a sigma interaction and then there is a possibility of a pi interaction and this can happen also the pi star orbital on the ligand and pi star orbital typically looks like this is very symmetric in orientation with the d orbital lobes over here because over there there are 4 lobes on the d orbital I was just drawing the half of it so far. So, the rest half of it will be here. So, this is actually happening with the pi symmetry. So, this is the sigma this is the pi. Now, if we want to look into the other important factor in the sigma interaction ligand gives electron density to the metal in the pi interaction metal gives electron back to the ligand. Now, when you look into the different set of ligands how we can differentiate whether it is a sigma interacting or pi interacting. So, that depends on the group number the coordinating element of the ligand. For an example if it is group 17 that means it is fluoride, chloride, promide, iodide those kind of ligands it actually has a lot of electron density no vacant orbital. So, it can mostly do sigma interaction and over here one thing I have also mentioned that if you have electron density present in the p orbital which is fulfilled there is a possibility that the ligand can also give electron density back. So, in this case of group 17 that is actually happening over here we see that ligands are actually sigma donating that is common for all the ligands that is possible because sigma interaction is the first interaction they will follow and the first electron is actually coming from the ligand to the metal side. So, this is going to be sigma donating and also pi donating. So, it is giving electron density both from the sigma and pi interaction. So, for both sides it is actually taking electron on the metal. So, typically the p symmetric orbitals give this so electron density is coming through there. So, it is a lot of electron density to be present on this ligand to the metal system. Then comes group 16 typically oxygen, sulphur, selenium this kind of system and differed it is found that if it is in the oxide, sulphide or thiolate this kind of orientation they are also becoming sigma donating and pi donating system. And this is happening because the electrons are coming from this ligand to the metal side and ligand has a lot of electron density present in the form of oxide, sulphides or thiols. So, group 16 and 17 are quite similar then comes the group 15 the next one. This is generally the amine ones where the lone pair of the nitrancer only available system or different kinds of amine primary secondary tertiary all of them can only give one electron that is from this lone pair and which is the sigma donating orbital because if you have only one pair of electrons available it is going to form the sigma interaction and there is no pi contribution at all. So, there is no orbital present in ammonia or any of the amines where the pi symmetric orbitals can come and interact with them. So, that is why if you have this kind of amine based ligands that is going to show only sigma donation character. Then there is other possibility of group 15 where the nitrogen is bound to an another ligand X this X can be carbon, nitrogen, oxygen all kind of system. For example, carbon means you take pyridine kind of system where this nitrogen and carbon has a double bond. And those nitrogen, carbon, nitrogen, nitrogen or nitrogen, oxygen double bonds can act as a pi acceptor orbital. So, here it is different metal gives electron metal having electron from the sigma interaction. However, the thing is a little bit different on the pi symmetry is now say the carbon double bond nitrogen has a pi star orbital. Let me put the nitrogen on this side carbon on the other and over here metal can transfer electron to this system and over here nitrogen double bond carbon or nitrogen double bond nitrogen or nitrogen double bond oxygen their low line pi star orbital play this role of this pi accepting orbital. Then comes a group 14 here you have carbon monoxide cyanides all these ligands this is also sigma donating and pi accepting where the C triple bond O or C triple bond N this pi star orbitals act as the pi accepting orbital. So, over there you can see by going through the group number group 17, 16, 15 and 14 we can easily follow out what kind of system would be there. So, all of them are sigma donating the difference is what is their pi symmetry it is pi donating for group 16 and 17 for group 15 which is amine there is no pi contribution and it is pi accepting for group 14 and 15 especially when the 15 has nitrogen double bond X kind of bonds present over there and nitrogen is actually coordinating to them. So, with that information in our hand we move back and take a look into all these systems and let me draw that one more time what we are actually getting over here. So, the ligands I have you know plus carbon monoxide, nitrite, ammonia, water and before going to their delta values I am just writing it over here it is 0 plus 1.5 plus 0.26 plus 0.28 and plus 0.31 units of millimeter per second. So, now we are trying to have an idea why this ligands are showing this particular isomership the answer is hidden in their character. So, let us find out whether it is a sigma pi character how we can differentiate them. So, NO plus so there is a nitrogen double bond O plus so that means it is a group 15 system and nitrogen double bond O is present. So, this is going to be sigma donating and pi accepting system. The next one carbon monoxide again a group 14 system carbon monoxide the carbon side is binding. So, you have C triple bond O kind of bond which provided the low line pi accepted orbitals the pi said orbitals. So, this is also sigma donating and pi accepting system nitrate on the other hand it is also group 15 you have N double bond O. So, it is also going through sigma donating pi accepting ammonia on the other hand it is simple group 15 only amine based systems it will be only sigma donating no pi interaction at all. Water it is belong to group 16 because oxygen is going to bind it is going to have a sigma donating and pi donating system. So, that is what is actually happening in the case of all these orbitals. Now, now we know it is either sigma donating pi accepting in the beginning only sigma donating and sigma donating pi donating. Now, how this pi interaction is hampering or regulating the isomer shift for that we look back to the interaction one more time this is the metal the orbital here is the ligand and the ligand can be pi accepting or pi donating. So, let me draw first this system it is a pi accepting system. So, electron density is moving from metal to the ligand. So, electron density is moving from metal to the ligand on the other hand there is a possibility a ligand is giving electron density to the metal the pi donating system. Now, in both cases where the electron density is coming or going from it is going from this D based orbitals. So, the D electron density in this pi accepting system will be low because it is losing D electron density towards the ligand. Whereas, the same thing the D electron density will be high in the case of pi donating system because electron density is moving from the ligand to the D orbitals. Now, this D electron density affects the S electron via the shielding effect. So, more will be the D electron more will be the shielding and less is the D electron less will be the shielding. So, if I already have a low amount of D electron density it will reflect with low shielding effect whereas, if it is a high electron density it will reflect a high shielding effect. And shielding effect is nothing but it is moving or stopping the S electron density to go towards the nuclei. So, S electron density on the nuclei that means the psi 0 square this value how it is going to change if the shielding value is low that means S electron density is free to move and go towards the nucleus. So, this value will be high and if shielding is high because of the high amount of D electron density it is going to stop the S electron to go to the nucleus and that is why the S electron density of the nucleus for this particular system will be low. And now we know the delta value depends on two factors one is delta R by R the change in the ionic radii when this is changing the nuclear state the nuclear radii it is going to change and as we know for iron 57 this is negative because it is actually going to get smaller when you go to the excited state. So, it is a negative value and the rest value it depends on psi 0 square sample minus psi 0 square source the source is constant. So, what it depends on is the sample value. So, now this value is going to be higher when you have a lot of S electron density. So, if it is high then it is going to multiply with a negative term. So, the effect would be that delta R value is negative. So, the delta value high multiplies the negative sign should be on the lower side of the negative side. Whereas if it is multiplied with a low number it will be on the higher side or positive number. And now if we look into these values now you can see I am slowly moving from 0 to plus 0.31 from positive direction and this is actually happening because of the pi interacting properties of the ligands when it is pi accepting it is moving a lot of electron density out of that D electron. So, it is actually less shielding. So, electron density can move and go towards this nucleus and it is showing high value but it is multiplied with delta which is negative that is why it is on the lower side. And if you have a pi donating lot of electron density coming to the D orbitals it is creating a lot of shielding effect lowest electron density in the nuclei and number in the positive side. Now over here how to differentiate between these three systems all are sigma donating pi accepting. So, NO plus because of this positive charge it is actually becoming actually wanting for more electrons it is positive charge so it requires more electron density towards it. And that is why a lot of extra electron density will move from the metal to the ligand during this pi accepting interaction with the NO plus. The positive charge is one of the any factor carbon monoxide is a neutral and nitrate nitrite is in the negative charge. So, now you can see as we go from positive to negative what will happen my electron movement from metal to ligand will be low because it is already having a lot of charge as you are trying to put more electron density the repulsion will be more whereas this is positive charge so that means it requires more electron density to come out. And that is why the amount of electron density movement will be different and that will be directly correlated with that how much electron density is remaining what will be its shielding effect and how it is going to affect the value of the isomershield. So, that is one of the example and how the properties of the ligand in terms of sigma and pi interaction can control the isomershield. So, very carefully look into all the pi accepting or pi donating or even some case of the M is no penetration at all. So, that is going to regulate where will be the isomershield for this particular metals example of iron in this particular case. So, with that we would like to conclude this particular segment over here. Thank you. Thank you very much.