 Hello and welcome to this new segment of CD spectroscopy and MOSBAR spectroscopy for chemist. My name is Arnab Datta and I am an associate professor in the department of chemistry IIT Bombay. So we are now looking into the applications of MOSBAR spectroscopy. So far we have covered how the ligand and its properties can regulate where the isomer shift of a MOSBAR spectra will come up. So today we are going to take one more example and we are taking this example of an iron 2 salt. However, there are different ligands bound to it iron fluoride, iron chloride, iron bromide and iron iodide and if I look into that delta value for the salts they are as following 1.40 for iron fluoride, 1.20 for chloride for the bromide for 1.15 and for iodide it is 1.10. So we can see as we move from the fluoride to iodide we are slowly moving towards the more negative direction of delta values. Now the question is why? So for that we want to take a look into the structure of this particular complexes and over here we are looking an iron 2 sample is interacting with fluoride ions 2 of them and this interaction and its properties are going to control how the D-electron density and the resultant D-electron density is going to change and how it will affect the MOSBAR spectroscopy parameter the isomer shift. So now if we look into all the system iron fluoride, iron chloride, iron bromide and lastly iron iodide and over here you can see all of them the common factor is the iron plus 2. So obviously the difference you are seeing is not for iron because it is remaining the same the difference is happening what is there on the anion side. So that is creating the problem I should say the differentiating factor is the anion now how we can explain that. If I look through this iron and this anionic interaction iron is remaining constant all over the changing factor is over here how it is changing. So iron plus 2 if I want to talk iron plus 2 in terms of hard soft acid based rule it is more actually on the intermediate side that means it is actually staying more on the softer side soft in nature compared to iron plus 3 whereas what will happen if I talk about hearts of system for this anions fluorides are actually hard chloride is also hard bromide is something in the intermediate and iodide is a soft in nature. So what is that particular factor that defines whether it is a hardened soft it is a polarizability all of them has a charge of minus 1 so charge density is going to be the important factor all of them has a charge of minus 1 but fluoride is a very small iron. So it controls its charge very strongly so it is very difficult to change the electronic distribution around the fluoride so that we can change the charge distribution. So it is very difficult to do that so they are less polarizable and in that case hard. So this is the polarizability factor would come into that is with heart. Chloride on the other hand is very much similar but its size is a little bit larger than fluoride. So over here it will still be hard but the hard nature between fluoride and chloride fluoride will be harder compared to chloride whereas bromide side the size is increasing further so that will be slowly moving to the soft nature and iodide is the largest of all charge is still same minus 1. So because it is a large size and same charge is much easier to control the charge density and even change it alter it so that is why the polarizability is on the higher side as we go down over there and on the lower side on the top of the group. So they belong to all in group 17 iodide is the highest polarizable molecule so it will be the soft in nature or softest in nature among theses fluoride on the other hand is the lowest polarizable and it will be the hardest among all the system. Now iron 2 it is interacting with all of them I am not going into which will be more stable or not that depends on the stability between the hard and hearts of interaction but I am trying to understand how the electron density is moving around. So when it is a softer nature iron bromide and iron iodide they will be more on covalent nature because they are softer in nature so they will be much more interested to share the electron density among them whereas chloride and bromide they will be more in ionic nature. So the ions will like to keep their charge and electron density more closer to themselves rather than sharing it. So that is defined over there this ionic this is in covalent nature and if it is ionic in nature this iron fluoride and iron chloride what will happen the iron is going to keep its D electrons to itself whereas in covalent nature D electrons will be shared and over here it is minimal sharing because all of them are iron plus 2 so it is having 6 D electrons to start with all the 6 electrons most or more or less remain as it is for the case of iron fluoride and iron chloride but iron bromide and iodide it is going to be sharing the electron density with the anions bromides or iodide and over here when it starts sharing if I lose S electron density it will be lowering the D electron density so it will lower the shielding effect again if I have a low shielding effect what is going to happen my S electron density in the nucleus is going to affect it which is nothing but psi 0 square it is going to be a high value when there is low shielding effect. So, psi 0 square value how it is going to differ it will be the highest for this iron iodide because most of the D electron is moving out as much as possible AC electrons are not feeling any shielding effect it has all the chances to go in the nucleus and psi 0 square will be high but it is going to be lowered on iron fluoride case because most of the D electrons are staying as it is there and as we know delta value is again depending on delta R by R and psi 0 square sample minus psi 0 square source and over here you can see the delta R by R is a negative value because iron 57 actually shrinks down in size when it goes to the higher excited state i equal to 3 by 2. So, this is negative and this value is constant I am using the same source the sample value higher the value it will multiply the negative number and it will go towards the negative region and that is what is actually happening over there because psi 0 square value for the sample as you figure it out is high for the iodine says multiply the negative number and that is why it is having a lower value on the negative side on the fluoride case it is on the most positive side that it can be because it has the lowest psi 0 square. So, even if it is multiplied with a negative number it is actually most on the positive side and that is what is actually happening over here the difference we are seeing it is mostly generating from the AC electron density how much it can be there on the electron which is connected to the shielding effect which in turn is connected to the D electron availability on the iron and that D electron availability of the iron is controlled by the ionic or covalent nature of the complexes over here iodide being a soft nature ligand it tried to bind with iron 2 plus much strongly doing a covalent bond and moving the electron density out of the iron and that is why in case of iodide we see that it shifted towards the relatively negative side from 1.4 to 1.1 again it is not the absolute negative value we are discussing we are discussing about which particular direction this is actually moving out. So, with that particular example we would like to stop for this particular segment where we have explained how we can take a look into the covalent and ionic nature of the ligand and the molecule to figure it out what will be the isomeric shift for the MOSBUS spectroscopy and over here this also show us that how we can use the MOSBUS spectroscopy to figuring me out whether my molecule is having a covalent nature or an ionic nature during the bond formation. So, with that we would like to conclude over here thank you thank you very much