 I'm sure you're all having good time in learning interpretive spectroscopy. So I welcome you all once again to MSB lecture series on interpretive spectroscopy. In my last lecture, I started discussion on charge transfer transitions. So let me continue from where I had stopped. First, let's look into metal to ligand charge transfer. If the metals are in low state, you should remember that they're electron rich. And the ligand, if the ligands that are coordinated to the metal possesses low lying, empty orbitals. Having pi star aromatic ligands or carbon monoxide or cyanide or olefins and also sigma star in case of phosphines. Sometime sigma star of hydrogen molecule as well. Then a metal to ligand charge transfer MLCT would occur. So MLCT transits are common for coordination compounds having pi acceptor ligands. You know that I have classified all the ligands that are at what is disposal into three categories, pure sigma donor ligands. They have low energy field sigma orbitals on the example, water and ammonia and other related ligands. In that case, what happens? The ligand field separation energy or homo-lomo gap is about average. We have another set of ligands. They are called sigma donor and pi donor ligands. For example, halides. They have low energy field sigma orbitals. And because of three lone pairs, they have low energy field pi orbitals. When they interact with metal, metals in their high valence state and low electrons. In that case, ligand to metal charge transfer happens. In that case, the CFSE gap decreases considerably compared to what we saw in case of pure sigma donor ligands. That the third class of ligands are sigma donor and pi acceptor ligands. In this case, we have low energy field sigma orbitals and high energy empty pi orbitals are there. So, in this case, what happens? Backbonding would be stabilizing in such a way that homo-lomo gap increases. That's what exactly happens in case of carbon monoxide and phosphines. Upon absorption of light, electrodes in the metal orbitals are excited to the ligand pi star orbitals. So, metal to ligand charge transfer can also be termed as backbonding. So, metal to ligand charge transfer results in intense bands. Example, tris 2-2-bipyridyl ruthenium compound. This is an orange-colored complex. It is being studied as the exegesis state resulting from this charge transfer has a lifetime of microseconds. And the complex is a versatile photochemical redoxy reagent. And other examples include a phenanthral ligand of tetracharbonyl tungsten and also 2-2-bipyridyl complex of iron tricarbonyl. Let's come back to now. Let's look into how this D and F-arbids will split. For D1 ground state is a 2D state. You can determine that one again using the method I showed you. We two states are there and then the states are T2G and EG similar to octahedral splitting. And T2G is lower in energy and EG is higher in energy. This we call it as D term. For a set of F-arbids, F-arbids should be split into three levels in an octahedral field to a triply degenerate one is single. So that means T1G and T2G and E2G. F-arbids will be split into this one. And there both of them are triply degenerate and this is a single one. Now let's consider spectra of D1 and D9 ions. D1 is one electron is there and D9 one electron less than completely filled electronic configuration. So they have similarities. That's the reason we are considering them together. So in a free gaseous metal ion as I already mentioned D-arbids degenerate and no DD transits are anticipated. In a complex D-genesis is lost and in an octahedral field this D-arbids will be split into T2G consists of DXY, DYZ and DXZ or T2 in case of tetrahedral. And then EG or E consists of DXY square and DZ square orbitals. Now let's consider a D1 system such as hexachlorotitanate 3 minus or hexa aquatitanium 3 plus D1 system. And then here I have given the absorption maximum value for titanium in plus 3 state but having different ligands. And you recall the spectrochemical series and also try to identify the position of these ligands in the spectrochemical series that would tell you some information. They are in the increasing order of the ligand field strength. Then you can see hexachlorotitanate 3 minus shows lambda maximum 13,000 whereas fluoroligand shows 18,900 increases there and hexa aquatitanium compound shows 20,300 and hexa cyanotitanate 3 minus shows 22,300. So this is in the increasing order of ligand field strength. And this gap is steadily increasing. And then in case of aqua compound hexa aquatitanium compound lambda maxim is 20,300. If you take the spectrum UVS spectrum of hexa aquatitanium this is how it's going to look like here. So the magnitude of delta O depends on the nature of the ligands and affects the energy of electronic transition and hence the frequency of absorption maxima. You should remember that one this term I have repeated several times. So now let's consider a D9 example such as hexa aqua copper 2 plus splitting of dr plus is very similar to D1 case. In D1 case one electron is there in T2 is a level. In D9 one hole is there in EG level. EG level we have dx minus y square and d z square. We have three electrons are there. So this is D1 system and this is D9 system. So here one of this electron would be promoted here. That means we have one electron here whereas here we have one hole here we are considering. So in D1 promotion of one electron from T2G to E while in D9 it is simpler to consider the promotion of hole from EG to T2G. So that means for D9 inverse of D1 energy diagram holds good. That means if you know the transition states in case of D1 system just if you reverse it that becomes automatically for D9 system for electronic transition. Now for D9 inverse of D1 energy diagram holds good. So this is for D9 system. So D1 and D9 in tetrahedral also same thing happens. If it is D2, E to T2 is there and in case of D9 it becomes T2E1. So this is what exactly happens. By considering this one what we are doing is we are simplifying for interpretation and elucidation of the structures using the spectral data. Let us look into now different electronic configurations under octahedral as well as tetrahedral and most of them we are considering are high spin complexes. D1 is a typical one and D6 we can see here one electron is here one pair of electrons are there and D4 will be something like this and D9 we have one electron here. And then if you consider tetrahedral high spin complexes it doesn't make much difference here D1 one electron D6 is very similar and D4 and D9. So that means all these electronic configurations have some similarities. So D1, D6, D4, D9. D1 and D6 is one electron and one more than half field. D4 one less than half field one less than completely field. Same thing in case of tetrahedral also. Now is it possible to combine all those things to make the interpretation simple? That's what we do in case of Argel diagram. In Argel diagram what we are considering is we are considering D1, D6, D4, D9 octahedral system as well as D4, D9, D1, D6 tetrahedral system. So all these cases can be combined into a single diagram called Argel diagram to interpret the data obtained from electronic spectra which describes the qualitative way of the effect of electronic configuration. That means all these electronic configurations of both octahedral and tetrahedral complexes can be combined into a single diagram because it is Argel diagram which describes the qualitative way of the effect of electronic configuration with one electron with one more electron than half field and one less electron than full shell and then one less than half field shell. So that would cover all the electronic configuration I have shown here D1, D6, D4, D9. A typical Argel diagram for all the system together is represented here and D4 and D9 here octahedral and D1, D6 tetrahedral is here. Again D4, D9 tetrahedral is here and D1, D6 octahedral is here. For example D4 system if one transition is there we can say it is from EG to T2G and then D9 also EG to T2G. But if you take D1 and D6 it is T2G to EG or T2EG. So this is how you can see the representation of all these electronic configurations for both octahedral and tetrahedral complexes in one diagram. This diagram can without any problem can explain the electronic spectra provided we have in the complex these electronic configurations. So now let's look into spectra of D2 or D8 I have to see some similarity similar to what we saw in case of D1, D4, D6 and D9 system. So in an octahedral field what we have is T2G0 and EG is the D2 system something like this. This is we can call this that EG0 and then when the electron is promoted this would change to T2G1, EG1. There are two possibilities for this transition to occur. Electrons may be promoted from DXY, DXZ or DYZ to DZ square or DXY square. So less energy is needed to promote an electron to DZ square than DXY square. So that Y probably you can go back to tetragonal elongation and tetragonal compression you would understand what is the benefit of promoting one electron to DZ square or DXY square. So now once you promote electron so D we have the electronic configuration of DXY1 and DZ square 1 then it is a less energy transition and the other hand when you promote electron to DXY square it's more energy transition. So electrons are spread around in all three directions XYZ reducing the electron-electron repulsion. So here electrons are confined to XY plane because four ligands are approaching in the direction of X minus XY minus Y as a result if you put more electrons what happens the method to ligand repulsion would destabilize and it will be more energetic results more electron-electron repulsion. In both the cases electrons are promoted and another high energy state will be formed thus four energy levels will be there. So as a result we can see four energy levels in case of D2 and D8 system. So now consider an example a specific example of D2 system hexa covaradium 3 plus and here D2 electronic configuration ground term is 3F already I showed you in my previous lecture and four excited states are possible here 3P 1G 1D and 1S and ground state contains two electrons with parallel spins. So that means the F state will be split into three levels 2 triple generate and one single 3D 1G 3D 2G and then 3A 2G these transitions are possible. So ligand field strength of water results in transitions occurring close to the crossover point between 3D 1P and they are not resolved but if you just see here why this is not resolved here I will show here vanadium 3 plus ion with three different ligands will show three distinct peaks if we have three different ligands are there we can see three different peaks here you can see this is the first one 3T 1G 2 3T 2G and then we have 3T 1G 2 3T 1G P and then we have 3T 1G 2 high energy 3A 2G of F. So three transitions are possible they are represented here and two have very narrow gap as a result they are coming here and then this is 3T 1G 2 3T 2G here. So now the way we combine D 1, D 4, D 6, D 9 is it possible to combine D 2, D 8, D 3, D 7 because of the similarities in case of nickel 2 plus a D 8 system has two holes in EG the way we had one hole in case of D 9 one electron in D 1 here in case of D 2 and D 8 if you consider D 2 we have two electrons whereas in case of D 8 system we have two holes in EG. Promoting one or two electrons to EG means transferring the holes from EG to T 2G level. So here 3P is not split 3P is not split because if you recall all T states P orbits are triplet generate we have T 1U we call it as SP 3D 2 when you consider or when you go to Ligand field theory the pre orbits are designated as T 1U. So they still degenerate they have the same energy. So they do not split whereas 3F is split into three states and will be inverted here. So 3A G 2 will be ground state term similarly D 7 is similar to D 2 and D 3 is similar to D 8 in octahedral environment. If you consider chromium 3 plus a D 3 system is expected to show three peaks. Here what we have shown is for electronic spectrum of hexa aquanical 2 plus D 8 system it shows three transitions as expected from these explanations. So here A 2G is the ground state A 2G 2 T 1G P and A 2G 2 T 1G F and then 3A 2G 2 3T 2G F. So we can see three distinct DD transitions here the same thing is shown here again. So now let us look into these systems to identify the similarities D 2, D 8, D 3, D 7 octahedral high spin complexes D 3, D 7, D 8, D 8, D 3, D 7, D 8, D 2 high tetrahedral high spin complexes. So now again is it possible to combine the octahedral and tetrahedral complexes having this electronic configuration to a single diagram to explain transitions happening in this type of complexes having D 2, D 7, D 3, D 8 octahedral as well as tetrahedral geometries. So here all these cases can be conveniently combined into a single diagram again called as Orgel diagram which describes the qualitative way of the effect of electronic configuration with two electrons, two more electrons than half field sub shell and two less electrons than a full shell and two less than half field shell. So that means basically that covers all and this is how you can write a typical Orgel diagram consists of D 2, D 7, D 3, D 8 electronic configuration for both octahedral as well as tetrahedral complexes in this fashion here. Of course here I have shown three transitions of D 8 system starting from 3A 2G 2, 3T 2G and 3A 2G 2, 3T 1G F and then 3A 2G 2, 3T 1G P. You can see some difference here if you see this energy level it has bent downward whereas the 3P state is bent upward. So why this is happening you can see here. So if they have been straight they would have gone like that. Why they are bending means they are deflecting the energy when bends further down and high energy level bends further up to minimize their interaction. Why that happens here? Explanation is shown here there are two three T 1G states one each for 3P and 3F state. Both T 1G states are curled because they have the same symmetry and they interact with each other. So inter electron repulsions lowest energy of the lowest state and increases the energy of the highest state. The effect is much more marked on the left of the diagram because two levels are closed in energy that is what we saw. If the lines have been straight they would have crossed each other which implies that at crossover two electrons in autumn have the same symmetry and same energy that is forbidden that is not allowed that is prohibited. This is impossible prohibited by non-crossing rule. So state of same symmetry cannot cross each other the state of same symmetry cannot cross each other. The mixing or inter electronic repulsion which causes the bending of the lines is expressed by rocker parameters B and C. So B and C can be calculated from linear combination of exchange integral and Coulomb integrals but they are obtained empirically from the spectra of the free ions. So that means basically to what extent that means when you predict from theoretical values they are different from the observed or experimental values. So in order to make the correction so that the experimental values would be same as theoretical values you have to incorporate rocker parameters. So these rocker parameters B and C can be calculated from linear combination of exchange integrals and Coulomb integrals and how to obtain them they are obtained empirically from the spectra of the free ions. So now let's look into the chromium 3 state how here splitting happens having 4 f and 4 p states here you can see here f state is lower in energy and p state is increasing energy because of mixing whereas these two states are not affected. So that means basically when we look into observed and measured data there is no change in this transition value whereas you can see decrease in the value of this one whereas increase to this extent here. So we have to account for this in both the cases decrease as well as increasing. For transition between the same multiplicity state B is enough to explain the position of the bands for different multiplicity we need both the value of B and C in case of D3 for V2 plus iron vanadium 2 plus iron separation between 4f and 2g is 4b plus 3c. So for B is approximately 700 to 1000 centimeter minus and C is 4 times that of B. So due to the mixing of p and f terms energy of 41gp is increased here by an amount x this you can call it as x and that of T1gf is decreased by an amount y this is y and this is x. So now let us come to understanding this anomaly here we use the term f laxatic effect. So now let us consider again chromium 3 plus iron D3 and B and C are known B value is 918 centimeter minus and C is 4133 centimeter minus. Now the observed value is 14900 and then this is 22700 and nu 3 is 34400. So predicted one is there is no change and then here it is predicted one is high 26800 and then this one is low we know that this is going up. So it is high and then this is going low so this is low so compared to observed values. So B relates to a free ion the apparent value of B prime in a complex is always less than that of a free ion value because electrons on the metal can be delocalized into m o's covering both metal and the ligands. So use of B prime improves the agreement this localization is called nephelacetic effect and nephelacetic effect ratio is given by beta equals B prime over B. Beta decreases as delocalization increases but always less than 1 B prime equals 0.7 to 0.9 B. If all the transients are there then 15 B prime equals mu 3 plus mu 2 minus 3 nu 1 or 15 B prime equals 15 plus 2 plus 18 minus 30 this is the value you can take and then of course raka parameters for B raka B for trans metals in plus 2 state and plus 3 state is available one can take directly from literature and then when you apply here as I mentioned this increases by x and this decreases by term y and then we add these values and do the correction of course no change here this is coming very close now 18 dq minus x and here 12 dq plus 15 B prime plus x it would come around something like that so now after correlation it comes very close to the observed value and magnitude is about 34400 plus or minus 400 is allowed and similarly here 22700 and 22400 plus or minus 300 is allowed and here it is very accurate because there is no change in the electronic levels so this is how we can use raka parameters to do corrections for the theoretical values so that that tal is with the observed value. So, let me stop here and continue in my next lecture about spectra of d5 ions high spin d5 ions. So, thank you so much.