 Hello everyone. Once again, I welcome you all to MSB lecture series on advanced trans-traumatical chemistry. In my previous lecture, after completing Welles-Born theory, I gave introduction to Christoph Field theory by going back to historical background and looking into the contribution of several physicists along with chemists to define this very important theory to explain almost all aspects of coordination compounds. So now, let me continue from where I had stopped. I shall introduce soon Jan Teller theorem also in this lecture. When we consider six coordinate octahedral complexes, this is how Christoph Field theory investigation began by considering six coordinated complexes. If you consider any trans-traumatal complex for that matter, the 5D orbitals of these cations are degenerate and have equal energy. The electric field around a metal cation results in the total energy level being lower than that of a free cation because of the electrostatic interactions and there will be some refulsive interactions because ligands are carrying electrons and also metals have electrons. As a result what happens the negative electric field destabilizes the system and compensates for the stabilization to some extent. Now, it would be interesting and informative to consider three geometries octahedral field, square panor as well as tetrahedral field. And for convenient and better understanding let us try to confine octahedral molecule, a square panor molecule and a tetrahedral molecule inside a cube with center occupied by metal ion and surrounding six ligands in case of octahedral and four ligands in case of both square penons and tetrahedral geometries. And these green spears represent ligands and their approach towards the central metal atom to establish metal to ligand bond and same thing here and same thing here. If you just look into octahedral molecule before we begin we should also consider this Cartesian coordinates and at origin we have kept the metal center. Now we can see how the ligand approach direction are coinciding with this Cartesian coordinates. Why this is important when we place all 5D orbitals at Cartesian coordinate we know that dz square is along z axis and dx square minus y square is along x and y axis and dx z dy z and dx y are between the respective axis this is how the orientation. And now if you just look into the direction of approach of ligands here and it is very easy to understand how these ligands will be interacting with 5D orbitals. That means in case of octahedral these six ligands will be interacting with two orbitals that are coming in their way that is dz square and dx square minus y square whereas other three remaining orbitals such as dx y dy z and dz x are in between the axis as a result what happens they are having much lesser interactions and this is where the splitting of the orbitals into EG and T2G with T2G having slightly lower energy and then EG having higher energy comes into the picture. Now let us consider a square planar complex in this one four ligands are approaching and if you assume this molecule is placed with principal axis being z. So in this case what happens when the four ligands are approaching the metal ion they are approaching from x minus x y and minus y direction. So in this case what happens since no ligands are coming along z direction that is least affected that means and then rest is very similar to what we saw in case of octahedral complexes. Now let us consider tetrahedral complex in which four ligands are occupying alternate corners of cube and when these four ligands are occupying alternate corners of cube and not this one you should ignore this one. So one here one here and one here and one here. So these four ligands are approaching the metal from alternate corners and when they approach and establish bond we will end up with a regular tetrahedral with regular tetrahedral angle here. So in this case again if you place all the five orbitals along this Cartesian coordinates depending upon their orientations and you will notice clearly that dz square and dx square y square are not overlapping directly with any of these four ligands. In contrast dx z and dy z and dx y would be having little larger overlapping with the direction of approach of ligand as a result. Here the energy levels reversal takes place compared to octahedral. This is what Kramer also depicted much before fully prepared crystal field theory was proposed by Van Bleyck. So in fact this observation was also made by Kramer and others when they were working on crystal field theory before Van Bleyck added all these ingredients to explain crystal field splitting in all geometries. So this is octahedral, this is square planar and this is tetrahedral. Now you can see in this one this is for octahedral and this is for tetrahedral and if you see the magnitude also in case of octahedral and CFS in case of tetrahedral a remarkable difference is there and in fact you can see the difference is quite considerable. That means usually weak field ligands form tetrahedral complexes whereas strong field ligands prefer octahedral complexes and whenever we come across a tetrahedral complex usually the ligand field is smaller and the splitting is smaller and there may be one or two exceptions but nevertheless this is followed all through uniformly. So now let us look into the magnitude of crystal field splitting and of course since it can explain the transitions because crystal field theory says that electrons are no longer degenerate in DR builds and it is split into two or more levels. As a result what happens one can anticipate electronic transition since one can explain electronic transition we can as well explain color of the complex using crystal field theory and color of the complex depends on magnitude of this crystal field separation energy when we consider a metal larger metal and we expect larger CFSC and metals with higher access state will be having larger CFSC and for understanding the color of the complex and magnitude of CFSC we should look into the positions of ligands in spectrochemical series and this is a spectrochemical series for few ligands I will be giving a consolidated spectrochemical series much later and if you see here among them chloride is the weakest ligand and then fluoride comes then water comes and then ammonia, ethylene diamine and nitrate N bonded one and the cyanide. So here crystal field supply agency is also increasing here and the ligand field strength is also increasing here. Now what it says is weak field ligands low electrostatic interaction as a result crystal field splitting will be very small and high field ligands high electrostatic interaction between the ligands and the metal ion as that results in large crystal field splitting you can see here how steadily it is increasing as the ligand field strength is increasing. So that means it can also give you the magnitude and also it can give you idea about the nature of the ligands whether they are weak ligands whether they are strong ligands whether they are of intermediate in nature. So this is the origin of spectrochemical series and here you can see here exofluorochromium 3 minus is there all are octahedral complexes only ligands have been changed here when the ligands have been changed from fluoride to water, water to ammonia, ammonia to cyanide and color also changing from green to violet to yellow to again yellow and you can see the separation is steadily increasing that means this clearly shows cyanide complexes are much more stable compared to fluoro or halo complexes in general. So now let us look into the electronic configuration in octahedral field when we try to ionize and generate cationic metal ions what would happen is first s electrons are lost and then that means 2 electrons present in n plus 1 s orbital will be lost and if further oxidation takes place then the electrons will be in a sequence removed from inner d orbitals. For example titanium 3 plus we have 3D2 forest electronic configuration we are left with one electron so it is a D1 system. Similarly vanadium 3 plus if you consider it is a 3D2 forest 2 electronic system if you remove 3 electrons it is a 3D3 forest 2 electron system and if you remove 3 electrons here will be left with the D2 system here and then in case of chromium we have 3D4 forest 2 system and if you remove 3 electrons will be left with D3 system and first 3 electrons are in separate d orbitals with their spin parallel that means while filling electrons Hund's rule is followed and fourth electron has a choice for example if you have 3 electrons automatically they will go to individual orbitals separate orbitals and the fourth electron comes it has a choice of going to fourth orbital or it can also get paired something like this. So that means here we have to introduce one more term the pairing energy higher orbital if delta is small that means high spin complexes and lower orbital if delta is larger low spin complexes will be there that means in case of weave field decant this separation is small as a result what happens electrons would be promoted easily rather than getting paid that means the CFSC is much smaller than pairing energy as a result what happens it is instead of getting paid so it will go and occupy higher position and then in case of strong field ligands we have larger separation is there as a result what happens pairing energy will be much smaller than the promotional energy as a result what happens they will be get paid instead of going to the higher orbital. So that results in low spin complexes in contrast in these cases where ligand field is small and this separation is small it is easy to promote electron we ended up with high spin complexes and you can see the again magnitude can be very nicely compared here one with hexafluorocobaltate here and your hexasino cobaltate here so you can see one is the weakest ligand one is the strongest ligand and that is reflected in the magnitude of this CFSC and this compares three important geometries we come across among coordination complexes this is say typical octahedral complex and here the corresponding orbitals have shown with relative orientations and here we have EG set we have T2G set and in case of tetrahedral exactly opposite takes place and E are lower in energy and T2 are higher in energy but the magnitude is smaller and then in case of a square planar what happens further splitting takes place and mostly d8 majority of them are low spin complexes with an exception of a few n equal to plus complexes for example if you take palladium 2 plus palladium 2 plus iridium plus and gold 3 plus and also rhodium plus most of them they possess the square planar geometry. So now let us move on to John Teller theorem Herman Arthur John and Edward Teller full name okay mostly books introduce them as John Teller but actually the full names are Herman Arthur John and Edward Teller published about distortions in the geometry of octahedral molecules in 1937 it states that I quote any non-linear molecular system in a degenerate electronic state will be unstable and will undergo distortion to form a system of lower symmetry and lower energy thereby removing the degeneracy. So that means this is called as John Teller distortion and especially when we have uneven occupation of electrons in EG orbitals or E orbital this is what exactly happens that means odd electrons if they are present in d4 system or d9 system this John Teller distortion is more pronounced ideal examples are d4 system chromium 2 plus and d9 system with copper 2 plus. Is it T2G orbitals or EG orbitals or both that means here the question is how John Teller distortion is affected is it because of filling of T2G orbitals or filling of EG orbitals or both let us try to answer these questions. In an octahedral ligand field the T2G orbitals are at lower energy than EG orbitals this is due to the orientation of T2G orbitals away from the ligand direction that you are all familiar already whereas the EG point along bond axis okay EG z minus z x minus x y minus y the shielding effect this has on the electrons is used to explain why the John Teller effect is only important for the EG level with odd number of electrons. The John Teller distortion is well documented in the case of copper 2 complexes as their d9 system with 3 electrons in EG level. So that means when we have 3 electrons in EG level we have 2 options of filling them that means I can put 2 electrons in dz square or I can put 2 electrons in dx square and 1 electron in dx square and 1 electron in dz square in this case. So what would happen let us say I put 2 electrons in dz square and 1 electron in dx square so that it has d9 electronic configuration in this case what is the sequence of this kind of arrangement on stability of the complex. Now when 6 ligands are approaching the metal ion what happens because I have put 2 electrons here those approaching from z and minus z direction would experience maximum repulsion and try to stay a little away from the metal center whereas here since only one electron is there 4 ligands approaching along x minus x y minus y would undergo or experience less repulsive forces as a result they come little more closer to metal ion compared to what we will see along z direction and contrast here other way around happens. So here if you put 2 electrons in dx square minus y square orbital and 1 electron in dz square here 4 electrons approaching the metal from x minus x y minus y direction experience larger repulsive forces and as a result what would happen they will stay away from the metal ion relatively compared to 2 ligands that are approaching along z direction they will come little closer as they have to experience repulsive forces because of only one electron present in dz square. So that means here we have 2 longer bonds and 4 shorter bonds and here we have 4 longer bonds and 2 shorter bonds you can decide yourself which would give more stability having 4 weaker bonds and 2 stronger bonds or 4 stronger bonds and 2 weaker bonds that means tetragonal elongation is more pronounced when metal ion has an option compared to tetragonal compression because here it will be little bit less stabilized because you have 4 longer bonds and 2 shorter bonds. So with this information we call this as z you can see something like this we can see this one z elongation 2 long bonds and 4 short bonds will be there whereas in this case z compression happens that compression means basically it comes like this case of z compression elongation means it goes like that when you are pulling up in z direction those ligands will come closer in the plane and that is when if you compress this z direction along the axis will be coming closer and they will be expanding. So z compression happens in this case. Let us look into some examples where we observe the John Taylor distortions I have listed some examples here CuF2, CuCl2, CuBr2 and here in all of them the metal ion copper is in plus 2 state and you should not assume just by looking into the composition that they are linear molecules no in 3 dimensional all of them are surrounded by 6 fluorine atoms or 6 halogen atoms in general in all these cases. So that means here if you look into the crystal structure of CuF2 that means copper is surrounded by 6 fluoride ions you should remember 4 fluorine atoms at a distance of 193 picometer whereas 2 fluorine atoms are at a distance of 227 picometer. So that means here they are 2 or longer and 4 or shorter you can say it is tetragonal elongation. But if you look into CuCl2 again 4 bonds are at 230 picometer and 2 bonds are at 295 picometer these 2 are longer and 4 are shorter again tetragonal elongation. In case of bromide you can see 4 shorter bonds and 2 longer bonds along that direction. So in all these cases we invariably tetragonal elongation and same thing is true in this case as well. And that can be extended to d4 system also d4 system John Taylor distortion is slightly less pronounced because we have only one electron here whereas here we have 3 electrons. So nevertheless it also shows John Taylor distortion that is reflected in this bond parameter. If you just look into it 4 bonds at 200 picometer and 2 bonds at 243 picometer. Similarly if you look into this molecule here 4 bonds at 214 picometer and 2 fluorines are at 200 picometer. So that explains that means is it possible to see tetragonal compression as well. Yes there are examples where we come across tetragonal compression as well but if tetragonal compression is there it is very difficult to predict probably it is because of solid state effect. What is solid state effect? By assuming tetragonal compression probably the packing will be very efficient and they can be densely packed and as a result what happens lattice energy increases and stability increases. In those cases wherever the energy gained through lattice energy by having compression if it compensates the energy loss due to you know John Taylor elongation in those cases what would happen one can observe tetragonal compression. There are examples for tetragonal compression as well but there you may not be able to explain unless you look into solid state effects or one can verify that one through spectroscopic measurements electronic spectrum one should record and look into transitions and sometime it can guide you. Complexes of the type I have shown here complexes of the type M2, PB, copper NO26 shows temperature dependent John Taylor distortion. For MEQCCM shows Z elongation below 285 Kelvin with complex ion adopting tetragonal symmetry whereas for MEQSK potassium distortion occur at below 273 Kelvin. So that means here temperature assisted John Taylor distortion one can see and similar distortions were also observed with MEQS rubidium at less than 276 Kelvin and in the case of MEQS thallium at less than 245 Kelvin and then that means above these temperatures the molecules appear octahedral due to the dynamic John Taylor effect. So that means low temperature what happened the splitting happens distortion happens tetragonal distortion happens but higher temperature it does not happen so this is called dynamic John Taylor effect. Let me stop here continue discussion on John Taylor distortion in my next lecture until then have an excellent time reading about crystal field theory.