 Hello everyone. I once again welcome you all to MSP lecture series on advanced transition metal chemistry. This is the 15th lecture in the series. In my previous lecture I did discuss about John Taylor distortion and I was telling you about complexes of this type. Here you can see clearly 6 anionic ligands are there and copper is in plus 2 state to compensate 4 charges we have led in plus 2 state and 2 alkali metals in their plus 1 arcane state. So, it is now it is a charge balanced complex and this complex shows temperature dependent John Taylor distortion. The interesting thing is with variation in the alkali metal or cation it shows distortion at different temperatures. When m is ccm it shows tetragonal elongation below 285 Kelvin with complex ion adopting tetragonal symmetry whereas for m equals potassium distortion occurs at below 273 Kelvin and once again similar distortions were observed for m equals rubidium at less than 276 Kelvin and when m is thallium it appeared at less than 245 Kelvin and above these temperatures the molecule appears octahedral due to dynamic John Taylor effect. So, it is not only John Taylor distortion we come across we can also see dynamic process with temperature. So, I have listed all electronic configuration d1 to d10 to just assess what electronic configuration really shows John Taylor distortion in complexes in case of both high spin and low spin if you just see when we have one electronic d orbital irrespective of high spin or low spin. So, they show weak John Taylor distortion and when we have d4 electronic configuration that means eg has one electron and also in case of d9 where we have 3 electrons we come across very strong John Taylor distortion that is what we saw in case of chromium 2 plus and copper 2 plus octahedral complexes. In case of d5 and d3 d8 and d10 we do not see at all and in case of d6 and d7 we come across again weak John Taylor effects and in case of low spin whether you take d1, d2, d4, d5 in all these cases we do come across weak John Taylor distortion whereas in case of d3, d6, d8 and d10 we do not see at all only in case of d7 and d9 we do come across John Taylor distortion in case of low spin complexes of course in case of d9 irrespective of there is low spin or high spin it is going to show strong John Taylor distortions. Then fine theoretically by just looking into the eg electronic configuration to an extent t2g electronic configuration we can predict the possibility of a complex showing John Taylor distortion but how to assess the presence of John Taylor distortion in complexes two important aspects that comes to our mind to assess this John Taylor distortion or electronic spectroscopy and crystal structure data of course crystal structure data is quite straight forward and the moment you determine x-ray structure you will come across variations in the bond length that is bonding parameters that should tell you whether its elongation is observed or compression is observed. How electronic spectra of you know these complexes vary let us look into this here in the absence of John Taylor distortion absorption spectrum shows only one transition whereas the complexes with the tetragonal distortion usually show two transitions and we call it as double hump pattern. How this double hump pattern appears ok this is the one without John Taylor distortion you can see simple one absorption here and when we see John Taylor distortion we see two intensities here and of course one is weaker and one is stronger then how to visualize this one through electronic transitions. So let us look into the electronic transition that are responsible for showing this kind of absorption without John Taylor distortion you can always anticipate one transition from T2g to Eg but irrespective of whether we have z elongation or z compression with tetragonal distortion due to John Taylor effect they show two electronic transitions and this is the one and this is the one here in case of elongated complexes in case of compressed complexes also we see two electronic transitions and this explains why we see double hump pattern like this in this pattern. So that means there is a way to assess and understand whether John Taylor distortion is there or not if it is there to what extent it is effective whether it is weaker or stronger can also be assessed simply by looking into the absorption spectrum of a particular complex. So what is second order John Taylor distortion? The second order John Taylor distortion occurs when the excited state of the transition metal has unequal occupation of the orbits with identical energies. That means once when you excite an electron from the ground state to excited state and if the excited state has unequal occupation of the orbitals very similar to D4 and D9 then we can anticipate second order John Taylor distortion. The ground state of the transition metal may not have unequal occupation of degenerate orbitals though so therefore the second order John Taylor distortion is also called pseudo John Taylor distortion and if you just want to look into the potential energy surface this is how it looks and this is at high symmetry point where there is no effect and this is in case of John Taylor effect whereas in case of this one it is pseudo John Taylor effect. You can see here splitting between the two electronic levels and this is again high symmetry point and this electronic state two and electronic state one similarly electronic state two here and electronic state one is here. So you can also see pseudo John Taylor distortion, regular John Taylor distortion and also second order effect will also be there this is because of the excited state in which uneven filling of electron or uneven electron occupation is observed. Now let us look into high spin and low spin complexes of the type hexa aqua M with a particular axis state for metals having D1 to D10 electronic configuration. So I have listed here for 3D series starting from D1 to D10 and the D1 the most common one here if you consider it is a titanium 3 plus and one electron is there and in case of titanium 2 plus D2 system we have 2 electrons are there and vanadium 3 plus 3 unpaid electrons are there and in case of D4 chromium 3 plus we have 4 unpaid electrons are there and this entire series is due to high spin complexes and in case of D5 we have 2 levels both are singly occupied. So maximum unpaid electrons can be seen example MN2 plus so this is 3D5 and 4S2 when you remove 4S electrons it will be D5 system similarly 3D6 4S2 remove 3 electrons so that your fee will be in plus 3 state this is D5 and in case of D6 again F2 plus and cobalt 3 plus we have 4 unpaid electrons and in case of cobalt 2 plus D7 system we have 3 unpaid electrons and nickel 2 plus octahedral we have 2 unpaid electrons D9 system we have 1 unpaid electron and in case of D10 zinc cadmium mercury so we do not have any unpaid electrons at all and of course only D4 D5 D6 D7 show both high spin and low spin complexes and the electronic configuration I have given here this series is for low spin complexes and this series is for high spin complexes and this is a consolidated diagram that shows various crystal field splitting of the d orbitals under the influence of different ligand field you can clearly see here this is for tetrahedral and of course this is degenerate system in the absence of ligand field and when you have this octahedral ligand field this is how the splitting is and then if you go for D A, B, C is this one and if you go for D I have listed here tetragonal elongation and when you have tetragonal elongation this T2G as well as EG will be further split and this is what exactly looks like and if you recall this is very similar to a square panner and of course when you have strong this ligand field the DZ square again comes below DXY and DXY goes little bit up so that means elongation has something to do with square panner geometry as it shows similarities and then the E square panner complex you can see here as I mentioned it comes little down here and this is a typical square panner splitting and in case of F this is for trigonal bipyramidal one and also you can see the relative CFAC, crystal field separation energy and also relative separation between the HOMO and LUMO in case of molecular orbital theory we frequently use highest occupied molecular orbital and lowest unoccupied molecular orbital the gap between that one is called crystal field separation energy same thing we use in ligand field theory also so keep that in mind and this is how one can show consolidated diagram for various ligand fields. So now let us look into the relationship between octahedral and square panner which goes through tetragonal distortion and then losing one ligand to HAU square pyramidal and eventually when you take off this ligand also in axial position we will end up with square panner so that means when you change from octahedral to distortion tetragonal distortion in particular tetragonal elongation and then get rid of one ligand to have square pyramidal geometry and then get rid of this one to end up with square panner geometry then let us look into the relative positions of various d orbitals when we try to do this transformation from octahedral to square panner you can see here the relative energies and how they are going to transform can be clearly seen here this is tetragonal elongation of course tetragonal elongation remains more or less same for square pyramidal there is no change that means the crystal field splitting whatever we write for octahedral with tetragonal elongation holds good for square pyramidal geometry as well so that means if you remember octahedral remembering for tetragonal elongation by looking into what actually happens to these orbitals and then that remains more or less same for square pyramidal geometry and then here only the difference between square pyramidal and square panner is dz square comes further down whereas dx by energy is slightly elevated that is it so that now remembering the crystal field splitting is rather much easier again I have listed here the relative energies of various d orbitals with respect to the corresponding ligand field I have shown here this is what exactly I wrote in the previous one and this is in this is written in a different format here okay and here I have included the square planar trigonal bipyramidal and square pyramidal and octahedral and also here I have included pentagonal bipyramidal and here square antiprism I have included and here this is tetrahedral so many textbooks give this kind of splitting diagram only for standard octahedral and tetragonal elongation and compression and square panner geometries but not for many others there is a reason I thought I should include crystal field splitting for most of the geometries we come across in coordination chemistry with coordination number varying from 2 to 9 so now I have listed here various ligands in the order of their ligand field strength and this we call it as spectrochemical series and there is a background for spectrochemical series that is crystal field theory so crystal field theory through CFSC that can be measured using electronic spectroscopy later I shall elaborate more about electronic spectroscopy so using that one you can look into the relative strengths of these ligands and you can put them at appropriate place or give appropriate rank in the spectro chemical series so this is in the increasing order of ligand field strength and among them I have listed here short one iodide is the weakest ligand whereas cyanide, carbon monoxide and tertiary phosphines are the strongest ligands and of course when you look into crystal field theory through CFSC you can judge whether a ligand is weak or strong with respect to another ligand but why a particular ligand is a weak ligand why a particular ligand is strong ligand that information really it does not come here it can only show through experiment what happens but when you go to molecular theory I shall tell you how to classify these ligands and why a given ligand is a weak ligand or a strong ligand or of intermediate strength that I shall clarify when I go to molecular theory so I have given an extended spectrochemical series here I have given an extended spectrochemical series here this includes most of the ligands we come across and also I have listed some important weak field ligands here for example water fluoride chloride and hydroxo and similarly I have also listed important strong field ligands carbon monoxide, cyanide, ammonia, triphenyl phosphine so before I try to make it very clear about how to write crystal field splitting diagram starting from octahedral so let me list some of those geometries we come across among coordination compounds the most common one is octahedral with coordination number 6 tetrahedral coordination number 4 and again square planar coordination number 4 linear we have 2 and trigonal planar 3 square pyramidal 5 trigonal bipyramidal 5 again trigonal prismatic 6 trigonal antiprismatic or octahedral same 6 hexagonal planar 6 pentagonal bipyramidal 7 and hexagonal bipyramidal 8 and cubic 8 do decahedral 8 bicarb trigonal prismatic 8 square antiprismatic 8 and tricapit trigonal prismatic 9 we have plenty of examples for each case among coordination compounds now to make you familiar with writing crystal field diagrams for any given geometry I will start with octahedral compounds this is what is very important you should be able to write Cartesian coordinates and then you should be able to identify different planes we come across for example if you assume this is z axis and this is x axis and this is y axis you should be able to distinguish between x y plane and x z plane and y z plane and then you should be able to assess the orientation of different orbitals with respect to Cartesian coordinates and also you can see their impact when the ligands are approaching the metal from different directions as per the defined geometry for that particular complex so first what you should do is write Cartesian coordinates and here the origin is there try to place your metal at this 0 and then identify the plane and you can see clearly x y plane and this is x z plane and this is y z plane now you start putting dz square orientation is in this direction now we know if the ligands are approaching in this direction what would happen to the energy of dz square in that particular ligand field can be clearly visualized here similarly I am going to put d x minus y square this is along x minus x and y minus y so whatever the ligands approaching in this direction would have impact if electrons are already present in this orbital now it is dx y dx y is slightly spread between x and y plane to take x minus y square and just rotate it by 45 degree that will change to dx y so this is also in x y plane but is between the planes now x z plane we have and then we have y z so this should be understood properly now once you understand properly so writing crystal fields between diagram for anything can be very easy and it is not memory based you should remember it is not memory based but you should have good analysis of impact on those things so that you can write relative energy of d orbitals with respect to any given crystal field or ligand field now I have written two octahedral molecules with the six ligands shown here and this is the place where metal is sitting so now I am putting here set of dz square and dx minus y square you can see here they are coming on their way directly that means energy of these two should be elevated higher increases and on the other hand if you see here dx y dx z and dy z and strictly speaking so they are not coming on the way of ligands but they are in between as a result the impact of ligand field is less on them their energy is lowered so now without worrying too much you should be able to write crystal field splitting doubly degenerate these two and triplet degenerate these three and this is how we arrived at the octahedral crystal field splitting here with respect to barycenter it is very easy so remember this one practice couple of times and you will not do any mistake in writing crystal field splitting diagram for other geometries. Now let us look into tetrahedral molecule and of course you can also write like this and to understand ligand field influence it is very ideal to imagine this tetrahedral molecule inside a cube something like this this is a cube here with the intention I have shown alternate corners with the different colored spheres. This is the tetrahedral molecule I am visualizing here this is the metallocenter and these are the red ones are the four ligands present at alternate vertices of this symmetry cube. Now once if you make connections to central metal from these four ligands then you will end up with a regular tetrahedral geometry and now what you should do is the way I placed Cartesian coordinates you place the Cartesian coordinates and then place all the 5d orbitals and you can see the influence of ligand field on this one. In this case what happens none of this dx square may square or dz square will be coming on the way of direction of approach of ligand in contrast dxy dyz and dxz would be having little larger overlap with the direction of approach of the ligand as a result reversal of octahedral splitting takes place in this one. Hence we write in this fashion crystalline splitting for tetrahedral molecules here. So energy is elevated here and if you just write you can see they have larger overlapping whereas here less overlapping so energy of E will be lower why we call E and T2 because it is non centrosymmetric so T2 dxy dy dxz and dyz. Now let us look into one more this is that tetrahedral molecule just I explained so here one system and I have another system here that means here I have used alternate corners in this fashion let me use other alternate corners to generate one more tetrahedral molecules what would happen if I superimpose this one on this one something like this if I superimpose these two it appears like the metal is common but it is making bond with all 8 corners having almost coordination number 8. This is what I am referring to cubic splitting so that means when the ligand 8 ligands are approaching the metal to form a metal complex with composition M L8 having cubic structure this is how you can visualize that means if I make an attempt to write crystal field splitting for this one it should be very similar to that of tetrahedral that means two tetrahedral molecules you superimpose on another one in opposite direction you end up with common metal and 8 ligands occupying 8 corners of cube so that means the crystal field splitting of cubic system is more or less is identical to that of tetrahedral so that means tetrahedral and cubic have the same crystal field splitting further it is simplified so in my next class I shall tell you more about several other geometries until then have an excellent time reading and digesting crystal field theory and especially chemistry of transient elements.