 Hello everyone, welcome to MSP lecture series on advanced transformational chemistry. In my previous lecture I discussed about the impact of ligand field on the energy of various d orbitals to make you familiar in writing crystal field splitting diagrams for various geometries and let me continue from where I had stopped. Again I did mention about how similarities one can envisage in case of tetrahedral as well as cubic crystal field splitting and having the same splitting pattern. Let us look into more examples now. First let us look into the square planar crystal field splitting that you are all familiar and also I showed in my previous lecture how they split. You can see here 4 ligands are in the plane and if you assume molecule is sitting with z axis perpendicular to the plane of the molecule that means molecule is placed along x and y plane so that we have 4 ligands approaching along x minus x and y minus y direction and no ligands are approaching along z direction. As a result what happens any orbit that is oriented in the z direction or in the z associated planes their energy will be low and hence dz square and dx z and dy z have relative lower energy but on the other hand 4 ligands are coming along x y plane as a result dx square minus y square will be having higher energy and then dx y has partial overlapping and it has little lower energy compared to dx square minus y square and this is the typical crystal field splitting diagram for square planar complexes here. So now let us look into an interesting geometry that is linear geometry. Linear geometry I did mention about a few examples diamines silver complex and also I did explain about how Welles-Bern theory explains in a very unusual way taking 3 orbitals and making 2 sp orbital through the mixing of dz square s and pz orbitals. Let us look into the splitting diagram using crystal field theory to understand the relative energies of d orbitals and let us assume the linear molecule we are considering is placed along z axis. So something like this is a linear molecule a metal center is coordinated to 2 ligands in a linear fashion. As usual we have to now write the crystal field splitting for which we have to understand the orientation of these 5 orbitals with respect to this linear geometry that we have given for this ml to molecule. And to begin with simply write Cartesian coordinates and identify 3 planes here x y plane and x z plane and y z plane here. Now I place like this now I have to see the consequence of replacing this one in this fashion on various d orbitals. This is dx minus y square and then this is a dz square of course it is greatly affected because these 2 ligands are also coming in the same direction and then we have dxy orbital and then again to make you familiar to give stress upon your understanding I put again linear molecule here and I will take it out and add x z and dxy is already added y z I have added and I have already x z. Now we should try to write the relative energies of d orbitals under the influence of linear crystal field. Let us consider so we are considering these 5 d orbitals obviously you should know which one is here because this is dz square the molecule is in this direction here and now the second one will be dx z and dy z and the least energetic ones are dxy and dx square minus y square. So this is for linear crystal field splitting so it is very easy is that right. So now another interesting one hexagonal planar geometry and if you recall Wernher's coordination theory he prepared a series of orbital complexes having different composition like MA6 and MAB5 and MA2B4 and MA3B3 to identify which geometries would give isomers and accordingly he try to isolate as many isomers as possible and later he concluded with that important experiment or work that for coordination number 6 the most preferred geometry would be octahedral and have you come across any examples of transmittal complexes having hexagonal planar geometry it is very difficult because you have to put lot of stress on orbitals to orient in this fashion by by distorting their original positions and that really makes unstable as a result we do not come across examples for hexagonal planar geometry especially when coordination number 6 is there the most preferred geometry is octahedral and the alternate one we have whatever disposal is trigonal-pismatic geometry but nevertheless let us try to look into it and find out whether any example is there or not in the literature. This is hexagonal planar so as usual I place Cartesian coordinates and look into relative positions of various orbitals here to understand the impact of them on the 6 ligands approaching in this XY plane you can see the 6 directions in which 6 ligands are approaching the matter to establish MA6 having hexagonal planar geometry it appears like a imaginary but later you will be surprised to see a result in the literature. So this is how I have placed some orbitals here you can see which are the orbitals I have placed here I have placed d x comma y square and d z square and d x y now I should place again Cartesian coordinates and add x z and y z here so that means if you go back here you can see that since 6 ligands are in the plane now whatever the 6 ligands are in the XY plane whatever the orbitals that are present in the XY plane will be affected more and once again if you see here d x y has maximum overlap with the direction of approach of 6 ligands compared to d x square y square as a result here the energy of d x y will be much higher compared to d x square plus y square next comes d z square we do not have anything and same things to in case of d x z and d y z so that writing in the crystal field splitting diagram for hexagonal planar geometry would be very easy once again right consider 5 d orbitals and then here it splits into 4 levels now I have shown 4 levels out of which third one is from top or second one from bottom is doubly degenerate and as I mentioned d x y has maximum overlapping with direction 4 ligands would have overlapping with d x y when they are approaching along the XY plane so d x y will be much higher in energy the next one is d x square y square next what we have is d x z and d y z and the least energetic one is d z square so this is how one should be able to write crystal field splitting for hexagonal planar molecule if at all if it exists among coordination compounds of 3d 4d or 5d it is very easy is that right so now I gave a list of several geometries I am going to discuss one or two more but nevertheless you make an attempt to write crystal field splitting diagrams for all geometries that I showed or you can look into various polyhedra and also you make an attempt to write crystal field splitting diagram for them the question is any metal complex is known in the literature with coordination number 6 having you know hexagonal planar structure yes there was a paper appeared in nature in 2019 that of a palladium complex having hexagonal planar geometry so this is the molecule shown here you can clearly see palladium is coordinated to 6 ligands 3 hydrates are there and 3 magnesium moieties are there and here magnesium is attached to a ligand called knack-knack and this ligand is derived by reacting with a very bulky primary amines with acetyl acetonide and these ligands have been extensively used in various aspects of both main group chemistry and transformative chemistry and here one interesting thing is they have made a magnesium complex this is a mono anionic so magnesium is still has one electron to donate and now three such bulky groups are attached to okay palladium in this fashion and in between we have three hydrogen atoms are there and then if I ask you to identify the oxygen state of palladium and oxidation of state of palladium in this molecule comes to 0 because three anionic are there and three cations are there eventually they cancel and the palladium is a detent and now all 6 ligands are according to covalent method one electron they are donating this is a D10 16 electron complex and this is how the structure looks like that you can see here how the entire metal coordination sphere looks planar here this came in nature in 2019 this is the only example we have to show that this very unusual geometry is also quite possible with transfer metals it is it is very interesting and it is a surprise result here so now let us look into hexagonal bipyramidal crystal field splitting in my previous slide I showed you about writing crystal field splitting diagram to hexagonal planar now let us see how to write the similar crystal splitting for hexagonal bipyramidal geometry here only the difference between the previous one hexagonal planar and hexagonal bipyramidal is we have two more ligands approaching along z direction so that means they have an impact on dz square orbital and probably energy of that one is elevated in contrast to what we came across in case of hexagonal planar that is going to be the only difference or anything else we shall see so this is the typical hexagonal bipyramidal molecule and the metal is at the center to distinguish between axial and the planar I have given different colors for the ligands does not matter it is a homolyptic or heteroleptic now again consider these 5D orbitals and place Cartesian coordinates and place this molecule here and place one you can see now the impact on dz square and then you can also see the impact of dx minus y square and then dx y the impacts are very similar to what we saw in in previous example of hexagonal planar except the influence of dz square rest would remain same so now let me write crystal splitting diagram for this one so 5 are there here so now I have written 4 energy levels with the least analytic one is a doubly degenerate obviously you can make out which is this one the orbits that are least affected when you visualize hexagonal bipyramidal molecule are dx z and dy z and the the maximum affected orbital is dz square because two ligands are coming in the right in the same direction so this is dz square and as usual dx y will be having maximum overlapping with 4 ligands in the plane as a result this is dx y the one left is without any problem one should be able to write what is this one this is dx minus y square so this is how hexagonal bipyramidal crystal splitting can be drawn in this fashion to show the relative energies of various orbitals hope you have understood how to write in a simple way quotation coordinates and then putting the geometry at the center and then try to place the ligands to understand their influence and writing the appropriate crystal splitting diagrams so let me summarize now crystal field theory crystal field theory provides a basis for explaining many futures of transmittal complexes that you saw in my last two lectures and examples include why transmittal complexes are highly colored and why some are paramagnetic while others are diamagnetic so that information about magnetism also comes the spectrochemical series for ligands he explains nicely the origin of color and magnetism for these compounds and there is evidence to suggest that the metal ligand bond has covalent character which explains why these complexes are very stable and of course that I did not really mention about that nephelacetic effect we say and that is coming under ligand field theory I am going to start that one and molecular orbit theory can also be used to describe the bonding scheme in these complexes to understand better about the significance of the ligand field one should go for either ligand field theory or I would say molecular orbit theory nowadays ligand field theory and molecular orbit theory are almost same a more in-depth analysis is required however to understand all aspects using molecular orbit theory as it involves tedious calculations so let me stop today and begin my next lecture on molecular orbit theory until then have an excellent time reading chemistry