 Hello everyone, I once again welcome you all to MSB lecture series on trans-traumatic chemistry. I am sure you are having good time reading chemistry and also enjoying life despite we have this COVID problem. Today I shall discuss about another important topic in coordination chemistry especially when it comes to the substitution reactions of square panner complexes that is trans effect. You all know that in case of square panner complexes if you want to do substitution reaction the preferred intermediate would be having a coordination number of 5 preferably having trigonal bipremial geometry. So now let us see how this trans effect influences the entry ligand as well as leaving ligands while we perform substitution reaction. You know that as I already mentioned when we perform substitution reaction on a square panner complex the preferred one is expanded intermediate that means coordination number having 5 and again the preferred geometry for that one is trigonal bipremidal. So in this one I have shown here x is the leaving group and y is the entering group here and in the intermediate what happens one of the ligands and leaving group and also the entering group will form this trigonal plane and other two become axial or apical and then once the elimination process is over x leaves and it will revert back to a square panner complex and now depending upon the type of complexes we have we can see geometry chisomers. Let us discuss now with this introduction with this background let us start looking into the trans effects. So what is trans effect I already defined in several ways the trans effect is the labelization of ligands trans to certain other ligands which can thus be regarded as trans directing ligands. That means the labelizing effect of a ligand trans to itself or labelizing effect of a group trans to itself during substitution reaction to allow incoming ligand to a position trans to it is called trans effect that means it is attributed to mainly electronic effects important in square panner complexes and also observed to an extent in octahedral complexes. So now as I mentioned earlier we come across three class of ligands you recall my discussion about classification of ligands by donor atoms. So all ligands whatever we have at our disposal as coordination commits or organometallic commits we have three types of ligands one is pure sigma donor ligands that means they have low energy field sigma orbitals they interact with metal orbitals to form sigma complexes. The other one is sigma donor and pi donor complexes. So in this case we have low energy field sigma orbitals and low energy field pi orbitals on entering ligand and these ligands would establish bonding with metals usually in high valent state which are necessarily axophilic or halophilic in nature. So that is called sigma donor and pi donor ligands. The third one is sigma donor and pi acceptor ligands in those cases what we have is we have low energy field sigma orbitals and high energy empty pi orbitals. So they interact with appropriate metals to form this type of non-classical complexes. So now all these three class of ligands would need three different type of trans theories to explain the substitution reactions. So those three here are polarization theory, kinetic electrostatic theory and pi bonding theory and of course you should remember I did mention about the trans effect order. So this is the trans effect series you can see here for example if OH- is there that can be readily replaced by NH3 and it goes like this. Now let us try to understand one theory at a time and which theory is applicable for what class of ligands. Now first is polarization theory. So this was proposed by Greenberg hence it is called Greenberg's polarization trans effect theory. Here you can see I have written two diagrams here this is A diagram and this is B diagram. A diagram represents symmetrical induced dipoles in square planar complex of the type MX4 that means here all ligands are symmetrical and also we can see symmetrical induced dipole. You should remember if they are neutral ligands and we have a cationic metal according to Christopher theory what the type of interaction we come across is ion dipolar interaction. Since it is based on electrostatic so this is ion dipolar. So according to that one if you see if you consider four ligands which are symmetric then the induced dipoles will be symmetric in all four directions. So in that case what happens we do not have any preferred elimination of one of the ligand. Any ligand can go during substitution reaction. On the other hand if you have unsymmetrical induced dipoles at metal center because of difference in the electro negativity of the ligands then probably what we come across is unsymmetrical induced dipole that results in weakening of one of the bonds. So you can see here in case of this diagram we see three X ligands are there and one Y ligand is there here and if we assume Y is stronger ligand in that case what happens it induces polarity or it polarizes the metal center also and as a result metal will be having dipoles like this plus and minus. If the stronger ligand would have positive so that this bond is stabilized due to electro static interaction minus and plus whereas here this one will be having negative dipole pointing towards the ligand. So here both are negative so what happens repulsion starts as a result what happens this group starts leaving. So this is called polarization theory and if you just look into Mx3Y where the induced dipole of Y is greater than that of X as a result what happens this is how the arrangement happens and then this bond is stabilized whereas this bond is weakened so this is ready for departure. So this is the departing ligand and once this is departed so entering ligand would come and substitution reaction is complete. So that means this polarization theory holds good for ligands such as ammonia, water which are neutral ligands, neutral sigmodonor ligands. I hope it is clear example I have shown here. So water and ammonia etc. So fine now let us look into another theory that is called kinetic electrostatic theory. What is this one? For example I have taken a complex like this homoleptic platinum say two complex and in the slow step entering ligand comes and it establishes a trigonal bipreminal intermediate as I mentioned here during the substitution reaction one group along with leaving group and entering group will form trigonal plane and other two will be axial. In the first step X is eliminated once X is eliminated what would happen is this trigonal pyramidal geometry will revert back to square panor complex. So that means of course that it is a SN2 mechanism bimolecular substitution nucleophilic and both the availability of a vacant orbital that is PZ and the general accessibility of the central autumn in square platinum two complexes makes SN2 mechanism very plausible involving a trigonal bipreminal configuration with the most electron attracting ligands are apical and the electron repelling groups are trigonal. This is very very important when we want to explain trans effect using the kinetic electrostatic theory what we should remember is we have to gauge and analyze the nature of the ligands present on the metal before we form a trigonal bipreminal geometry that is electron attracting ligand pairs are apical and the electron repelling groups are trigonal. So that means if we have two negative ligands trans to each other they are electron repelling groups they will be trigonal planar. So in the first step this happens now let us look into couple of examples here recall how we made cis and trans platinum complexes we took first tetra chloroplautonate added ammonia when ammonia is added so here there is no preferred leaving of any of the chloride. So it is in symmetrical any one of these ligands can leave and then NH3 comes and again when you add another ammonia so now ammonia is going to cis position here. So this we observed whereas on the other end if you take tetramine platinum two complex add chloride again first one can go anywhere and next when we add another chloride it goes to a position trans to the chloride that is already present that results in trans complex. So now let us see whether we can explain this one in a more satisfactory way using kinetic electrostatic theory. Kinetic electrostatic theory holds good for anionic ligands such as halides. So now let us try to consider this trichloroamine platinum anion so now we want to add one ammonia before that one we should convert this into along with ammonia into a trigonal bipreminal geometry. For that what we should do is we should remember this one electron attracting ligands are apical and electron repelling groups are trigonal planar. So now let us try to see which are electron repelling groups and electron attracting groups electron repelling groups are these two both are negatively charged and because ammonia is there you can consider relatively electron attracting group is this one this is electron attracting group. Now we have entering group is there so with this one what happens now this electron attracting ligands are apical so this one is this should be apical. So the moment you put apical you know the fate of the isomer what kind of molecule we are going to get what kind of isomer we are going to get. So now this will be here this is coming here apical or axial now electron repelling groups are trigonal planar. So these two chlorides along with ammonia will be forming this trigonal plane here. So this trigonal plane of course in the first step you should remember this is the slow step and this is the rate determining step addition of ammonia. Now once chloride goes what would happen is already the fate of this molecule is decided so these two are there. So basically when it comes back this is going to be see anything in the plane is going to be cis to these groups here as a result what happens we are getting cis isomer here. So now let us consider the opposite of that one mono cationic triamine chloroplatinum. So now it is very different electron attracting ligands are apical here and then electron repelling group because one chloride is there. So this has to be the plane we can see here. So this is a has to be in the plane and this is in apical position. This is in the apical position now. So now NH3 comes and of course here plus is added chloride is added. So now once this ammonia is eliminated now what would happen is it will revert back since we have two ammonia here itself in the intermediate itself we have in mutually trans disposition we can always anticipate that we are going to get the trans compound that is what exactly happens and if you go back same thing happened here but obviously we had no explanation here with better explanation came into picture because we have understood what is the trans effect and how trans effect is playing when we are using anionic ligands such as halides using kinetic electrostatic theory. So what is important is this one should remember when we are generating an intermediate having coordination number 5 with geometry trigonal bipyramidal we should remember electron attracting ligands are apical electron repelling groups are trigonal planar. We have to consider we should not take literally meaning of this one among this pair of ligands which are electron attracting and which are electron repelling we should see. If we have anionic ligand pair in the trans disposition they are essentially electron repelling groups and if we have neutral ligands are cationic ligands probably they will be electron attracting ligands. So based on this assumption if we generate intermediate trigonal bipyramidal intermediate obviously we can explain very nicely which isomer is going to form and that is what exactly happens. So this is kinetic electrostatic theory holds good for anionic ligands such as halides. Now let us move on to the third theory this is called pi bonding theory. As name suggests this holds good for substitution reactions having pi back bonding ligands. For example if you have triphenyl phosphine if you have carbon monoxide if you have ethylene such back or even anionic hydro cyclic carbons you can anticipate the role of pi bonding theory in the substitution reactions of square planar complexes. So here I have mentioned again you can see here I have written this trigonal bipyramidal transition state are intermediate and now if you just look into this complex and let us say we have a ligand here this is capable of having back donation from the metal and if you consider one of the orbitals say DXY is the electron filled metal orbital that is ready for interaction with appropriate pi bonding orbitals present on the metal. So it may be sigma star with tertiary phosphines or pi star with an hydro cyclic carbene, carbon monoxide, cyanide and etc. So the three ligands in the plane and the metal center can communicate electronically through pi bonding in the transition state or intermediate, this intermediate I am talking about. This implies that pi coordinate species must be trigonal bipyramidal rather than square based pyramidal. One should remember if L is a strong pi acceptor, if L is a strong pi acceptor like carbon monoxide it will stabilize the transition state by accepting electron density that the incoming nucleophil donates to the metal center and will thereby facilitate substitution at trans to it. So that means when the ligands are coming you should remember ligands are always coming with a pair of electrons and they are looking for nucleophilic site. So here in the pi bonding theory how efficiently a trans directing pi acceptor ligand generates a vacant site we are having less electron density at the position trans to itself. How to explain this one? Let us consider interaction of say sigma star or pi star of CO with one of the metal pi bonding orbitals preferably DXY, DYZ or DXZ let us consider since we have placed in this molecule in DXY plane let us consider DXY you can see here. So here since it is a better pi acceptor ligand so this electron density is moving as a result what happens since entire electron density is pulled towards strong pi acceptor ligand here what happens here less electron density is left here, here less electron density is left as a result let us say this is leaving group and this is entering group what happens this can leave whereas this one is coming with a pair of electron and this is an excellent nucleophilic site so substitution is completed. So this is how pi bonding ligands facilitate substitution at a position trans to itself that means first of all by taking electron density what happens it leaves less electron density so that it can go and then next one it is coming with a pair of electron this is looking for a nucleophilic center it generates nucleophilic center trans to itself and once the substitution is over and this is eliminated it will divert back to square feather complex with having appropriate geometry or conformation cis or trans in case if there is a provision for isomerization or formation of two isomeric species. So now using this trans effect let us see how we can make different all possible so let us see by using this trans effect how we can make some isomers for example if you consider this complex here if you consider this complexity this one is something like this A B C and D so all ligands are different all ligands are different and it is a neutral complex and if all ligands are different and also whatever the ligands that are present I have also provided trans order for these things and starting with simple metal complex of our choice let us try to make first of all we should know when we have four different type of ligands in a square feather complex how many isomers are possible first we should make out that one with this one we can have a maximum of three geometric isomers let us see how we can make or how we can prepare these three isomers using the trans effect order that is provided here NO 2 minus Cl NH 3 and methyl amine. Let me start with the complex of this type the simplest one having three chlorides and one NO 2 and this is 2 minus now we shall add first ammonia when we add ammonia we should know where it goes since NO 2 is the strongest trans directing group among all obviously NO 2 labelizes a group trans to itself means this should say goodbye this chlorate should go he has no option other than leaving that place so now we have one negative charge now let us add methyl amine now we should see where methyl amine goes now we have two chlorides are there among chlorides what happens one of the chlorate should go so we made one this is one isomer of course it is convenient if we write down first all possible isomers and then look for the preparative method starting from an appropriate substrate molecule or metal complex now what I would do is I would start with again this the same now instead of adding NH 3 what I would do is I would add first CH 3 NH 2 when it goes to the position does not change in place of ammonia here methyl amine would go so no change rest of the ligands would remain same at their respective positions so now I shall add ammonia now ammonia can go here so we ended up getting a neutral molecule this is the second isomer so we are left with one more isomer for this one I can start with one of these compounds that we have already made for this one what I would do is I would take this itself this compound itself I shall take for that one NO 2 position remains same chloride remains same I am taking NH 3 here and then here NH 2 CH 3 so now I am adding one more methyl amine to this one so that it becomes very clear where it is going this would not change and of course this is already there this would not change so this comes here so now we have positive charge so now we will add CL if we add we should tell where it goes now the next question is if I am adding here where it goes now just go back to this one among all this is the strong NO 2 minus is the strong trans directing group obviously this chlorate should go to your position trans to NO 2 this is the third isomer so now you know how effectively we have used this trans directing series and we have prepared all the three isomers possible for this combination of four ligands of general formula A B C D so similarly it is very interesting whatever the complexes you come across having A B C D R A A B C R A A B C D of course in case of A B C D you can have three when you have A 2 B 2 you can have two isomers in these cases you just look into a combination of different type of ligands having different trans directing abilities and try to familiarize with this trans effect series of performing substitution reaction. So let me take up another interesting example before I conclude trans effect with the square pressure complexes and move on to looking into the substitution reactions in octahedral complexes until then have an excellent time reading and understanding in algorithmic reaction mechanism. Thank you for your kind attention.