 Hello everyone, welcome to MSP lecture series on advanced transformational chemistry. This is 19th lecture in the series. In my previous lecture I discussed about the application of molecular orbital theory to main group elements with a couple of interesting molecules where there are controversies when you compare valence bond theory or VACPR theory. Now let us continue from where I had stopped. Now let us try to use molecular orbital theory or one can also call it as ligand field theory to explain bonding among coordination compounds. So I have just shown a generic M-O diagram that depicts bonding in case of metal complexes with ligands having pure sigma bonding properties. A typical M-O diagram can be written in this fashion. I am just overwriting on the diagram I have shown to make you familiar with drawing yourself M-O diagrams for complexes. For example in this case what we are considering for any given transition metal Nd orbitals and N plus 1s and N plus 1p orbitals are considered as valence orbitals and the intention needs to utilize them for making bonds when the ligands are entering coordination sphere. Now here of course to begin with we are considering atomic orbitals of metal here. We are considering this 5 d orbitals and we are considering N plus 1s orbital and then we are also considering N plus 1 3p orbital for which the mullican symbol is T1u and here we are considering ligand group orbitals. There are 6 ligand group orbitals are there and each one is coming with a pair of electrons. So we have a total of 12 electrons that has to be accommodated. Let us not worry how many electrons we have in the d orbitals of transition metals that is immaterial now. So now if the 12 electrons are coming from 6 ligands as 6 pairs they will be put here. So now you can see this is how this molecular orbital that have combined ligand orbitals along with 2 d orbitals that is in this case it is dx square minus y square and dz square to generate this many bonding molecular orbitals in which these 6 pair of electrons are accommodated. If any electrons are left in T2g that is dxy dyz and dzx they remain non-bonding and they are placed here. And now this gap what we have here here so this one so this is called delta O here and then from here onwards if you consider they are all anti-bonding properties they have. They are anti-bonding molecular orbitals Eg star A1g star and T1u star. So this is how one can write a typical M O diagram for a metal complex with octahedral geometry. Of course this the same M O diagram what I showed is also good for ligands having only pure sigma donor properties. So again I have shown here is the same what I showed here except for the fact that I have also included the symmetry of the ligand group orbitals that so that you can match appropriately with appropriate metal orbitals to generate molecular orbitals A1g. This A1g is coming here along with A1g is this one and then when you go for T1u so 3 orbitals coming here and 3 orbitals are coming here and then T2g Eg is there Eg 2 orbitals are coming here and then Eg 2 orbitals from this d group will be coming here and then this will remain and this is T2g so rest would be same. Once you know how to write this diagram you should be able to fill the electrons according to the number of electrons you have in a given metal complex. So this molecular orbital diagram represents ligands having only pure sigma donor properties. Now let us look into ligands having both sigma donor and pi donor properties. So in this case already told you in my previous lecture ligands have low energy filled sigma orbitals and low energy filled pi orbitals when they interact with metal atomic orbitals to generate molecular orbitals this gap between T2g and Eg or T2 and E shrinks this is what exactly happens in case of metal chlorides and fluorides. So here relatively these compounds are less stable compared to pure sigma donor ligands. Then the other ligand group we are talking about is bonding with sigma donor and pi acceptor ligands such as carbon monoxide triphenyl phosphine and hetero cyclic carbenes and etc. In this one here it is again metal valence orbitals Nd N plus 1s and N plus 1p and here we have again ligand orbitals they are low energy filled sigma orbitals and high energy empty pi orbitals. So in this case you can see the magnitude of this separation has increased remarkably that explains why these complexes are very stable compared to other 2 class of ligands that I had described. For comparison I have put all the 3 types of you know diagrams here you can clearly distinctly you can see the separation between these 2 and of course he in this case we also use the term HOMO and LUMO this is highest occupied molecular orbital and this is lowest unoccupied molecular orbital between which electronic transition takes place. Once again 6 sigma donor ligands such as water or ammonia and they are again coming with 12 electrons that these are the symmetry of the ligand group orbitals that are going to combine with metal orbitals and here I have just put 6 electrons in the orbital and these 6 electrons remain here as non-bonding and if there is any electronic transition is there these electrons can be promoted to this higher energy state here. So this represents a typical hexamine metal complex or hexa aqua metal complex here. Here you can put whatever the electrons you think the metal possess after required axis state is achieved with that metal. So this one you can guess what geometry it represents and of course the moment you see here 2 orbitals and the symmetry corresponds to A1 and T2 obviously this is for a tetrahedral complex. In tetrahedral complex we are talking about 4 ligands coming with 8 electrons and here we have 5D orbitals again and here the symmetry will be E and T2 and as usual N plus 1 S orbit having A1 and this N plus 1 P will be having T2 in case of tetrahedral. So here very similar to Christofield theory the energy of E will be lower compared to energy of T2 and this remains non-bonding here and in many cases what happens tetrahedral complexes also have electrons in the anti-bonding orbital here. If they have more than 4 electrons obviously the 5th electron has to be placed here and these compounds are relatively unstable compared to tetrahedral complexes also to an extent square pen are complexes. 4 pairs of electrons coming from the ligands would occupy these 3 plus 1 4 molecular orbitals here and electrons present in the D orbitals will be filled in this one if it is more than 4 2 electrons then it would go here usually here pairing does not start here because this gap is much smaller so electrons will be promoted here and they will be usually high spin complexes. And also I am showing you the orbits involved in this one you can see these are the orbitals having this kind of orientation of course you can also compare them to PX, PY and PZ and this is A and all of them what is putting tetrahedral geometry can be seen here. This is called symmetry adopted linear combination of atomic orbitals you can see here this is composite of DZ square and DXMY square you can clearly see and also you can see how DXMY square orbitals are oriented with respect to direction of approach of ligand again once again Christopher theory whatever we discussed it can be seen here why the energy of these things are lower and then if you see here again it is very clear why the energy of T2 is higher because they are almost coming on the way of ligand approach here in all these cases. And this is ML bonding molecular orbitals filled with 8 electrons for 4 ML sigma bonds and then we call them as anti-bonding molecular orbitals and this is how the looks like anti-bonding orbital the another one of course you can see here this is for square planar complex in case of square planar complex again 4 ligands are coming but they are having different symmetry or mulligan symbols depending upon what kind of orbitals that are available for bonding from metal ion or metal autumn. So, here ND you will be further split into different sets that I shall show you and then A1G does not change here and since one of the orbitals participating in mixing and others 2 does not if you recall Wellesborn theory of DSP2. So, 1p will be mixing as a result what happens it has 2 different type of p orbitals here then accordingly they are further split of course tetragonal elongation if you remember how it splits one can visualize here also in case of ligand field theory then these are the mulligan symbols given for various d orbital A1G, B1G for d x y square and B2G for x y and d x z and d y z still degenerate they are called EG set and they are metal to ligand bonding orbitals again 8 electrons are comfortably placed here as 4 pairs they are responsible for making 4 metal to ligand bonds and they are anti-bonding and these are non-bonding orbitals here this is very very important when we talk about reactivity and let us now compare bonding in metal carbonyls and metal phosphines and if you see metal carbonyl in my previous lecture I showed you about the lone pair present on carbon that is responsible to perform as a sigma donor these electrons from CO would be overlapping with one of the orbitals from M essentially d z square or d x y square to form a sigma bond then the field metal orbitals will overlap with pi star atomic orbitals to pi star orbitals of carbon monoxide to take electrons from the metal through back bonding and this is how carbonyl ligands stabilize metals in their low valence state in their low valence state and overcome inter electron repulsion in the metal by taking comfortably the electrons from metal to pi star of carbon monoxide orbitals. So this is a typical metal carbonyl bonding scheme here and let us assume a pair of electrons are coming from carbon monoxide these 2 electrons here this orbital and they interact with appropriate metal orbital to form a sigma bond so we established a metal to carbonyl sigma bond now the energy of this one quite comparable with pi star of this one that is t 2g I would say and then they overlap they interact again to generate a set of bonding and anti-bonding molecular orbitals one thing one should remember here is pi star is not going to take directly electrons from the metal t 2g orbitals when the metal autumn or anion is ready for giving electrons from its t 2g t 2g orbital should be treated as atomic orbitals and pi star or anti-bonding orbitals present on carbon monoxide should be treated as atomic orbital they 2 combined together to generate again a set of bonding orbital and anti-bonding orbital to the bonding orbital what happens the metal electrons goes so this is how back bonding takes place if somebody says that sigma star or pi star in phosphins or carbon monoxide directly take electrons that is incorrect they are not going to take they have to be treated as atomic orbital and t 2g orbits or with pi symmetry are essentially t 2g or dx y dy z and dx z they have to be treated as again atomic orbital they combine together to generate a set of bonding and anti-bonding orbital and they have pi symmetry and we call it as that is the reason we call it as pi back bonding so same thing I have shown here for in case of phosphorus again phosphorus lone pair is there that goes to metal to form a sigma bond and then the sigma star when we write pr 3 molecular orbitals pr 3 has a sigma star that sigma star interacts with field d orbital to take electrons and this is called back bonding here again in this case also sigma star orbitals you should treat as atomic orbitals they interact with field d orbitals to generate a set of bonding and anti-bonding again they have pi symmetry and here electrons will be transferred from metal to this bonding molecular and hence we call it as pi bonding very similar here except for the fact that here we are using pi star anti-bonding orbitals and here we are using sigma star as anti-bonding orbitals and then you can see symmetry pi star and pi so electrons would be transferred to this place so that what happens inter-electron repulsion is minimized. So now I have shown here for chromium hexa carbonyl you can see here 6 carbon monoxide are there and the symmetry is referred to A 1 G T 1 U and E G here and as usual they combine here and then we have also given T 2 G pi star orbital as I mentioned these orbits are now combined with T 2 G and you can see now this is a bonding and it is anti-bonding and the electrons are placed here and now you can say the moment it is involved in bonding the energy of electrons present here previously it was non-bonding somewhere here it is lower so since it is lowered the gap is increased so we can say now the compound the complex is more stabilized because the energy of this one is lower. You can see here this is the pair I have shown here this is the one so that I am considering from 6 carbon monoxide this 6 pairs of these electrons are given shown here and then this pi star whatever I have shown here this is the pi star I am referring to here this is same as this one. So I have an interesting case here nickel tetracharbonyl and normally you do not see a more diagram for nickel tetracharbonyl here I have written too many electrons you may be surprised to see why I have written so many electrons instead of writing just 8 electrons from 4 carbon monoxide they are making bond with Ni to form NiCO 4 but I have written extra electrons here you can see 1 2 3 4 pairs are there and 4 and 8 pairs and another so basically I have 16 pairs are there so that means about 32 electrons I have considered here 32 electrons where they are coming from yes let me show you here let me write Lewis dot structure again you know how we are establishing a triple bond between carbon and oxygen. So this pair is 1 2 3 and 4 pairs are there 4 pairs means 8 electrons are there and 8 electrons into 4 ligands 32 electrons are there out of 32 electrons what happens this 4 8 electrons ok this 4 pairs means 8 electrons equals 4 pairs they are responsible for making bond. So this is what wellness bond theory says but is it really true let us examine that one whether the sigma the electrons really participate and interact with appropriate metal orbitals to make NiCO bonds you can see here these 4 pairs are I am referring to this one this is the 4 pairs I am showing and then here 8 pairs whatever I am showing here these are the electrons I am showing here together they remain non-bonding and to our surprise if you see these 4 orbital should have interacted with this one as well as this one to generate a set of 4 molecular orbitals to place these 8 electrons very similar to what we saw in the tetrahedral complex in earlier case but if you see these 4 pairs of electrons remains almost non-bonding here the energy is much lower compared to any of the metal orbitals here that indicates there is no sigma bond at all between nickel and carbon monoxide there is no sigma bond in order to have sigma bond we should have pulled these orbitals much lower and to connect with this one but that is not happening. So that indicates the 4s and 4p energy are too high to interact with this sigma electrons from carbon monoxide then in that case there is no 4 sigma bonds at all in NiCO4 then how NiCO4 is formed? NiCO4 is formed through only back bonding you can see here 5 star of carbon monoxide that is interacting with here T2 E T1 T2 here and as a result what happens the electrons here degenerate prior to the formation NiCO4 are split into two levels as usual E and T2 and the electrons are placed here 10 electrons present in nickel nickel 0 means 3d 8 4s 2 0 means 10 electrons these 10 electrons are placed here that means just by splitting this one what happens this is stabilized. So Wellesband theory says that we are utilizing these orbitals from metal to establish NiCO4 that is not the case at all and of course this one can see if they want to read more about that one you can refer to these papers and not many textbooks talk about this one and I have not come across in many textbooks describing a more diagram for NiCO4 in this fashion but this is very true and also this explains why NiCO4 is highly volatile and more reactive for the same reason this is stabilized by only back bonding and there is no sigma bonding in NiCO4 at all whereas in case of chromium hexacarbonyl we have sigma bond that I showed you in previous slide. So there it is a solid and reasonably stable high melting point white powder moderately are reasonably stable atmosphere whereas this one is highly volatile and it can decompose readily. So some of these properties can be explained nicely and bonding also can be explained and that normally we do not see in case of Wellesband theory and also crystal field theory of course it does not explain anything about such complexes where metal is in zero valence state. Let me stop here and continue more interesting stories about trans-metal elements in my next lecture.