 Once again, welcome you all to MSP lecture series on Advanced Transformation Chemistry. This is the 11th lecture in the series of 60 lectures. In my previous lecture, I started discussion on valence bond theory concept and its utility for explaining bonding in coordination chemistry. Today let me start from where I have stopped. We all know that valence bond theory was developed by Linus Pauling using hybridization concept and essentially he started hybridization theory to explain bonding in methane. In case of methane, he thought that we have S2 P2 electronic configuration. What he thought was first one electron from S orbital will be promoted to P orbital and then these three P orbital would interact with 1S orbital of hydrogen to form three bonds having very similar properties and the fourth one would interact with one electron left in 2S orbital with 1S 1 orbital of hydrogen and then in that case you are supposed to have two type of bonds. But the studies showed that methane has all the bonds equivalent as a result the assumption was thought of flaw and then they thought that prior to the bond formation between carbon and hydrogen this S electron will be promoted to P orbital and then three P orbital and S orbital would combine together to generate a set of four hybrid orbitals having the composition SP3 and these four SP3 would interact equally with four hydrogen atoms to have methane molecule with tetrahedral geometry and later that was extended to all main group compounds and then they made an attempt to expand that to coordination complexes as well. So for his hybridization theory he was awarded Nobel Prize in 1954 and then again in 1962 he got another Nobel Prize but not for chemistry but for peace and he is one of the four individuals to have one two Nobel Prizes okay. The first one being Mary Curie so let us try to understand about hybridization concept. So according to Weyland's Bond theory the number of hybrid orbitals formed equals number of atomic orbital mixed for example if you consider SP3 we have S orbital and 3P orbitals are there that result in the formation of four SP3 hybrid orbitals that says that number of hybrid orbitals formed equals number of atomic orbitals mixed that type of hybrid orbital formed depends on the type of atomic orbitals mixed as I mentioned if you take S and 2P then it will be having SP2 with you know two third P property and one third S property and many types of hybridization are also known and the most common type of hybridization observed among main group compounds are SP, SP2, SP3, SP3D and SP3D2 and of course we can always debate about SP3D and SP3D2 among main group compounds but now let us focus on simple compounds to understand SP, SP2, SP3D hybridization before we extend this concept to coordination compounds. Now let us consider SP hybridization always we should remember to write this one and now let us consider simple compound having simple electronic configuration having 1S, 2, 2S, 2 and then 2P okay so here 2P, 2S, 1S so 1S we have 2 electrons let us say we have 2 electrons and here we do not have any electron. Now what would happen is let us try to mix 2S, 2 electrons with one of the 2P preferably PZ in that case let me write 2 like this and then other 2 unutilized ones would remain same so then here we have 2 electrons that means we generated 2SP3 hybrid orbitals having one electron each okay they are valence orbitals and SP. So now what would happen to this one so since this is core electron does not participate in bonding so bonding we should consider only valence electrons and this we call it as SP hybridization and of course if you are interested in I have also given here orbitals wave functions and here S and P characters in SP hybrid are 50% each and this is how you can write wave function so here you can see 2S is there and then 2PX it is combined to generate SP hybrid in X direction and similarly another hybrid in X direction of course with turning exactly opposite to each other and it appears like the 2 would be looking something like this okay now we have 2SP orbital disposed at an angle of 180 and now autumn coming should overlap here and overlap here having a linear geometry. The generation of 2SP hybrid orbitals from 2SP and 2PX orbitals are shown and if we choose 2S and 2PY hybrid will be along Y axis and if we choose 2PZ it will be along Z axis. Now consider SP2 hybridization this is for SP2 and let me write here how we can arrive at SP2 in the same way again consider 1S and 2S we have 2 electrons here and 2 electrons here and we have 2P orbitals and what we have is 1 electron here and 1 electron here so that means we are considering carbon with 2S, 2P to electronic configuration and now if it undergoes hybridization let us SP2 so 1 will be left we have 3 orbitals 3SP2 orbitals are there having 1 electron each and of course here as usual this core electrons would remain so they do not participate in bonding and one should write like this wave functions for X direction and Y direction and Z is not used here. So this how you should be able to write for SP2 and here you can see 2S is interact in 2PX to have an SP2 hybrid in X direction then what happens it remains intact in X direction then when 2S is overlapping with 2PY that what happens it is initially it is orthogonal it makes a anticlockwise rotation by 30 degrees to orient in this direction so that angle between them is 120 and then when 2S interacts with 2PY another one so it can interact in opposite direction with clockwise 30 degree away from the orthogonality to have 120 so basically you will be having something like this with each SP2 is 120 degree apart so of course here one should be very clear you can use any of this orbital to explain but you should be very clear about directions and the plane in which you are talking about these molecules are you replace this orbital in that plane so if you are considering XY atomic orbitals it will be in XY plane and if you are considering Y and Z orbitals it will be the trigonal planar molecule will be along YZ plane or if you are considering X and Z so the molecule should be considered kept on XZ plane that one should remember now let us consider SP3 hybridization 1S, 2S and 2P so we have 2 electrons here 2 electrons here and 2 electrons here and if I am considering SP3 what would happen I have 1, 2, 3 and 4 and now we have one electron each and this remains as it is 4 and we do not have any orbits left now so these are SP3 hybrid orbitals this is how you can write SP show SP3 hybrid orbitals so this is not a real observation and it just a formalism and valence state cannot be observed by spectroscopic techniques that one should bear in mind and then here the wave functions are written for 4 SP3 hybrid orbitals of course when S is overlapping with PX, PY, PZ you know that it is symmetrical so everything is positive and then with PX and PY and it is PZ direction so that means now they are disposed at an angle of 107.8 this is how this 4 SP3 would look like and it is very convenient if you imagine these 4 orbitals inside a cube and if you place the central atom at the center of a cube and place the ligands on alternate corners this is how it looks like they are 4 peripheral atoms are disposed to 4 alternate corners of a cube and it makes a perfect tetrahedral geometry and in this one as I mentioned S and P characters are respectively 25% and 75% in each of these orbitals. So let us now extend this valence bond theory to metal complexes let us start with D3 electronic configuration let me consider a metal ion having 3 electrons in its d orbital. So now in this case what happens of course you should also consider its 3D we have to consider 4S and 4P also this is 4S this is if it is this is 3D this is 4S and this is 4P. So in this one what you can do is if it is complex is forming a tetrahedral compound with coordination number 6 in order to explain the bonding we have to go for utilizing all these 2 plus 3 plus 3 6 orbitals and 6 orbital we should use here and remaining this would remain intact. So now this is the hybridization we are talking about and here if 6 ligands are coming with 12 electrons they will be accommodated something like this. So since here inner 3D is used for accommodating electrons through hybridization so here 2D and 1S and 3P orbitals are used this is D2 SP3 hybridization. So you can explain this is called inner orbital complex complex and also this is paramagnetic because we have 3 unpaid electrons are there. So some of these properties can be explained and the geometry is octahedral. So that means here whether we are considering homo-liptic molecule or heteroliptic molecules or we have more than one type of ligand even MABCDEF if you consider 6 different type of ligands still the hydration is D2 SP3 and it still is an inner orbital complex or still it is paramagnetic and still it is octahedral. But to what extent octahedral geometry is regular or distorted that information does not come from Wehrmacht's bond theory. Let us look into D5 electronic configuration. So we have 5 electrons and of course if I am considering 3D I have here 4S and 4P is there. So now again once again if I assume that D5 metal ion is forming an octahedral complex then 6 ligands are coming with the 12 electrons that has to be accommodated in appropriate metal orbitals for that one. Now what happens inner orbital we do not have any vacancy as a result what we should do is we should use 4D orbitals here and of course 4D also we have 5 and we have to choose 2 orbitals here and then we can consider this for bonding. In that case we have 1, 2, 3, 4, 5, 6 are there and then here we have 5 as usual metal ligand electrons are accommodated in these 6 hybrid orbitals. Now we are using S and P and outer D orbitals as a result what happens in the same sequence we can write the hybridization as SP3D2 hybridization. So here we have used outer D orbital as a result it is called outer orbital complex complex and still it is paramagnetic also you can call it as high spin complex, high spin complex. Before we look into D8 electron configuration is any alternate hybridization is possible with this one if you use strong field ligand what would happen say something like this is there and in this case what happens metal electrons from these 2 will be getting paired here and we have an as a result what happens 2 orbitals will be vacant here. So now again you write 4S and 4P ok. Now there is a possibility of utilizing inner D orbital for hybridization same thing we shall do it now something like this and now we try to accommodate 12 electrons coming from 6 ligands again we have octahedral geometry and now it is once again S D2 SP3 hybridization. So that means with the D5 electronic configuration both SP3D2 as well as D2 SP3 hybridization are possible and how it is possible VLS bond theory does not say how inner orbital complex are formed and how high spin and low spin are formed it does not say but when the electronic configuration is like this that information comes from magnetic moment and one spin only magnetic moment from that one one can speculate whether a D8 D5 metal ion would prefer SP3 D2 or D2 SP3. The ligand field does not come into picture at all that means ligand field strength or the strength of the ligand is not pronounced in VLS bond theory. Now let us look into D8 electronic configuration this is 3D this is 4S this is 4P so consider 1, 2, 3, 4, 5, 6, 7, 8. So here we have this electronic configuration is there now and in this case what we can do is if the since more electrons are there we assume that metals having more electrons in their D orbital tend to have lower coordination number. Let us look into first 4 coordination in 4 coordination without disturbing these electrons how to make bond that means SC is available 4S is available 4P is available if the coordination number is 4 we can think of simply SP3 hybridization that means 4 ligands coming with 8 electrons can be accommodated in this fashion. So that means a D8 system can have SP3 hybridization with 4 coordination number 4 and in contrast so let us consider like this and this also we shall try to pair up as a result we will be left with one D orbital. Now so now we have 4P now one more is there again the strategy is not different still it prefers 4 coordination number but inner when the D orbital is available as a result what happens it try to utilize this one in this fashion. So that means 1D orbital and 1S orbital and 2P orbitals are utilized now if the 4 ligands are approaching now so D is there and ES P2 is there that means this 4 ligands would give 8 electrons that are accommodated in these 4 DSP to hybrid orbitals. So here it is tetrahedral here it is square planar geometry but on the other hand if the metal still prefer to have 6 coordination in that case what would happen and then we have 4D 1, 2, 3, 4, 5 so in this case what happens since even if you pair one of the electron one orbital alone does not help in making inner orbital complex if the coordination number is 6 in that case what happens it would try to go for outer orbital complex. So here 12 electrons are accommodated so now this if you see again this is ESP 3, D 2 and octahedral. So this explains now the possibility of having different geometries and different coordination number for a given electronic configuration for D 8 you can have ESP 3 tetrahedral arrangement for D 8 you can have DSP 2 square planar arrangement of the same 4 ligands or if it is ready to accommodate 6 ligands then it has to be a outer orbital complex with ESP 3D 2 hybridization so that it can have octahedral geometry. So this kind of information comes from wellness bond theory and let us try to extend for some more molecules before we proceed further to take up crystal field theory until that have an excellent time reading chemistry.