 Hello everyone, once again welcome you all to MSP lecture series on Advanced Transmittal Chemistry, this is 12th lecture in the series, in my previous lecture I had initiated discussion on valence bond theory and I did discuss about various hybridization concepts and also with respect to transmittal compounds, I discussed selected electronic configuration such as D5, D8 and I showed how one can form a different type of complexes and one can explain the bonding using valence bond theory. So now let me continue from where I had stopped, you can see the table I have given here, I have covered almost all hybridization you come across for transition metal complexes, having different ligands and also having different geometries and hence different hybridization they have used. Look into the first one, here arrangement of donor atoms or ligands is linear with respect to central metal atom and when the coordination number 2 is there invariably we use SP hybridization example, diamine-silver complex, next trigonal planar with coordination number 3 here example I have given is HgI3- with SP2 hybridization and of course tetrahedral you all know that we use SP3 hybrid orbitals, we have several examples just I have given one example here tetra bromo ferrate, then square planar and all most of the D8 metal complexes invariably show square planar geometry along with D7 S2 system with plus 1 oxen state for example rhodium and iridium along with nickel palladium and platinum that is square planar the hybrid orbitals we are using is DSP2 and trigonal bipyramidal the hybridization is SP3D, I have given one example of copper here and you should remember for example when we move to square based pyramid again we are using SP3D that means we should be able to differentiate between the D orbitals that are involved in these 2 hybridizations, if you look into trigonal bipyramidal we have 2 ligands in the axial position, so we need to use DZ square orbital and hence here the hybridization of SP3D involves DZ square whereas in case of square based pyramidal 4 ligands are in the plane so we need to use lig orbitals that are oriented in the plane or along the plane so that is the reason we use here DX square minus Y square, so hybridization is same but the D orbitals are different in these 2 cases that one should remember and of course in case of octahedral we are using SP3D2 for outer orbital complexes or D2SP3 for inner orbital complexes just I have given one example hexamine cobalt we have numerous examples for both the cases inner orbital as well as outer orbital complexes, when we go to coordination number 6 we come across another important geometry that is trigonal prismatic and here also the hybridization is essentially the same SP3D2 but the orbitals we are taking among D are different, one such example is we can take S PX PY PZ and then XZ and YZ are taken here not DZ square and DX square and Y square we prefer in case of octahedral complexes but when the early metals exist in their highest possible access state we can as well consider inner D orbital instead of taking P orbital and hence many trigonal prismatic complexes utilize all D orbitals along with S to have a hybridization of SD5 instead of SP3D2 and this is true with metals in their high valence state especially metals having D1, D2, D3 electronic configurations and then when we have coordination number 7 we have pentagonal bipyramidal geometry so in this case since we need 7 orbitals we are using additional one more orbital from D here it is DXY DX square may be Y square and DZ square along with S and 3P orbitals example I have shown here SP3D3 pentagonal bipyramidal but when you have 7 coordination we can also have another geometry that is called capped or mono capped octahedral geometry so in this case also SP3D3 is there but the orbitals are different here you can see here. In the same way when coordination number 8 is there we come across 3 different types of geometries one is cubic, one is duodecahedral, one is square anti prismatic. In case of cubic we use SP3D3F that comes to our F block complexes and then in case of duodecahedral coordination number is 8 we are using SP3D4 here we are using SP3D4 and then I have listed all DZ square and all DXY, DYZ and DZX are used and then duodecahedral geometry when we have square anti prismatic geometry again SP3D4 and here except for DZ square all D orbitals are employed in this hybridization scheme and example is this octaphoro tantalate 3 minus and then the last one with coordination number 9 the most symmetric geometry you can think of for coordination number 9 is trigapid trigonal prismatic and here this is an example where all valence orbitals have been utilized in arriving at a hybrid scheme having the composition SP3D5 this is SP3D5 hybridization example is REH9 2 minus here 9 metal 2 hydrogen bonds are there and this is one of the rare examples of homolyptic hydrates having as many as 9 metal to hydrogen bonds and this has tricapid trigonal prismatic. So now to make you familiar with some of these geometries I should show you it becomes very clear and then also you should be able to distinguish between trigonal prismatic and octahedral geometries and what are the difference between them. So if you can see here this is trigonal prismatic geometry you can see this one this is a trigonal prismatic geometry and now I have here octahedral geometry here and then is there any alternate name for octahedral geometry yes this is trigonal prismatic and this is trigonal antiprism if it is trigonal prism this is trigonal antiprism so then how they are related it is very simple okay you take this one and if you see these two triangular phases they look eclipsed right they look eclipsed here and whereas if you look into this one if you just try to see this triangular phase and this triangular phase they look like staggered that means this is a trigonal antiprism staggered one and where we have eclipsed one this is trigonal prism but is it possible to convert yes just see keep this one and turn this phase something like this and then you have to connect this one with three more sticks then you can get octahedral geometry so this is the relationship between trigonal prism and trigonal antiprism or octahedral so now this is octahedral and then you go back and see them eclipsed this is trigonal prism okay you take this trigonal prism and make one of the phase staggered and you can visualize octahedral geometry there should not be any confusion between these two geometries I have shown those things here so this is trigonal prism you can see the relationship of these two triangles they are on same side that means if you just bring it towards the screen then what happens they will be eclipsed whereas here you can see you take any two and they are staggered similarly we can compare this tricaprio trigonal antiprismatic geometry I mentioned in case of ReH9 2 minus for coordination number 9 you consider this one you can see here we have 1, 2, 3, 4, 5, 6, 6 vertices are there and then on each of rectangular faces if you put one more vertex and then connect it with four bonds then this will be monocapital and then if you put one more here and connect it with four bonds this will be bicapital trigonal prism and then one you put here and make bonds with here then this become tricapital trigonal antiprism okay this is what the geometry adopted by ReH9 2 minus 2 accommodate 9 hydrogen atoms surrounding rhenium atom you can see this one this is monocapital and this is bicapital and this is tricapital trigonal prism and you can distinguish these three new vertices added so total vertices are 9 so this is tricapital trigonal prism and similarly you can take for coordination number 7 octahedral is there and besides pentagonal bipyramidal another possibility is monocapital octahedral geometry so you consider one of these faces and add one more vertex and connect it with the three more bonds then that becomes monocapital because something like this yeah you can now distinguish the 7th vertex coming here this is monocapital octahedral okay and this is cube now in the same thing with cube is there and in the cube you take this square then turn it okay so that both of them are not eclipsed so you take this yellow face and rotate it so that this will be staggered that means vertex will be pointing here okay turn it so something like this so now now connect them in this each one each square you put one more so that you will end up with two equilateral triangles and on four sides you will end up with two equilateral triangles so that you will be having eight equilateral triangles and then you have two square faces this is called square antiprism sometimes coordination number 9 one can also have monocapital square antiprism sometime you can also have another one below so that you can have bicapital square antiprism so this is for coordination number 10 okay this is the most common one in some cases coordination number 9 instead of adopting this one they can also adopt this geometry having monocapping on square antiprism it is bicapital here on this square faces then it becomes bicapital square antiprism even I have that example here you can see this is how it looks like so this is one face okay and this is another face and they are staggered to each other when they are staggered to each other you can clearly see these eight faces here eight equilateral triangles so you can count them there are eight so this is all about I wanted to tell you about the geometrical difference between various polyhedral so in the beginning I made a statement number of hybrid orbitals formed equals number of atomic orbitals mixed is it really true or is there any exception I shall tell you in a couple of minutes answer for this one and then type of hybrid orbitals formed depends on the types of atomic orbitals mixed that is absolutely correct no issues and many types of hybridizations are known you saw all possible hybridizations I mentioned and one more thing you should remember when we have octahedral geometry with metals in their high valence state especially with early metals it is not just sp3 hybridization you should remember one more thing when we talk about tetrahedral complexes of early transfer metals or metals in their highest possible ox states not necessarily they utilize sp3 hybrid orbitals sometimes they also utilize all the d orbitals okay thus showing d3s hybridization that is quite common with high valence metal ions example titanium tetrachloride if you take titanium is not sp3 hybridizer you know environment it is utilizing but it is utilizing d3s and similarly if you take potassium dichromate chromium is using d3s because this inner d orbital and it is essentially using 3d and 4s so it should be d3s and similarly in Kmino4 manganese utilizes d3s and osmium tetroxide osmium is in plus 8 ox state this also utilizes d3s while forming osmium tetroxide however the most common types of hybridization observed among main group elements are sp sp2 sp3 sp3 d sp3 d2 or in some cases d2 sp3. I put a question mark here you can see number of hybrid orbitals formed equals number of atomic ions mixed it is not true there is an exception here that is what I want to show you here look into this one here we come across 2 spd hybrid orbitals that means 3 orbitals are involved one pz one s and one dz square is involved and but still we are ending up with only 2 hybrid orbitals spd oriented opposite to each other in this direction. So these empty orbitals can take readily from ligands to establish a linear molecule the example is diamine silver plus complex so how that happened the hybridization of spz and dz square with the choice of phases shown here produces a pair of collinear orbitals that can be used to form strong bonds in order to form strong sigma bonds some metal ions utilize this kind of hybridization very rare ones you can see now so these 2 spd hybrid orbitals are oriented in this direction now if 2 ammonia ligands are coming they donate a pair of electrons to these empty orbitals to form strong diamine silver complex this is the complex here and in order to interact the energies of Nd that is 4d and N plus 1s 5s and 5p should be similar true with heavier elements so energy should be very comparable and if you go for heavier elements the energy difference marginally remarkably decreases as a result and also the size increases they diffuse into each other because of this one what happens in order to strengthen metal to ligand bond they utilize this kind of hybridization in fact molecular theory also suggested significant mixing of spd and dr builds in this complex now I will show you another interesting hybridization looks very similar to ethylene but how they utilize in a different way to facilitate metal to metal multiple bonding especially in case of main group elements so tin 2 organometallics of the type of SN with bulky organic ligands containing tin to carbon sigma bond containing tin to carbon sigma bonds are stabilized only if R is sterically demanding that means in order to stabilize metals R elements in their low valence state you have to have bulky groups they will give umbrella protection one such example I have shown here when you react SNCl2 with very bulky ligands such as this one it forms decoordinated tin compound and then this is monomeric in solution but dimeric in solid state but the dimer does not possess a planar yes sent to R4 framework very similar to ethylene ethylene molecule is planar you can see simply something like this okay it is something like this in contrast okay this tin dimer has a different structure and another interesting thing is SNSN bond distance is 267 picometer this is shorter than a normal single bond having a distance of 276 picometer that means it is slightly smaller but that is good enough to evoke a multiple bond character that means this has some multiple bond character between two tin atoms and then does SN2 R4 this dimer has a trans-bend structure with a weak tin-tin double bond now using hybridization concept is it possible to explain this trans-bend structure if it is a planar structure like ethylene no issue simply you can say it is sp2 hybridization and there is 3 sp3 are there on carbon 2 sp2 hybrid orbitals are utilized 3 sp2 hybrid orbitals are there on carbon and 2 sp2 are utilized in making 2 CH bonds and 1 for cc bond from each CH2 fragment okay now what it happens but the structure is something like this okay and in case of ethylene we have something like this so no issues with this one sp2 can very easily explain the bonding here but if you see here what would happen similar to carbon it also undergoes sp2 hybridization here but in this case what happens one of the p orbital is left unutilized but not left with an electron but empty and now we have 3 sp2 hybrid orbitals are there and 2 sp2 have 1 electron each whereas the third sp2 has 2 electrons so that we have 1 p orbital without having any electron so now they orient in this fashion in order to see this kind of Lewis acid base type of bond formation now this acts as a Lewis acid and now this acts as a Lewis base similarly this empty p orbital on this tin acts as a Lewis acid whereas this field sp2 acts as a Lewis base so in order to facilitate this kind of overlapping this orientation has to be in this fashion see it is not like this it should be something like this when they rotate like this what happens empty p orbital on this thin atom will be aligned towards this one so that they can have overlapping so now we have 2 electrons here and 2 electrons here it is not really a strong pi bond it is not a sigma bond either but the it is intermediate between that one we have still 4 electrons between 2 tin tin bottoms 2 tin tin atoms but because of this kind of overlapping what happens they do not really represent 2 strong bonds whereas they are relatively weak nevertheless they add some multiple bond character and hence the SNSN bond distance decreases to 267 from 276 and now you can see very interesting sp2 hybridization and in fact if you look into multiple bonded compounds of main group elements whether you take carcinic carcinic bismuth bismuth even lead lead or silicon silicon in all these cases this is what exactly happens this kind of sp2 hybridization takes place and in order to facilitate this kind of overlapping they have to take this kind of alignment as a result they have a bent structure so that means successfully you can explain this trans-bent structure using this hybridization concept and what are the limitations of valence bond theory when we talk about limitations that limitations are more or less confined to its utility among coordination compounds that means it does not explain the color of the compounds that is spectral properties it does not explain fully the magnetic properties especially the temperature dependent magnetic properties and also it gives emphasis for spin on the values when you go for 4d and 5d most of the metal ions do not obey experimentally determined values experimentally determined values are always different from theoretical values calculated from spin on the and valence bond theory does not explain these things and does not explain relative stability of complexes once you have coordination number 4 it can depict whether tetrahedral or square pin or but relative stability of complexes with different ligands answer does not come from valence bond theory that means it does not explain about strong ligands and weak field ligands does not distinguish between different types of ligands it does not distinguish as far as valence bond theory is concerned whether you take cyanide whether you take carbon monoxide whether you take triphenyl phosphine ammonia it is just a ligand having a pair of electrons that is coming towards the metal that is it. In general VBT gives emphasis for localized bonding model again what happens it will try to confine a pair of electrons between the two atoms through overlapping that means if there is some delocalization there is no answer from valence bond theory so these limitations are sufficient to look for a better bonding concept to explain various aspects related to metal complexes that is where crystal field theory made its entry into coordination compounds. So in my next lecture I shall tell you more about crystal field theory with an interesting background to this crystal field theory as well with this have an excellent time reading chemistry.