 Welcome you all to MSP lecture series on advanced transformative chemistry. This is the 10th lecture in the series. In my previous lecture I was discussing about the coordination concept brought out by very systematic and meticulous experiments with critical and analytical evaluation of his own work Alfred Werner and how nicely he postulated coordination theory. When a time there was no methodology or any instrumental facilities or any spectroscopic means that was available for him to convincingly understand the futures of coordination compounds nevertheless whatever the postulations he made by emphasizing for quantitative understanding was proved to be precisely correct later with experimental and spectroscopic evidences. Once coordination chemistry started evolving people started looking into various bonding concepts to explain how to characterize compounds and how to explain the bonding that happens between metal ions or metal atoms and ligands. In that context let us look into early bonding concepts. Before that let us look into few points about what are the factors that affect coordination number. The size of the central atom or ion has an influence on coordination number because if the size is larger it can accommodate more ligands surrounding it as a result coordination number can be more. Steric interactions between bulky ligands is also quite important. Despite the size of the metal is appropriate to have 4 or 6 ligands if the ligand size is much bigger like triphenyl phosphine it is very difficult to accommodate 4 ligands. In that context steric interactions are very very important and steric bulk of the ligands plays a major role in deciding the coordination number of a metal ion. The electronic structure of the metal atom or ion is also equally important. If the oxidation number is high the metal can accept more electrons from the ligands that is Lewis base because it has more empty orbitals metals with many D electrons will have lower coordination number. That is the reason when we go for to later transfer metals after D6 electronic configuration the coordination number the tendency to have less coordination number will be more pronounced compared to early metals which have a tendency to have a larger number of ligands surrounding it. So now let us look into some methodologies developed to explain bonding. By the time coordination chemistry was evolving after systematic understanding from Werner's coordination theory people began looking into bonding concepts. As I said by the time main group chemistry had very interesting concepts such as VSEPR theory wellness bond theory to explain the structure and bonding among main group elements and same strategy was attempted to expand or extend to coordination compounds. In that context electroneutrality principle developed by Pauling is very very important. Pauling's electroneutrality principle is an approximate method of estimating the charge distribution in molecules and complex ions. What it states is the distribution of charge in a molecule or ion is such that the charge on any single atom is within the range of plus 1 to minus 1 ideally close to 0. So that means in a metal complex all species involved should have a charge between plus 1 and minus 1 ideally close to 0. This is what the definition of electroneutrality principle. To test this principle let us consider one well known example hexamine cobalt 3 plus ion. So I am considering hexamine cobalt 3 plus ion. Let us look into first covalent method. If you consider covalent method we have 9 electrons in the well and shell that is 3D7 4S2 and assume that 6 electrons are used for covalent bonding leaving 3 charge on cobalt. That means out of 9 electrons from the valence orbital of cobalt utilize 6 for making covalent bond with amine or ammonia ligands. So that 3 electrons will be left and that should be considered as 3 minus charge on cobalt. This is the first assumption using covalent model. So this is simply unrealistic as cobalt cannot be negatively charged going to its positive, electro positive nature. We all know that in this one since it is positively charged it is not very appropriate to assume as negatively charged. So moving to this okay this covalent model was ruled out. So this is how the covalent model was depicted. Out of 9 electrons 6 are utilized for making the bond with ammonia and leaving 3 as excess considered as negative charges so that now we have a charge distribution like this. This is simply unrealistic. So let us look into ionic model. So plus 3 charge remains localized. We have 3 plus charge so cobalt is in plus 3 charge. So ionic model assumes that plus 3 charge remains localized on cobalt and 6 ammonia ligands remain neutral. Again this is flawed because the experimental evidence shows that the complex ion remains intact in solution or retains its identity in solution. That means a complex ion is not a double salt. That fact we know because here primary valence is, secondary valence everything is clearly spelled out and the electrostatic interactions implied by the ionic model are unlikely to be enough to all these things to happen. So that means if you just consider that cobalt 3 plus remains as an entity in that case what happens if you put into solution that disintegrate. So that is not going to happen as a result ionic model was also overruled. So this is how the ionic model was assumed. It appears like a double salt and in solution it will disintegrate. Since that is not the case this is also overruled. Then came electro neutrality model. What it says is we know the net charge on the metal center should be 0 or between plus 1 minus 1 close to 0. So that is cobalt 3 plus ion may accept a total of only 3 electrons from 6 ligands giving the charge distribution shown here. I am going to show you the electro neutrality principle results in a bonding description for the complex having 50% ionic character and 50% covalent character. So this is what the electro neutrality principle means. That means if you see the charge distribution all on all entities is well within plus 1 to minus 1 and cobalt being 0 here. Using this method let us try to understand one more problem. So let us consider hexa-sinoferrate a realistic charge distribution results in each ligand carrying a charge of minus 2 by 3. In this model what charge does the FE center carry and why is this charge consistent with the electro neutrality principle. That means whatever the example we saw from Hexamine cobalt 3 plus let us try to extend same analogy to this one okay in which ligands carrying a charge of minus 2 by 3. So that means here each cyanide is carrying a charge of minus 2 by 3 and that means we have now 6 cyanide ligands are there. So this will be minus 4 okay if it is minus 4. Now if you consider FE 3 plus we have 5 electrons 5 minus 4 equals plus 1. So in this one FE carries plus 1 charge. So according to electron neutrality principle the charge carried by iron center is plus 1. Let us look into another problem here. If the bonding in CrO4 2 minus were described in terms of a 100% ionic model what would be the charge carried by the chromium center and explain how this charge distribution can be modified by the introduction of covalent character into the bonds. That means we have to examine this example chromate 2 minus using both 100% ionic model and also a covalent model. If you just look into 100% ionic model the CrO4 2 minus here charge will be minus 2 okay in this case the charge will be minus 2 here. But if you consider covalent model so 6 electrons are there and these 6 electrons are utilized in making 6 metal to ligand bonds according to electron neutrality principle. In that case what happens number of electrons left on chromium will be 0 so that here the charge on chromium will be 0. Next after successfully applying VACPR theory to explain bonding among main group compounds okay attempts were made by Keppert to utilize VACPR theory to explain bonding in trans metal complexes. So for this one following examples were considered and here like all 6 coordinated compounds but having different valential electronic configuration were considered for example vanadium 3 plus has 2 D electrons and manganese 3 plus has 4 electrons and then cobalt 3 plus has 6 electrons and then hexa aqua nickel 2 plus has D8 8 electrons and zinc hexa aqua zinc okay here we have 3D 10 4 S2 so 10 electrons are there that means we are considering examples of vanadium manganese cobalt nickel and zinc having octahedral geometry homoelectric hexa aqua compounds but having different electrons left in their D orbitals that means D2 D4 D6 D8 and D10 however each of these species has an octahedral arrangement of ligands that we know in spite of having different electron configuration so obviously if you go by VACPR theory to count all electrons VACPR theory cannot explain the bonding among D block metal complexes as a result some assumptions were made in VACPR theory to accommodate the explanation of bonding in D block element complexes so what is the assumption that was made metal lies at the center of the sphere and the ligands are free to move around the surface of the sphere so that means metal assume as a sphere and the ligands are free to move about the surface of the sphere okay and occupy positions as far away from each other as possible to minimize repulsion between the ligands the ligands are considered to repel one another similar to those in VACPR model okay where we were using steric numbers but unlike VACPR model Kepert model ignores non-bonding electrons and assumes coordination geometry of a D block species is independent of ground state electronic configuration of the metal center so this is the assumption according to their convenience they postulated this assumption to employ VACPR model to explain bonding in coordination complexes okay so as a result what happens ions of the type mlnm plus or mlnm minus irrespective of their cationic in nature or anionic nature if the number of ligands are same they are bound to have the same geometry again we know that this cannot be used due to various reasons because most of the complexes if you look into its geometry reactivity properties and everything depends on the electronic configuration the oxygen state and many other factors as a result this initially made early method called Kepert model to imply VACPR theory did not work well. Kepert model rationalizes the shapes of D block metal complexes okay and by considering the repulsion between the groups L okay lone pairs of electrons are totally ignored and here also if two ligands are there they propose linear geometry if three ligands are there they propose trigonal planar geometry if four ligands are there they consider tetrahedral geometry and five are there square pyramidal or trigonal bipyramidal geometry and for six octahedral geometry so what they did was the number of ligands surrounding the metal ion was considered as steric number ignoring the electrons present non bonded electrons present in the orbital then it is bound to show results very similar to main group elements but certainly it does not explain and then when we have different type of ligands it can further deviate as a result it was concluded that Kepert model is not a suitable model okay and VACPR theory is not a suitable theory to explain bonding in coordination compounds. So example they showed this trees or trisino cuprate that we can use VACPR theory ignoring the D electrons to arrive at the geometry of course here trigonal planar geometry is correct but citing one or two example it is not an ideal way to accept the model. So now let us look into the bonding concepts that were later used to explain okay bonding in transformation metal complexes the first one was valence bond theory it was proposed in 1930 by Linus Pauling and he brought hybridization concept to explain bonding and also a couple of properties and using this bonding he could explain inner orbital complex outer orbital complex similarly high spin complex and low spin complex and only spin on the magnetic moment. So in fact when Linus Pauling wrote his very famous and popular book nature of bonding he gave emphasis for his theory and demonstrated giving importance to the recognizing magnetic properties of transmittal complexes using valence bond theory but however it can predict spin only magnetic moment and beyond that many other properties including color and effective magnetic moment geometrical distortions all those things Linus Pauling's valence bond theory could not explain as a result people were looking for an alternate that time crystal field theory was proposed by Beth and Van Blic mainly based on electrostatic forces that means if you take a metal ion and when you put a metal ion into the electric field generated or originated from ligands what happens the degeneracy of orbital will be twofold and this is how this work was started and then early electrostatic interactions and their utility was mentioned by Bakurl who is known for discovery of radio activity along with Mary Curie and Peary Curie for which he got a Nobel Prize in 1903. Crystal field theory looks very ideal and here it explains different type of interactions between metal and ligand if the metal is cationic and ligand is anionic then one can talk about ion-ion interaction and if the metal is cationic and the ligand is neutral one can bring ion dipolar interaction and but if the metal is neutral and ligands are neutral like what we see in case of zero valent metal carbonyl compounds such as chromium hexa carbonyl iron pentacarbonyl where metal is neutral and ligand is also neutral in that case we have to bring or evoke dipolar-dipolar interaction unfortunately crystal field theory does not explain the bonding arising out of dipolar-dipolar interaction when the metal is neutral and ligands are neutral okay that was the only limitation and later to accommodate this dipolar-dipolar interaction the covalency was included and accommodated in crystal field theory and that become ligand field theory and ligand field theory is a beautiful theory it is a very nice combination of molecular herbal theory crystal field theory and also to an extent valence bond theory. The advantage of crystal field theory is it can explain absorption spectra that means color of complexes and it can explain magnetism and also it can explain geometrical distortions and also relative stability because it brings a spectrochemical series and gives definite position in the spectrochemical series for a ligand to say whether it is a strong field ligand or a weak field ligand and bonding in neutral complexes was also brought through ligand field theory and overall molecular herbal theory explained bonding in all type of complexes whatever the bond molecular herbal theory presently we are using is nothing but ligand field theory having good parts of all these theories valence bond theory and Moulican's molecular orbital concept and also crystal field theory very nicely developed by Bieth and van blik okay and it is a very complete theory but it may appear like time consuming but nevertheless it can give you literally all information that is needed to understand the reactivity or their application in various aspects. So before we start digging deep into these bonding concepts it is better to understand the shapes of the orbitals and their relative orientation in space. So we know that S is spherically symmetrical and we have px orbital dumbbell shaped p y and pz they are orthogonal to each other and then we have dx z dy z and dz square and dx myy square and dx y. So these 5d orbitals are there. So S orbitals are spherically symmetrical and their orientation does not affect bonding but in a in a bond involving p orbitals d orbitals or f orbitals the orbitals will be oriented in a direction that maximizes the overlapping so that means their orientation matters if oriented in a direction does not maximize overlap the bond will be weaker. Let us start looking into valence bond theory. Let me tell you about the central theme of valence bond theory I am sure most of you are familiar with valence bond theory. The space generated due to the overlapping of orbitals has a maximum capacity of accommodating two electrons with opposite spins that means when you overlap orbitals from two atoms that can accommodate two electrons with opposite spins that means between two orbitals essentially electrons are localized that is the first concept. If the overlapping is greater the bond formed is stronger and more stable bond strength depends on attraction of the nuclei for the shared electrons that means if the overlapping is greater that means if the electronegativity is comparable and the size of the orbitals are comparable then these two nuclei can come very close to each other and establish a bond in that case what happens the bonds will be much stronger. When two atoms combine to form a covalent bond the valence orbital present in each atom overlap to form new hybrid orbitals with different shapes from the original spd or f orbitals from which they are made up of that means before two atoms combine to form a covalent bond the valence orbital in each atom overlap to form a new set of orbitals called hybrid orbitals that carries the properties of all the orbitals involved in that hybridization. For example if you consider sp hybridization in that one one s and one p involved so that sp hybrid orbit would have 50 percent s character and 50 percent p character and this is how it goes and in sp to what happens we have one third s character and two third p character whereas in case of sp three hybrid orbitals we have one fourth s character and three fourth p character that means 25 percent s character and 75 percent p character that means valence bond theory uses the concept of hybridization of atomic orbitals prior to the bond formation okay a covalent bond forms when orbitals of two atoms overlap and the overlap region which is between the nuclei is occupied by a pair of electron this is an important concept okay that was proposed by valence bond theory one is hybridization concept and the second one is concentration of electrons with opposite spin between the two atoms or that are combined okay or two nuclei okay that means it gives emphasis for localization of electrons between the two atoms okay let me stop at this stage and continue in my next lecture more discussion and with more examples on valence bond theory