 In this lecture, we will be discussing cyclopentadienyl complexes. Many of them are so called sandwich compounds, but not all of them are. In fact, cyclopentadienyl complexes initiated the second wave of an ammetallic chemistry. It is only after the discovery of cyclopentadienyl complexes that there were several new discoveries made and laboratories started working earnestly on organometallics and they realized the importance of these compounds. So, let me just briefly show you a variety of structures that are formed by these compounds. They are ubiquitous in organometallic chemistry. They are very good supporting ligands and many times they are purely supporting ligands, but they can also involve themselves in reactions in a very subtle fashion. This allows them to be used in catalysis very effectively. So, it is important that we understand how these compounds are formed, how they can be made and how they can be used in organometallic chemistry. As I mentioned earlier, the most important compound or the compound that really started off this new wave of organometallic chemistry is the molecule ferrocein. You are definitely familiar with this particular molecule which was in fact accidentally discovered. The other compounds which are shown here, some of them are dinuclear, bridged and having some exotic structures are very common in organometallic chemistry as well. They are probably as common as ferrocein, but nevertheless because ferrocein initiated this race in organometallic chemistry for making newer and newer compounds, it is very often considered as the icon of organometallic chemistry. So, let us start first with this accidental discovery that I talked to you about. Two groups in 1951, one was a group of miller and the other was a group of poisson. These two people were trying to make full valine. Full valine is a simple organic compound which is pictured here. They wanted to make this compound by oxidizing cyclopentadienyl anion. To do that, they made bromocyclopentadiene and then made the grid nut which is shown here and tried to oxidize the grid nut using ferric chloride. Ferric chloride is indeed a good oxidizing agent and their idea was perfectly sound. Surprisingly, instead of making this full valine molecule, they got something where the properties of the compound did not match the expected properties. So, as they were wondering what the structure would be, poisson in fact proposed that the molecule was in fact an ion containing compound. It had an ion in it by analysis and so he proposed this compound where the ion had replaced the magnesium as an electropositive element, a simple trans-metallation compound and he suggested that it was a sigma bonded organometallic compound with ion in the middle between two cyclopentadienyl units. At that time, there were no known examples of extended pi systems coordinating to metal atoms. So, this was the beginning and it was natural that they think about a sigma bonded system as a possible organometallic compound that could be isolated through this reaction. However, all the properties of the compound, especially the diamagnetism did not match the type of the diamagnetism was not matched by the structure that they had proposed. Two people, both in England and in Germany, in the US and in Germany, two groups, two major groups. One was the group of Wilkinson and Woodward and Harvard and the other group in Munich headed by Fisher. These are the two people who proposed a different structure. The radical structure that they proposed was the double cone structure which was proposed by the double cone structure was proposed by Fisher. The sandwich structure was proposed by Wilkinson. Wilkinson and Woodward often had lunch conversations about their research and this was an exciting piece of result that had come out in the literature and probably the sandwich they had proposed suggested this new structure for them, a sandwich structure for the iron cyclopentadienyl complex. Nevertheless, over the years the double cone structure proposed by Fisher did not, that name did not stand the test of time and it was the sandwich structure which was preferred. The sandwich structure, the name sandwich structure was preferred by people. So, we now call most of these compounds where a metal atom is in between two planar molecules as sandwich molecules. The double cone structure in fact came from a scientific background. He looked at the X-ray density that was there and he found that the X-ray density was in the form of two cones and that led to the double cone structure. So, Wilkinson used the diamagnetic behavior, chemical behavior of this molecule to propose this sandwich structure whereas, Fisher proposed on the basis of the X-ray and the infrared evidence that in fact it was a double cone structure. So, after this structural analysis was completed it was quite clear that the structure of the molecule was indeed unique and it was realized that planar pi systems can coordinate two metal atoms in a symmetrical fashion. The coordination number of carbon which was originally restricted to the value of 4 need not be preserved in these molecules. So, the bis cyclopentadienyl structure, the sandwich structure is pictured here is indeed a celebrated structure because it was the first one that was discovered with M equals Fe. It was discovered and it was shown that it was in fact a pi system which was extensively delocalized and at the same time coordinated to the metal. The bond distances between the metal and the carbon, the bond distances between the metal and the carbon were all symmetrical and it was also found that in some of the structures the cyclopentadienyl groups could be eclipsed or they could be staggered. We will see some of those structures after a few slides. So, after Fisher and Wilkinson realized that the double cone structure was in fact possible with iron there was a historic race where both groups tried to make the metallocene of every other metal. So, they tried to make the periodic table of metallocenes with all the transition metals and in fact at the end of this race and after discovering several new compounds and their chemistry they were awarded the noble prize in 1973. It was around 1956 that they had proposed the structure for cyclopentadienyl, bis cyclopentadienyl iron compound and it was in 1973 that both of them were awarded the noble prize. Wilkinson was the one who in the group Wilkinson and Woodward it was Wilkinson who started this race and Woodward who had predicted the chemistry correctly for these compounds did some studies on the organic chemistry of these molecules, but then he did not pursue the inorganic chemistry further. So, what is happening in these molecules? Let us just take a closer look at cyclopentadienyl group C 5 H 5 minus should in fact be an aromatic system. So, cyclopentadienyl has got 6 pi electrons that is 4 n plus 2 pi electrons and where n equals 1. So, it should be a simple aromatic planar ring and in fact it is found as a planar ring in some of the structures that have been characterized in even in the free state that means C 5 H 5 minus as a counter ion can be found in crystal structures and in fact it is planar and it is found to be an aromatic system. The chemistry of C 5 H 5 minus is extensive and sometimes it is advantageous to have the hydrogen substituted by methyl groups. So, it has become very popular to have pentamethyl cyclopentadienyl group and this is C 5 M E 5 minus the per methylated analog of cyclopentadienyl and this group is called the C P star. In fact it is a star ligand it is used in a variety of reactions where you do not want the cyclopentadienyl group to have any electronic influence or a steric influence. It is essentially an anionic R minus group, but has the ability to coordinate through all the 5 carbons. If you have a simple alkyl group we alkyl anion it has only a single point of contact whereas, the allyl has 3 carbon atoms which can coordinate to the metal simultaneously the allyl anion which we just saw a few lectures ago and the C P as a cyclopentadienyl has got 5 carbon atoms coordinated to the metal atom. So, if you look at the structure we very often indicate the cyclopentadienyl metal structure using this representation where we have a planar C 5 H 5 unit coordinated to the metal with a metal having a bond to the center of the ring. There is no atom which is present at the center of the ring it is only to indicate that all the 5 carbon atoms are equally bonded. If you were to draw structures where all the 5 carbon atoms are indicated with bonds then the structure will look very clumsy. Now you can see how that representation might look. So, that seems to be a bad representation and so the representation that has come about that is used by the the representation that is used popularly by chemists all over is the representation which I just showed you where only the central bond which is to a ghost atom at the center is indicated by a line. Now when we indicate it like this we intend to convey the message that all 5 carbon atoms are interacting with a metal atom. So, the preparation of metallocene is something that is of interest and it is of value for us to consider because it is an aromatic ion. The cyclopentadienyl anion is a stable moiety on in contrast with the linear or the acyclic version of the pentadienyl unit pentadienyl anion is also known and it is also known to form complexes. But the cyclopentadienyl anion is stable because of the aromaticity and it can be readily generated by removing a hydrogen from this carbon. So, if you have a hydrogen C H 2 group cyclopentadiene then we can remove it easily as a proton and generate the anion. Any base will do in fact a wide variety of bases can be used. Simple organic base like triethylamine can work very well and can deprotonate the cyclopentadienyl group and generate the anion easily. And we can have several trans metallation reactions possible. It is possible for us to have thallium cyclopentadiene made and stored in the laboratory. Thallium is a univalent metal and so T L plus C P minus is an extremely stable molecule that can be stored. And it is a convenient source of cyclopentadienyl group. The only problem being that thallium is in fact toxic. The toxicity of thallium should be born in mind and one should handle it with extreme care if one is using that as a source of the cyclopentadienyl anion. But it is possible to generate the cyclopentadienyl anion rather readily in the laboratory by a simple grinna reaction between cyclopentadienyl bromide. And one can also use lithium metal and cyclopentadiene itself. C P L I is usually made from cyclopentadiene and lithium sand. The lithium sand is what is made when you bring lithium to the melting temperature and shake it vigorously when it breaks down to form very fine globules. These globules are so fine that they have a large surface area and can react readily with the cyclopentadiene molecule. Now the after the discovery of the structure and after understanding the chemistry behind the cyclopentadienyl complexes ferrocene was made fairly rationally and very effective means of making ferrocene were generated. The simplest way by which you can generate it in the laboratory is by reacting ferrocene is by reacting F E C L 3 with iron powder. The iron powder is merely there to reduce the iron 3 to iron 2. This is the purpose of using the iron powder and F E C L 3 together. In conjunction with cyclopentadiene and triethylamine, one can in fact triethylamine and cyclopentadiene in stoichiometric quantities, one can in fact form the ferrocene molecule fairly readily. The amine is there to remove the H C L that is formed between iron trichloride, the iron trichloride and the cyclopentadiene. So this reaction is fairly effective and you can replace this whole system by a simpler system but that requires a slightly higher temperature, which is the direct reaction between iron and triethylamine hydrochloride at reasonably higher temperature in the presence of cyclopentadiene. So if you use cyclopentadiene triethylamine hydrochloride as a catalyst and iron it is possible in fact to generate this molecule in a very good yield. Now you always have to generate cyclopentadiene in a fresh fashion. There is no substitute like having a molecule that can be stored in the laboratory unless you want to store it as thallium cyclopentadiene and that is because cyclopentadiene readily dimerizes to give you C 10 H 10 and this molecule of C 10 H 10 is readily available. It can be cracked so to speak by heating it to a high temperature in the absence of any other reactant. When you do that it splits back into C 5 H 6. So C 5 H 6 is formed and C P H twice that is C 5 H 5 H twice and that should be 12 here. C 5 H 6 twice is the dimer of cyclopentadiene is the one that is readily available in the laboratory. So let us look at the bonding in these molecules. It is possible to make these molecules by a simple reaction between cyclopentadiene. Let us look at the bonding in these molecules. If you remember the orbitals of the iron atom or any transition metal you know that we have a 4 S orbital. We have a 4 S orbital for the n plus 1 p orbitals and the n d orbitals. These are the d orbitals. These are if they are n d then these are n plus 1 s n plus 1 p orbitals. So if you are considering the first row of transition metals then this will be 3 d and this will be 4 S n 4 p. These are the orbitals which are available for the metal atom for interaction with the cyclopentadienyl group. The cyclopentadienyl group itself the molecular orbitals of the cyclopentadienyl group need to be simplified significantly. One only needs to consider the pi system which is available on the cyclopentadiene. One need not look at all the molecules all the molecular orbitals of cyclopentadienyl anion. So there is a simple way by which one can arrive at the energetics of these molecular orbitals the pi m o's of the cyclic ring systems that cyclic pi ring systems that we are talking about. One has to take a sphere which is shown here on the screen and if you inscribe the appropriate planar molecule that you want to examine. So one can in fact take a circle and inscribe a square if one wants to find out the energy levels of the cyclobutadiene. So if you have a square cyclobutadiene inscribe the square cyclobutadiene inside a sphere and that will give you the energy ordering of the molecular orbitals of the pi system of cyclobutadiene. Similarly benzene and here we have to look at today the cyclopentadienyl group. So the pi system the Huckel molecular orbitals can be simply derived by drawing them inside a circle and that is called the phrase circle. So now here are the m o's now generated for the cyclopentadienyl molecular orbitals and if you remember you have 6 C 5 H 5 would be radical, C 5 H 5 minus would be an anion which has got the 6 pi electrons. So these are the 6 pi electrons which are available for the cyclopentadienyl group and you will notice that the pi system is completely symmetrical in the case of the lowest energy molecular orbital and all the orbitals are of the same phase. That means they are pointing in the same the p orbitals which are pointed in this direction in the upward direction perpendicular to the plane of the molecule have the same phase and the next set of molecular orbitals have got a single node. So this node for example here you will notice that there is a node which is going through in this fashion. So 2 orbitals have the same phase these 2 orbitals have the same phase and the other 2 orbitals have the opposite phase but in the same fashion the next m o will also have a single plane a nodal plane and that nodal plane is indicated as passing through this molecule you can notice that the 2 orbitals are closer to you these 2 orbitals have the same phase and the other 3 orbitals have the same phase. So you can see that there is a single node for psi 2 and psi 3 if one might write this as psi 2 and psi 3 you have a single nodal plane and for psi 4 and psi 3 you have a single psi 5 the number of nodes increase further. So you have 2 nodes one node in this fashion and the other node in this fashion. So you can see that that is true for this also this molecular orbital also you have 2 nodes but they will make the diagram clumsy so I am not drawing it right now but you can see that the number of nodes keep increasing as you go up the energy diagram right here and these molecular orbitals of the pi molecular orbitals of the cyclic systems can interact very efficiently with the metal orbitals. So first let us can take a look at what is going to happen I will briefly switch the views. So now let us look at the bonding in these molecules in order to get the orientation right. Let us just refresh our memories about the nature of orbitals on the metal atom. The metal atom has got Ns, N plus 1, Nd, N plus 1s and N plus 1p orbitals these are pictured here for the first row transition element you would have 4s that would be 4s, 4p and then 3d orbitals. Now these orbitals are arranged in such a way that the dz square orbital is going to be along the z axis which is in this particular orientation. Now the x axis is the one which is the x z plane is the one which is the plane of the screen and the y axis is going in and out of the plane of the screen. So now let us look at the energy levels of the cyclopentadiene anion or for that matter any of the cyclic pi systems they can be readily derived by drawing a circle and inscribing the polygon in that circle. So if you want to derive the energy levels of for example cyclobutadiene you would just draw a figure you would have to draw a circle and then inscribe the polygon in that case the cyclobutadiene would be a square. So you would inscribe that and then the energy levels of the various orbitals would be just the places where the square intersects the circle. So in the case of cyclobutadiene cyclopentadiene anion you would have the orbitals having the energy levels which are indicated here and since you have 6 electrons you would just fill them up in order to get the appropriate energy level diagram. So let us now look at the actual form of these molecular orbitals of the cyclopentadiene anion and these are pictured here. Notice that the cyclopentadiene anion is placed in such a way that it is in the x y plane. The metal atom itself is in the z axis along the z axis and so the metal atom is placed along the z axis and that is placed let us say at a point here and the cyclopentadiene anion is placed in the x y plane. So that would be placed as if it is going in and out of the plane of the screen. So here are the form of the molecular orbitals of cyclopentadiene anion you will notice that the lowest occupied molecular orbital that is this orbital right here has got all the p orbitals oriented in such a way that they are in phase which means that there is no node when you look at these MOs or these atomic orbitals all of them are facing in the same direction and there is no node. The next higher energy molecular orbital of the cyclopentadiene anion has got one node and this node nodal plane is running perpendicular or it is in the y z plane it is in the y z plane. So the nodal plane is passing in the y z plane and it is perpendicular in such a way that you now have a single node in the cyclopentadiene p orbitals. Now you have another set of orbitals which are even higher in energy and these orbitals have got two nodes two nodal planes and these nodal planes are easy to analyze right here they would the two nodal planes are going in this two directions perpendicular to one another and if you look at this molecular orbital they would be slightly diagonal but you can see there are two nodal planes right here for this MO also and in this MO the nodal plane is running in this direction. So that would be the x z x z plane is a nodal plane here you have the y z plane as the nodal plane. So now you see that as you go up in energy the number of nodes increases. Now these molecular orbitals will now have to be matched with the molecular orbitals the atomic orbitals of the metal atom. So let us do this let us take a look at the MO's that are available for the metal and these are pictured here with only one cyclopentadiene anion the MO's of one cyclopentadiene anion. So that we can proceed in a stepwise fashion and see how they match what you have to do is to make sure the symmetry of these orbitals are well matched. Now let us first take the 4 s orbital the 4 s orbital in the case of the 3D transition series. So the 4 s orbital is pictured here you can see that you can put the metal s orbital on the top of this lowest energy molecule orbital and it would have a perfect overlap with all the p orbitals because all of them are in the same phase and s orbital is also having the same phase. And if you look at the p z orbital that also has a proper match and you can see that this can be matched so that in put in the center of the cyclopentadiene anion in such a way that it would have a good match with these orbitals as well. So similarly if you take the d z square orbital that also has a good match with this molecular orbital. So in other words the lowest energy molecule orbital of cyclopentadiene anion has got the right symmetry to interact with all 3 of these orbitals and you will notice that it has got sigma symmetry which means that you can rotate the cyclopentadiene anion as much as you like with respect to the metal atom along the z axis and there would be no break in the overlap between the metal orbital and the cyclopentadiene anion orbital. Now if you go to the next step you see that the p x orbital has got the right symmetry to overlap with the cyclopentadiene anion and the metal p x orbital. So the metal 4 p x will match perfectly in a pi fashion this time with one of the molecular orbitals and similarly if you take the p y orbital that would be matched with this orbital right here. The p y would match with this and the p x would match with this molecular orbital. The d x z orbital also has the right symmetry. So in this particular case it is easy to visualize if you place the metal atom in the center of the cyclopentadiene anion slightly below the plane in which the cyclopentadiene anion is present. You will see that this now has got a pi symmetry because if you rotate the cyclopentadiene anion some of the symmetry would be some of the overlap would be lost. If you rotate it by 90 degrees it would be completely lost. So this two orbitals d x z p x and the p y will match with these two orbitals right here and you will notice that the d x z is pointed towards the cyclopentadiene anion and so we will have better overlap whereas the p x has got only lateral overlap since one orbital is lying in the plane of the metal atom and the cyclopentadiene anion is in a parallel plane. Now similarly there are two d x squared minus y squared orbital which will have a suitable overlap here but that would be very poor overlap also because the metal is in a different plane and it is not pointed towards this set of orbitals. So these are the orbitals which you can see by matching the symmetry you would be able to find out which orbitals on the metal can overlap with the orbitals of the cyclopentadiene anion. Now let us proceed further. We know that in ferrocene there are two cyclopentadiene anions which are present in two parallel planes. So if you take the lowest molecular orbital on the cyclopentadiene anion you can either place them in such a way that the two orbitals are oriented in the same way. So in such a manner that you will get the shaded portion of the lobes pointed towards the unshaded portion of the lobes of the other cyclopentadiene or you can invert them in such a way that the unshaded portion is facing the unshaded portion of the other cyclopentadiene anion. This is equivalent to saying that you take psi a that you take psi a minus psi b in order to get this combination. And not get this combination you would get psi a plus psi b. So these are two combinations that can be present because these two molecular orbitals are far away from one another the energy of these two orbitals are very close to one another. We will see this in the next diagram but now let us mix and match the molecular orbitals. Here is the s orbital here is the s orbital and if you want to use the s orbital if you take the s orbital and if you take the s orbital and place it here you can see that it has got the right phase to overlap with the two molecular orbitals of cyclopentadiene anion. Similarly if you take the d z squared orbital that also has the right symmetry to overlap with this symmetry molecule orbital where you have combined psi a minus psi b. Now if you take the p x orbital then you notice that this molecule orbital where we have taken psi a plus psi b that has got the right combination but all of them have got the sigma symmetry in order to overlap with these two molecular orbitals. Now you will also notice that you cannot use a combination of the p x or the d x z to form any combination because the overlap net overlap would be 0. The symmetry of that will not allow you to match the d x z with this combination or this combination. So as a result you can see that by just by matching these molecular orbitals you can arrive at a qualitative molecular orbital energy diagram and that is what we have done here. We have transformed the matched m o's into energy level diagram and that tells you that if you have 6 plus 6 12 electrons on this two cyclopentadiene anions they can be filled up to this level. Now in the case of iron we know that there are 6 electrons on the d orbitals also that is Fe 2 plus has got 6 d electrons and they will be filled in the d x squared minus squared the d x y and the d z squared. So that is what we have done here we have filled it up up to this level. Now you will notice that because you have 6 electrons here and you have 12 electrons from the cyclopentadiene anions you have filled up up to the energy level where there is sufficient stabilization. If you fill up more electrons you will end up filling the d y z and the d x z and those orbitals are the destabilized orbitals and so the system itself will be destabilized. So 18 electron rule holds good in this case also the 18 electron system which is formed by iron 2 plus and 2 C p minus units form a very stable molecular orbital setup and it is the one which is the most stable molecular orbital which is available for the metallocenes among the various metal atoms that you can think of. So you can see that the filling up of less number of electrons or if you fill up more number of electrons you would lose the total stabilization that is present and you can also see that the d z squared is almost present in a non-bonding molecular orbital and you can remove one electron from this d z squared and you still have a stable system. So ferrocenium ion Fe C p 2 can be oxidized to Fe C p 2 plus and you still have a stable molecular orbital. Another interesting feature of this molecular orbital diagram is the fact that the cyclopentadiene ions are stabilized mostly by the sigma interactions that are present and so you can have free rotation of the 2 cyclopentadiene ion groups around the z axis. If you remember the orientation of the 2 cyclopentadiene ions they are located in such a way that the metal is in the z axis and the 2 cyclopentadiene ions are on the x y plane parallel to the x y plane and so they turn out to be freely rotating systems and for many metallocenes you can rotate the cyclopentadiene ions freely and still have the stability of the whole molecule due to electronic stabilization. So this helps us understand the electronic structure. Now let us look at the filling up of the electrons and the properties and how they come about in the next slide. So the properties of the metal complexes are all satisfied by these by this particular simple molecular orbital diagram of the picture that I drew for you here. So a variety of metal 2 plus compounds can interact with C p, C p minus 2 C p minus units and form sandwich compounds. Many of them have electron counts of 18 or more and when they have more number of electrons then they end up with let us say extra unpaired electron as in the case of cobalt which has one electron more than ion and this electron is right here as an unpaired electron leading to a magnetic moment of 1.76 Bohr minotrons and this is exactly what you would expect for a molecule with one unpaired electron. The interplanar distance between the 2 C p rings so that means the distance between the 2 C p rings is 332 picometers and in the case of cobalt it ends up with distance of 340 picometers. So in the case of nickel you have 2 unpaired electrons and you are populating some of the anti-bonding orbitals and as a result you have a paramagnetic molecule with 2 unpaired electrons a both magnet ferromagnetism of 2.86 is of 2.76. A paramagnetic molecule is observed having a magnetic moment of 2.86 Bohr magnetons and the distance between the 2 C p rings is also increased to 360 picometers. You will notice that ion has got the shortest distance between the 2 C p units. So in other words they are tightly the 2 C p minus groups are tightly held to the ion and that is natural because that is the one way you have the perfect combination of metal electron count and organic component electron count. Manganese and chromium have got less than what is required and very often they acquire an extra pair of electrons from another ligand and stabilize the system. But nevertheless chromosine has been crystallographically characterized and we will look at this molecule in a moment and that has got a bond distance which is slightly higher and is as good as nickelosine which is 360 picometers. Venadozine is again a molecule which has got 3 unpaired electrons and the expected paramagnetism for this molecule is exactly what you observe in the system. So you can see from this list of properties that in fact it is possible to predict the experimental magnetic moment from the molecule orbital diagram that we drew just now by mixing and matching the appropriate molecular orbitals. Now ferrocene as I mentioned to you is the most stable molecule it is got it is stable up to 300 degrees which is remarkable for simple organic molecule it sublimes and can be in fact if you crack it or heat it very to very high temperatures in the absence of air it will not it will get converted to carbon nanotubes and that is one of the popular uses for ferrocene nowadays. But during this process some iron atoms get occluded in it and in the case of ferrocene the bond distance between as I mentioned earlier the iron carbon bond distance is the shortest. And it is also to be noted that in the case of nickel and zinc you end up with molecules which are going further and further apart the two C P rings are going further and further apart. There is a different way by which you can move the metal atom away from the center and so reduce the number of electrons that are donated to the metal atom. So let us take a look at some of the structures which we have and I will first show you the structure which is formed by one of the metal atoms. So this is the metal atom which is chromocene. So chromocene has got a structure where the two cyclopentadienyl units are perfectly eclipsed so the two C P units are perfect perfectly eclipsed. And as I told you it has got electron count which is not perfect and so the carbon, carbon interplanar distance between the two, between the two rings is slightly more than what you have for ferrocene. So nevertheless this shows you how the structure can exist and here is another molecule which is co-crystallized with chromocene and this molecule is in fact a dysprosium element which I will show you in a moment. The dysprosium is also forming a cyclopentadienyl complex but it has got two iodides which are shown in purple here. These two iodide atoms are also coordinated to the dysprosium but you will notice that because the dysprosium requires more number of electrons it has got the cyclopentadienyl units coordinated to it and in addition it has got two iodide molecules also coordinated to it. So this is the species which has got an anionic charge and the chromocene has lost an electron to the species and so this is how you have an anion and a cation co-crystallized in this particular molecule. So let us take a look at another molecule now. We first looked at the, so here is, here is another molecule. In this case the molecule is a venidocein and you will notice that the venidocein is staggered. The two cp rings which are present in, they are parallel to each other. Here you can see that they are perfectly parallel, the plane in which the C5 H5 units are placed are perfectly parallel with one another but in this instance they are perfectly staggered and in the case of the previous structure the chromocene we found that it was perfectly piclipsed. So the nature of these molecules allows for very easy rotation of one ring with respect to the other ring. So the cyclopentadienyl ring is very easy to rotate along this vanadium cyclopentadienyl centroid axis. So it is very easy to rotate. It is possible to spin the Cp molecule very fast and as a result it is this has some consequences for the spectroscopy of these molecules. You usually end up seeing equivalents for all these 5 hydrogens which are present in the cyclopentadienyl moiety. So now let us proceed to get another molecule. If you move to systems where you have more than the required number of electrons on the metal atom, in other words if you take a molecule like zinc, if you take a molecule like zinc, zinc is also a 2 plus metal atom and here you have zinc coordinated to 2 Cp rings. So if you have 2 Cp rings it should be having the right number of the charges to balance. But in this species you can see that there are 3 Cp rings which are coordinated to the zinc and these 3 Cp rings coordinated to the zinc render an extra electron to the system. As a result there is a counter ion which is sodium which is present in this molecule. So here is a sodium atom which is sodium ion cation which is present as a counter ion for this Cp3 zinc 2 plus molecule. The Cp3 zinc 2 plus molecule is also not symmetrically coordinated as we saw in the previous case. So let us just take a look at a closer look at how the zinc is interacting with the Cp unit. So here I have rotating it in such a way that you can see that this Cp molecule, this cyclopentadienyl unit is interacting mostly to the zinc which is labeled as zinc 1 here through only a single carbon. I will mark it so that you can see the type of interaction that you have. Let us measure this distance between this carbon and the zinc it is 2.06 angstroms. This is 2.06 angstroms whereas if you take this carbon which is much further away from the zinc you can see that it is 3.3 angstroms. So the cyclopentadienyl group is displaced from the centroid of the cyclopentadienyl unit. The zinc is displaced from the centroid in such a fashion that it is interacting with only one carbon atom. But if you look at the second cyclopentadienyl unit that it is interacting with you will see that two carbon atoms are interacting with it. So here is one carbon atom which is at a distance of 2.4 angstroms and here is another carbon atom which is at a distance of 2.08 angstroms. But these three carbon atoms which I have marked out for you these three carbon atoms are at a much longer distance from the zinc one. So in one side zinc is interacting with two carbon on the other side it is interacting with only one carbon and with yet another C P unit it is interacting with only one carbon. So you can see that you can have very unsymmetrical bonding with the cyclopentadienyl units when you have metal atoms which have more number of electrons than necessary for forming a stable metallocene structure. But nevertheless these molecules have fascinating structures. Now in the case of beryllium where the beryllium it is a main group element here I am showing you beryllocene. Beryllocene is a case where you have V e 2 plus and two C P minus units and these two C P minus units retain a perfectly planar structure and so you have only C 5 H 5 units which are in two planes and these two planes are parallel to one another also. So unlike ferrocene where the two cyclopentadienyl units are eclipsed or staggered you will notice that these two C P rings are slipped with respect to one another. So they move away from one another and the beryllium is interacting with only two carbons. So you can see that beryllium is interacting with two carbons and the remaining carbons are not interacting with the beryllium as much as these two. So you can see that there are a fascinating range of cyclopentadienyl structures that can be formed. In many cases the cyclopentadienyl unit is interacting in a eta 5 fashion that means all 5 carbon atoms are interacting together with the metal atom. But when the metal is either too small as in the case of beryllium or as in the case of zinc when it has got too many electrons, zinc is a 2 10 plus D 10 system. So that means there are 10 electrons then the metal slips from the centroid of the C P minus and moves away. So that the number of carbon atoms it interacts with is less and nevertheless many of these molecules are stable and form interesting molecules for study. So let us get back now to the structures. The bonding as I mentioned depends very much on the electron count. If you have a beryllium then it tends to interact with only one part of the cyclopentadienyl ring and because it has got only S and P orbitals to interact with the cyclopentadienyl unit. In fact, beryllium is a fairly very complex system because you have different structures. The one that I showed you had 2 carbons interacting with the beryllium. You also have beryllium where one of the carbons is interacting strongly with the beryllium and on the other side 2 carbons are interacting with the beryllium. So there are also molecules where you have for example, copper which is a detent system again. This structure turns out to be completely fluxional where it appears as if one carbon is interacting strongly with the copper. But the copper is oriented in the center of the towards the center of the ring as far as crystallography goes. So many interesting phenomena are observed in the case of cyclopentadienyl metal complexes. Importantly the C P ligand can adjust the number of electrons it can give to the metal. So if you draw for example, you draw a line which goes in between the cyclopentadienyl ring in this fashion like the arrow that I have shown you. You can orient it in such a way that only the first carbon atom which is only this carbon atom is interacting with it and then you would have a eta 1 structure and you can have systems where a lilac coordination is present. So that means the molecule is oriented in such a way that 3 carbon atoms 3 carbon atoms are interacting with the metal and then you would also have systems where you have 5 carbon atoms which are interacting with the metal. In which case of course, we draw a circle and then we indicate it as a eta 5 structure or if it is like an allyl we say it is a eta 3 cyclopentadienyl unit or we can say that it is a eta 1 unit. So, it is very rare to have 4 carbon atoms interacting with the metal. But as I just showed you one structure with verulocene it is possible even to have structures where you have 2 carbon atoms of the cyclopentadienyl unit interacting with the metal atom. So ferrocene can be oxidized and protonated FECP 2 for example, is very readily oxidized with silver tetrafluoroborate and that generates FECP 2 plus ion and this species is quite stable. One of the few ionic cyclopentadienyl molecules one of the few sandwich molecules metallocenes that can be very readily oxidized and studied and in the ferrocene unit can also be protonated a proton as an H plus can be added to give you FECP 2 H plus. It is not very clear where the hydrogen is present in these molecules. Let me just show you one option for this molecule it is possible to have ferrocene pictured like this and the proton can come from the top phase of the C P. So, in fact it was suggested that it has a structure where you end up with protonated species like this and then the ferrocene migrates the proton migrates on the ferrocene. So, that you end up with protonated species which is shown right here. So, initially the proton arrives at the cyclopentadienyl unit as one would imagine because that is a negatively charged species and then the proton migrates from the top cyclopentadienyl unit to the metal atom to give you iron protonated species. This is in fact a resonance of minus 2.1 ppm suggesting the fact that it is sitting next to the metal atom itself. So, you can see that there is some very interesting chemistry. What is important is that the highest occupied molecule orbital is in fact located on the iron. So, you would have expected the proton the electron to go away from the iron and you would have an unpaired electron on the iron and that is exactly what you see. But you would expect the proton also to go directly to the iron because that is the highest occupied molecule orbital. But in fact it seems to go to the cyclopentadienyl unit first and then migrate to the iron. That is in fact a kinetic phenomenon. Now, it is also possible to also possible to do aromatic type reactions with ferrocene and that was what was predicted by Woodward after looking at the structure because it is a cyclopentadienyl anion which is aromatic and that is coordinated metal and he treated it with acetyl chloride. Acetyl chloride and aluminum trichloride are classic combinations of for doing the Friedelcraft's reaction and he used ferrocene and treated it with acetyl chloride and lo and behold you could do a reaction on the cyclopentadienyl unit as if the ring is an aromatic benzene ring. So, that was another reason why the name ferrocene stuck because it was like benzene. So, you have an aromatic ring system which is present. So, yet another reaction to show you how the molecule is in fact aromatic. If you take ferrocene and treated with lithium metal and tetramethyl ethylene diamine, tetramethyl ethylene diamine is an extremely good complexing agent. So, there are four methyl groups on the two nitrogens and that makes the nitrogen extremely electron rich and it coordinates to the lithium very readily and stabilizes the lithium when it is attached to the cyclopentadienyl ring. So, this lithium atoms are in fact stabilized by the ring system that I just talked about. So, here is the ring system that is coordinating to the lithium and stabilizing it. In fact, there are three of them three tetramethyl ethylene diamine units coordinated to the two lithium molecules. This system is so stable that you can lithiate it, you can treat it with even one equivalent of lithium and you will end up with the dilithiated species just as much as you have the monolithiated species. So, it is very difficult to prepare pure forms of the monolithiated species, but nevertheless the dilithiated species can be prepared in a pure form by treating it with an excess of lithium metal in the presence of trimethyl ethylene diamine. So, this is again an aromatic behavior of the ferrocene. You will remember that molecules like benzene can be lithiated very readily to give you lithiobenzene. So, this is again an aromatic property and that is coming about because the acid there is some acidity associated with the hydrogens on an aromatic ring system. So, this brings me to the end of today's lecture where we have looked at a variety of sandwich complexes and as I told you the chemistry of this sandwich complexes are still fresh. In 2010 there was a review article that was published by Paul Chiric and that talked about how for 40 years the transition metal complexes of titanium have fascinated chemists. And this is a interesting story because in 1956 we had this cyclopentadienyl, this cyclopentadienyl titanium synthesized by Wilkinson, but many people could not reproduce that result. There was in fact a green paramagnetic compound that was reported. Later on it was shown that the molecular weight was twice as what was expected and then it was after nearly 20 years that the NMR spectrum showed titanium hydrogen bond. Finally, in 1992 the structure of the molecule was solved and it was shown that it was in fact a fascinating dimeric structure. So, this brings us to the end of a brief introduction to cyclopentadienyl complexes of metal atoms. As I told you that this is like a periodic table all the metal atoms have been synthesized and studied. So, we will continue with this in the future.