 In this lecture, we will talk about sandwich complexes again. This is in fact, the third lecture in the series, where we are talking about metal atoms or metals and a few ligands, which are sandwiched between two organic pi systems. These pi systems could be aromatic or could be even anti-aromatic as in the case of cyclopyridine. Now, in today's lecture, we will also talk about some variations in these sandwich complexes, where the sandwich is not complete as in the case of a half sandwich complex or may be a bent sandwich complex or even something that is open, where the cyclic pi system is not there, but it is only an acyclic system, but it is still sandwiches, a metal atom and its components. There are still more complicated and exotic structures such as multiple metal atoms between organic pi systems or even multiple metal atoms, which surround a cyclic pi system. So, the type of complexes that are formed by organic ligands are quite large in this category, where we classify them as sandwich complexes. Today's lecture, we will explore some of these variations. To start with, I would like to point out a study that was done by Longwood Higgins and Orgel way back in 1956. If you recollect the first lecture in the series on sandwich complexes, it was in 1956 that the first sandwich complex was identified. Although these compounds were several sandwich compounds were known, the correct structure was identified through x-ray and through the intuition by Wilkinson, Woodward and the groups in Germany, which correctly identified the double cone structure or the sandwich structure. So, at this time it was Longwood Higgins and Orgel, who studied cyclobutadiene. Cyclobutadiene was already known to be an anti aromatic system. So, they looked at the nature of a metal complex that would be formed by this cyclobutadiene, which is an anti aromatic system with a metal compound. They conjectured that suitable transition metal ion for the formation of a sandwich or rather a pie complex between cyclobutadiene and a metal would be of the type Mx2C4H4, where M is nickel either nickel palladium or platinum and X is actually a univalent ligand. So, they arrived at this structure where they said NICN2C4H4 for example, could be a stable complex or be an intermediate in the rapace synthesis, which involved acetylene and nickel disynide. So, they predicted before it was actually synthesized that a complex of cyclobutadiene could be in fact formed by the ion group metal atoms. So, the synthesis of the 4 membered rings the cyclobutadiene rings in the coordination sphere of the metal atom was done in a couple of different ways. And we will just look at a few ways, because it illustrates some different methods by which these molecules are synthesized. You can see here that nickel literally substitutes for the tin, but there is no oxidation state change on the nickel. You start with nickel dibromide, you start with nickel dibromide and that is replacing the tin dimethyl compound that is being eliminated in the system. So, and you have a dimery complex of the nickel which is formed, where you have tetraphenyl cyclobutadiene coordinated to the nickel. Similarly, you could also have a dichlorocyclobutene which reacts with ion di carbonyl, ion here however is in the 0 oxidation state. And one ion gets oxidized to ion 2 plus, this ion gets oxidized to 2 plus and it forms a f e c l 2. And the cyclobutadiene is coordinated to ion tri carbonyl. So, you can see that variety of reactions are possible, all of them generate this very stable system where cyclobutadiene is coordinated to a metal complex metal fragment. Now, here is the structure of cyclobutadiene and here it is coordinated to ion tri carbonyl and you have 3 carbon monoxide molecules which are in fact symmetrically coordinated or the carbon monoxides are like a pianostool which are holding up the butadiene unit. So, these are in fact called pianostool complexes, where the butadiene unit is in fact, you can just draw out to show that. So, here is the butadiene part which is the stool of the pianostool and these are the 3 legs of the pianostool. So, this is a typical complex that is formed by cyclobutadiene and ion tri carbonyl which is quite stable. There are several methods as I told you, here is a template method where you would take diphenyl acetylene and in the coordination sphere of ion pentacarbonyl, ion is in the zero oxidation state here and ion is in the zero oxidation state here. So, no redox reaction is involved. The templated synthesis of the cyclobutadiene however is achieved in the coordination sphere of ion tri carbonyl. No intermediates were isolated although one can conjecture that ion complex would go through an intermediate where ferrocylopentadiene would be an intermediate. This would collapse as in the case of the nickel system that we talked about. This would collapse to give you the appropriate number of carbon monoxides attached to the ion. You would end up with the ion tri carbonyl complex. So, in fact there are examples of complexes where ferroles as they are called and in conjugation with the acetylene has been isolated. Such complexes have been isolated and so this is quite an acceptable intermediate in this reaction. The generation of cyclobutadiene which is an unstable molecule is difficult. So, one has to generate it in the coordination sphere of the metal. That is what we have seen in the last three methods of synthesis. So, here we have a photochemical reaction in which a mole of carbon dioxide is eliminated from this bicyclic molecule. So, here is a mole of molecule of carbon dioxide which is eliminated and that generates a cyclobutadiene. Now, because you are doing it in the coordination sphere of the ion you end up isolating a stable molecule of cyclobutadiene coordinated to ion tri carbonyl. So, you will notice that these reactions have to be carried out. So, that the cyclobutadiene is captured by the metal atom before it is too late. Otherwise it dimerizes and it generates a molecule which is cyclic and which is just a dimer of the cyclobutadiene. So, this is the molecule that you would isolate if you are not careful enough to provide a stabilizing for cyclobutadiene which is anti aromatic and it would dimerize rapidly. Now, although cyclobutadiene itself is anti aromatic it seems to exhibit reactivity that is characteristic of aromatic compounds. One of the characteristic reactions of aromatic compounds is the electrophilic aromatic substitution. So, you would remove one of these hydrogens which are here on the ring and replace them with another electrophilic group and that in this case is the acetyl group which is generated by A L C L 3 and C H 3 C O C L. So, you end up generating the acetylated version of cyclobutadiene which is still coordinated to the ion tri carbonyl and the yields in these reactions are quite good indicative of the fact that you can have a smooth electrophilic aromatic substitution of the C H group on the cyclobutadiene coordinated to the ion tri carbonyl. So, this is very characteristic of the aromatic behavior of cyclobutadiene which is completely new. It is a changed reactivity all the way from the anti aromatic behavior of cyclobutadiene to the aromatic behavior that we have observed here. Now, other half sandwich complexes are also known. What we have talked about is butadiene because that was the first one which was predicted even before it was synthesized. A very simple reaction of ion pentacarbonyl with cyclopentadiene. If you remember cyclopentadiene is the molecule that has to be generated by cracking dicyclopentadiene. If you heat the two together what you end up with is a ion hydride cyclopentadiene molecule where two carbon monoxide just still retain in the coordination sphere of the of the ion. You seem to have accomplished an oxidative addition of cyclopentadiene which of course is this molecule. So, you have two hydrogens here and now you have ended up adding one of those hydrogens to the ion atom. So, you have a hydride which is present on the ion and the cyclopentadiene ion. So, ion has gone from the zero oxidation state to the plus two oxidation state. This is an oxidative addition but at the same time there has been substitution of the carbon monoxide on the ion which is pentacarbonyl to start with and it is now become a di carbonyl molecule. What you have replaced the three carbonyls is the cyclopentadiene unit which in fact will donate 3 into 2 6 electrons. So, this donates 6 electrons in the ionic method and you can do the electron counting to show that this is in fact going to be a stable 18 electron molecule. Although this system is stable what happens is it dimerizes rather rapidly and loses a molecule of di hydrogen. That means two hydrogens from the two ion atoms are eliminated as H 2 and you end up with a dimer which is a very convenient source of C P F E C O 2 unit when units when you need them. So, here I have shown you what you can do with this dimeric species. You remember the reaction which we have encountered several times in this series which is M N 2 C O 10 which has got a manganese-manganese bond and this manganese-manganese bond could be reduced with sodium and mercury. Similarly, this system also can be reduced with mercury and you would end up with a very reactive ion di carbonyl anion and this very reactive ion di carbonyl anion has been called super nucleophile. So, this is extremely reactive and will react with a variety of electrophiles. Here I have shown you the reaction with R x and this does a simple nucleophilic substitution now on the R group and that leads to the ion R bond. So, this ion R bond is formed and X leaves as X minus and this X minus and N A plus end up with as N A X in the reaction medium. So, you can see that there are some very interesting half sandwich complexes and ions that can be generated by a simple reaction of ion pentacarbonyl with cyclopentadiene molecule which is shown right here. So, let us proceed now as I told you earlier it is possible to generate the half sandwich complexes in a variety of ways. One of them is to treat a sandwich complex which is not an 18 electron species. So, in this case for example, this is not an 18 electron species if you reacted with carbon monoxide at high pressure. Then you at high pressure this is fairly high pressure 200 bar which 200 times the atmospheric pressure you treat a magnetosine with carbon monoxide at high temperature and high pressure. What you end up with is a molecule called siman trine siman trine and this molecule is a very interesting molecule because the substituted variety of this the methyl siman trine turned out to be a possible additive a possible additive for here is the molecule that I am talking about. This is the methyl siman trine you have 3 CO groups attached to the manganese and you have a stable 18 electron system and this turned out to be a additive for fuels. So, that you can have smooth burning of the fuel in the internal combustion engine. Remember tetraethyl lead was used for a long time and now it has been completely removed from the market and in between people were looking for alternatives as fuel additives. So, that there will be less knocking in the fuel and one of the molecules was siman trine and that is this molecule where you have a methyl cyclopentadiene which is coordinated to the manganese and 3 carbonyl units are there along with the manganese to give you a stable 18 electron system. So, this siman trine molecule again has got the familiar half sandwich structure where you have one cyclic pi system which is coordinated to the metal and the other side has got ancillary ligands to support this metal atom. Now, here I we cross over into rhodium one chemistry and this is just adjustment of the number of electrons rhodium has in its plus 1 oxidation state. This is plus 1 oxidation state 8 electrons in its valence shell. This has got 8 valence electrons and so it would require a total of 10 valence electrons and those are provided by a cyclopentadienyl anion and 2 carbon monoxides. A simple displacement reaction happens of the chloride, but that now generates a monomeric complex from the dimeric system that we started out with and it is again a stable 18 electron complex 18 valence electron complex that we isolate. So, you will notice that this molecule is looking very similar to the C P F E C O 2 minus ion and that is exactly isoelectronic with this molecule. Although this is not as good a nucleophile as C P F E C O 2 minus this molecule also behaves like a like a nucleophile and it attacks r x molecules. So, before we proceed further with cyclopentadienyl systems I should mention that there are some as you go down the group or as you go to heavier transition metals you end up with some very interesting reactions. In this reaction which was in fact discovered fairly recently or at least utilized fairly recently you have a carbonyl species the analog of the simantrene. So, the simantrene had manganese here you have renium. This molecule surprisingly reacts with hydrogen peroxide and hydrogen peroxide is a fairly good oxidizing agent and it oxidizes the carbon monoxide to carbon dioxide. So, carbon monoxide is converted to carbon dioxide and the hydrogen peroxide not only oxidizes the carbon monoxide it also oxidizes the renium. Renium is in the plus 1 oxidation state here and it has oxidized renium all the way to plus 7. So, here is the metal atom which is formally in the plus 7 oxidation state and it has got 3 oxo groups which are coordinated presumably through the double bonds to the renium metal. So, that makes an oxidation state of 7 and this unit that is R E O 3 turns out to be a very stable unit. This stable unit is been found in many organometallic molecules. It is doubtful that the renium is in fact in the has lost all its D electrons but nevertheless the formal oxidation state has to be plus 7 on the basis of oxygen which is electronegative and is considered as a O 2 minus ligand in this case. Surprisingly, very surprisingly you can convert this renium trioxide in the presence of carbon monoxide back to the renium tricarbonyl species and it is also possible to make methyl trioxide renium which means it is M E renium with 3 oxygens. So, this incredible molecule is in fact not only methyl but any long alkyl chain can be attached. So, this is in fact strange molecule where you have an inorganic oxide which is attached to an organic molecule. So, this has been called organic metal oxide. The renium turns out to perform several catalytic functions and we will look at some of these reactions in a later talk. Now, up to now we have looked at cyclobutadiene and cyclopentadiene. Let us look at some arene half sandwich complexes. Arene half sandwich complexes can also be made by a templated synthesis. Here we have reduced chromium trichloride with ALR 3. ALR 3 is a very powerful reducing agent and it reduces the chromium from plus 3 to 0 and because you have in the reaction medium dimethyl acetylene, the dimethyl acetylene is quickly polymerized or oligomerized I should say to trimerized to give you hexamethyl benzene. So, this would be a convenient way of making hexamethyl benzene and if you do this in the presence of carbon monoxide then you end up with a half sandwich complex where CRCO 3 has got hexamethyl benzene coordinated on the top. So, all of these molecules have this pianos tool and they have either 2 or 3 legs for this pianos tool. So, that you have this familiar framework and that is why they are grouped together as pianos tool complexes. Now, the aromatic ring system that is coordinated to the metal can in fact be synthesized in the coordination sphere of the metal atom through a reduction reaction which is accomplished by the organic ligand. In the previous instance we used an inorganic reducing agent to reduce the chromium to chromium 0. Here in this example what we are doing is reducing the ruthenium 3 we are going to reduce the ruthenium 3 to a ruthenium 2 species, but we will do this using the aromatic ligand system that is present here. So, 2 hydrogens are here which in fact can be used for the reduction reaction and that is what is happening you lose those 2 hydrogens as HCl and you end up with an aromatic ring system coordinated to the ruthenium. Notice that here ruthenium is in the plus 2 oxidation state and you will also notice that this is completely an inorganic compound except for the organic aromatic ligand that is present here. It is possible to reduce it or rather convert it from the dimeric state to the monomeric state by using a good ligand. So, if you add good L plus by which I mean a strong ligand which will coordinate very effectively to the metal. It could be anything from a phosphine or a pyridine or triethylamine or any ligand you which is suitable for the stabilizing the ruthenium and you end up with a piano stool complex where the aromatic ring system is now coordinated to the ruthenium. So, here is the ruthenium coordinated to the aromatic ring system and supported by the 3 legs 2 of which are chlorides. So, 2 of them are chlorides and 1 of them is a ligand that you added. This provides a very convenient way of generating the half sandwich complexes because this reaction is conveniently performed in a refluxing ethanol. You can just heat ethanol and the phylandrene which is this molecule to generate this cyclic aromatic ring system coordinated to the metal. Now, not only is it possible to generate eta 6 molecules in the previous aromatic ring system you have 6 carbons attached to the metal atom. In this case you have 7 carbon atoms attached to the metal. You will notice that C 7 H 8 is a cyclic molecule which is not conjugated to start with and you have 3 aromatic or 3 non-aromatic double bonds which are capable of coordination to the molybdenum. Molybdenum of course requires only 3 into 2 6 pi electrons from the cyclohexatriene and it has got a saturated carbon center which is indicated by 2 colored hydrogens. Now, 1 of them can be removed from the carbon as H minus along with the spare of electrons. So, this is accomplished very effectively with a treetile cation. The treetile cation is a stable cation which is capable of removing hydrogen as hydride ion. So, if this hydrogen is removed as H minus you will end up with a cyclohexatriene cation. This cation would be pseudo aromatic would be aromatic because it now has 6 pi electrons and 1 p orbital which is vacant. So, you have 6 pi electrons in a cyclic system and all the criteria required for making an aromatic pi system is satisfied. Only difference is that you are now attached you are now attached the pi system to molybdenum atom and this gives you a cyclohexatriene molybdenum tricarbonyl. Now, because you have removed this molecule you have removed the treetile cation which is now become tri phenyl methane. You have removed tri phenyl methane by removing this hydrogen as H minus and P H 3 C plus you are left with B F O minus which is the counter ion present here. You will notice that in this molecule all the 7 carbon atoms are almost equally bound to the metal atom. So, that is why we call it eta 7 complex and all of these are again half sandwich complexes meaning the system has got 1 pi system pi ring system which is coordinated to the metal. And not only is it possible to expand the ring size from 6 to 7 it is also possible to go down from 4 to 3. Remember we talked about 4 5 and 6 being the most popular ring systems, but it is also possible to have 3 and 7. 7 is what we discussed earlier and here we have cyclopropenyl bromide which now needs to lose Br S Br minus. It needs to lose Br S Br minus to form cyclopropenyl cation which would be aromatic. And in the coordination sphere of nickel tetra carbonyl you end up with a dimeric complex where 2 nickel atoms are interacting with the cyclopropenyl group and the bromide is in fact a bridge. Notice that in this molecule nickel bromide Ni 2 Br 2 exists in an unusual oxidation state of nickel 1 if you only take this fragment, but normally by convention you consider the cyclopropenyl cation which is coordinated to the nickel as an anionic group. So, this confusion is therefore most organic molecules because by convention we take them as the negatively charged species which would be anti aromatic, but we know that from the way in which it was made that in fact it has been made by generating the cation. Nevertheless the molecule can be treated with ligand like pyridine and you can generate once again generate now a piano stool which is only 3 pointed it has got a 3 pointed leg and it also got a 3 pointed stool to support the support the person sitting on it. So, here is the nickel atom now attached to 2 pyridine ligands and it has got a bromide and a cyclopropenyl group. So, if you were to use the anionic method or the let us use the neutral method this time you have 3 electrons from the C 3 P H 3 unit and you have 2 into 2 from the pyridine unit. So, that will give you 4 electrons and you also have the bromine which is in the neutral method giving 1 electron. So, nickel has got 10 electrons and so you would form this nice 18 electron system. So, the total works out to be 18 valence electrons. Now, obviously you do not have a cycle when you have only 2 atoms which are coordinated to the metal, but I just wanted to point out here that the 18 electron rule turns out to be reasonably important. You can have 16 electron complexes, but nevertheless going beyond 18 is more difficult than having complexes which have less valence electrons than 18. So, here I have shown you a platinum complex which looks like a piano stool complex, but only 1 of the double bonds in the benzene ring is coordinated to the platinum atom. So, platinum is in the 0 oxidation state and you have 2 triphenyl phosphines which are coordinated to it. So, you have 16 valence electron system and you do not go to the 20 valence electron system. You do not go to the 20 valence electron system which would be formed if you were to have a symmetrically eta 6 coordinated benzene ring. You will also notice that this now has got bond alternation that is a very clear indication of the double and single bond that are isolated and not really conjugated with one another. So, this the bond which is right next to the double bond which is coordinated is in fact a long bond and then 2 short bonds are there. One is 1.33 angstroms and the other is 1.36 angstrom units. The bond that is coordinated to the metal is in fact lengthened significantly which is indicative of the fact that double bond has been weakened quite a bit by interaction with the platinum. Electron density from the pi molecular orbital has been donated to the metal and the pi star orbital has been populated. As a result this bond has been weakened significantly. So, it is no longer like a double bond. It is almost like a single bond. So, this brings me to a conclusion of the chapter of the series of half sandwich complexes that I wanted to talk about. I just want to point out that this forms a series where we talked about mixing and matching the number of electrons required for the metal to form 18 valence electron systems. You could do this by simply replacing the neutral ligands, neutral aromatic or non aromatic pi systems which are present on the metal by carbon monoxide units. Because each carbon monoxide gives two electrons you can replace say a benzene ring with three carbon monoxides and that is exactly what we have done here. Similarly, you can have here a cyclo octatetrene which is giving you eight valence electrons. We just replace it with 4 into 2. So, four carbon monoxides replace the cyclo octatetrene. If you replace a cyclo heptatrienyl unit this is a cyclo heptatrienyl unit. So, you replace it with three carbon monoxides and one chlorine. So, that also seems to be an acceptable solution. So, you can just see that you can have a huge variety of metal complexes which are formed. Let us just take a look at the last one which is mentioned here where I have replaced the cyclo propenyl unit which will give you three electrons. A three electron donor among the carbon monoxide type molecules is NO. So, you can simply replace C 3 H 3 with NO. In the neutral method this is so convenient for us to understand this isoelectronic substitution. So, C 3 H 3 is just replaced by a NO molecule. So, a multitude of metal sandwich complexes can be made which are all half sandwich in nature. Now, I want to move on to bent metallocenes. What are bent metallocenes? The metallocene or the sandwich complexes are systems where the metal was in fact symmetrically held in between two flat pi systems. So, there are two flat pi systems and the metal was sandwiched between the two units. Just now we have looked at a series of systems where the metal is just attached to attached on one side with a pi system and the other side is supported by a variety of other ligands. So, instead of combination instead of supporting it in this fashion with three ligands one can also think of supporting the metal or bending the sandwich in this fashion. So, that if you have vacant if you have vacant orbitals on the metal atom these can also coordinate. This happens very readily in the early transition metals. So, early transition metals have in fact three orbitals in this on the metal the d x squared minus y squared the d x y and d z squared. So, there are three different orbitals on the metal which are available for bonding if you have a titanium 4 plus just interacting with two cyclopentadienyl units. So, all of them will be empty all three orbitals will be empty. So, these orbitals are now available for coordination with the metal atom. So, here I have shown you a set of molecules which are capable of interacting with more ligands just because they have got these empty d orbitals which are available for interaction. First let us take the tetrachys cyclopentadienyl titanium. So, you have C p 4 T i. So, you take T i C l 4 and treated with four molecules of cyclopentadienyl anion and you end up with this molecule which if you want to write an 18 electron structure it would be difficult to do. So, but if you bend this cyclopentadienyl unit sufficiently you can attach the cyclopentadiene unit in a sigma fashion. You can attach the cyclopentadienyl units in a sigma fashion or a eta 1 fashion in this molecule. Notice that there is nothing which distinguishes the ring A, ring B, the ring C and the ring D other than the hapticity that is associated with them. So, there is a very rapid exchange of A with C or B with C and that leads to some fluxional behavior which we will talk about in a future lecture. But these molecules right now this is a 16 valence electron system. These molecules are quite stable and turn out to be an interesting class of molecules where the cyclopentadienyl ring systems are just bent a little bit backward from the usual orientation. So, this bending back allows for exposure of these orbitals which are there in the x y plane and so allows for interaction with a metal atom with other ligand atoms. So, I have shown you a variety of molecules here, but let us take some specific examples now. Let us take C p 2 T i C l 2 and C p 2 T i C l 2 is titanium 4 plus. This is titanium 4 plus you can reduce it with zinc and use carbon monoxide in the reaction medium and that gives you T i C O twice. So, here is a molecule which is titanium in the plus 2 oxidation state. Remember there were three orbitals there were available on the titanium. So, we can in fact add six electrons if it was titanium 4 plus. Now, you have a plus 2 system. So, you end up adding only two other ligands. So, that gives you two other ligands in the coordination sphere of the titanium and you have a bent sandwich structure. So, this is again the bent sandwich structure which we just talked about where the cyclopentadienyl units are coordinated in a eta 1 fashion. This is the way you would end up making it. Now, having talked about bent sandwiches species which we will encounter later during catalysis. Let us take a look at what would happen if you have a open sandwich. These are molecules which are not appreciated a lot because you do not have a very stable cyclopentadienyl unit which is aromatic. So, this cyclopentadienyl unit is aromatic. Whereas, if you have a acyclic version this is the acyclic version then it turns out that this is not an aromatic molecule. So, but still you do have situations where you have cyclopentadienyl units which are coordinated to the metal. These are stable molecules I have shown for you on the screen the half sandwich complex. We can also have this type of complex in a bis form that means two acyclic cyclopentadienyl units can coordinate to the iron or a ruthenium. Those molecules have also been isolated and characterized. Let us take a brief look at why these molecules would be different from the cyclopentadienyl molecules. If you treat FeCl2 with a cyclopentadienyl anion you end up isolating a well characterized molecule which is exactly like ferrocene, but it is not as stable as ferrocene because of its open structure. You can also form a half sandwich complex by treating a tin substituted molecule. So, here you have MnCO5Br and SnMe3 reacting together to form SnMe3Br. So, you end up with a cyclopentadienyl unit sorry you end up with a acyclic pentadienyl unit which is coordinated to the manganese. So, this is the acyclic variety of Simon-Trem. So, these molecules are not as stable as the cyclic variety for two simple reasons. One is the fact that the cyclopentadienyl unit has got a covalent bond between carbon atoms 1 and 5. Here the covalent bond is being replaced by two hydrogens. So, here are the two hydrogens and they are pointed towards one another. So, as a result there would be repulsion between the two hydrogens. Let us just indicate that repulsion with different color here. So, this is the repulsion that is going to happen the place where the repulsion is going to happen between the two hydrogens. To avoid the steric interaction you would either have to twist the CH2 group or you would have to widen the two ends of the pentadienyl unit and both of these movements of these cyclic pi systems would destroy the metal interacting with the pi bonds. So, what will happen is you will end up weakening the metal pentadienyl bond and it would make the pentadienyl unit more reactive. That is probably one reason why ruthenium complex has been isolated and characterized crystallographically and has been found to be more stable than the iron system itself. It is possible to make pentadienyl complexes, but it is also possible to make a bifurcated pi system. Here I have shown you trimethylene methane and the trimethylene methane is formed by reacting a bis allylic system. So, here is a double bond which is here is a chloro atom which is allylic or just a double bond that two chloro atoms and this forms a Y shaped molecule. If you remove the CL as CL dot then you end up with a bi radical. So, this is a bi radical which can now react with iron and it forms a molecule which is shown here. This is called trimethylene methane because you have three trimethylene groups. There are three trimethylene groups which are attached to the central carbon atom. So, these three are in obviously in resonance and are equivalent and you can coordinate it to an iron tricarbonyl molecule. Now, you can also do this by without doing an oxidative addition. You can do a ring opening reaction where the ring opening reaction can happen by the coordination of iron tetricarbonyl unit to the double bond first. Then that allows you to have a ring opening reaction and the trimethylene methane tricarbonyl complex is formed. Now, I will show you this complex. Here is the iron tricarbonyl unit coordinated to a trimethylene methane. Now, you will notice that there are four carbons which are marked here. These are the four carbons. All of them are within the bonding distance of the iron atom which is shown here. So, the iron atom which is marked in red is almost equidistant with all the four carbon atoms. But, the central carbon atom is slightly above the plane which is formed by the three other carbon atoms which are closer to the iron atom. So, three carbon atoms are closer and the central carbon atom is in fact slightly above the plane. Recently, it has been shown that the bonding nature of these iron tricarbonyl unit to trimethylene methane is quite interesting. It is a very interesting molecule where people have questioned the nature of the bonding between the central carbon and the iron atom. But, nevertheless the bonding distance happens to be slightly more here. The distance is slightly more, but it is still within bonding distance. So, this is the molecule which is called an inverted which is almost like an umbrella. So, it is like an umbrella which is coordinated to the iron tricarbonyl. So, let us proceed further now. This molecule has got this inverted umbrella or it is also in one sense it is like an umbrella structure and it has got a staggered orientation of the three carbonyl units with respect to the three methylene units. So, it is almost like octahedral geometry around the iron if you consider the three carbons of the trimethylene methane and the three carbons of the carbon monoxide. You could also have some exotic sandwich structures where the heteroatom which is the bread part of the sandwich structure that we are talking about. So, the cyclic pie system that makes the bread part can be different. It can be a heteroatom and some of the simple replacements that we can talk about is converting the C H into M. So, from benzene from simple benzene we can move on to pyridine. So, that will give you a pyridine complex or you can replace it with a phosphorous. So, this will be phosphor benzene. So, we can have phosphor benzene which is coordinated to a metal atom. Now, it is also possible to replace C H by S C H minus by S. So, in other words if you can have cyclopentadienyl anion cyclopentadienyl anion you could convert it into thiophene. So, there should also be a good replacement to do. So, these are possible replacements and you can have these molecules where you can have substitution, appropriate substitution to make sandwich complexes. Thiophene can replace cyclopentadienyl units and here we have chromium which is coordinated to thiophene and it forms a nice half sandwich complex. So, you can imagine how the cyclopentadienyl unit has been replaced by thiophene. To form this half sandwich complex, to make a complex with pyridine and which something that would be equivalent of this benzene chromium it has been quite difficult. In fact, it was this was accomplished fairly recently by Elsenbroek and co-workers. What they did was a co-connensation reaction of this substituted pyridine where you can have a pyridine. The two ortho positions were blocked. So, much so that the nitrogen was not able to interact with the chromium. So, this pi system was now able to interact with the chromium in pi fashion or in other words it was forced to interact only in this fashion where it forms a sandwich structure. After the sandwich structure was formed the SIME3 groups were oxidatively removed just by bubbling oxygen into the benzene water mixture of this molecule. It was possible to knock off these SIME3 groups and replace them with hydrogen. So, these hydrogens were generated from this water molecule or taken from this water molecule and you have this nice sandwich structure where the pyridine is not coordinating with the nitrogen. Otherwise it was not possible to make complex simple complexes of pyridine which were coordinated in a bisbenzene type fashion. Now, I also told you about making a p5 phosphobenzene complex. Instead of making a phosphobenzene even more exotic structure would be to replace all the molecules of all the molecules all the atoms of benzene which are all CH units. If you can replace all the CH units with a phosphorous then you would end up with a molecule which would be a phosphobenzene. And in here I have shown for you a phosphocyclopentadienyl system. So, here you have eta5 p5 unit. So, let us take a look at some of these structures because there are several structures where the CH unit has been replaced by a phosphorous molecule phosphorous atom. So, here I have shown for you two chromium atoms which are interacting with cyclopentadienyl groups here on one side and five phosphorous atoms. So, that is equivalent to C5 H5 minus. So, it is possible to make these molecules. So, here is the other molecule which is shown on the screen. Here we have five phosphorous atoms. So, we have five phosphorous atoms which are substituting for five CH units and you can have isomorphic replacement of a CH group with a phosphorous atom. So, phosphorous sandwiches are also known. It is also been possible to make multiple metals coordinating to a single pi system. So, here I have a replacement of cyclo dodeca trine which is a weak ligand coordinating to nickel zero by cyclo octatetraene. But because cyclo octatetraene needs less number of electrons and you have more electrons in the nickel system, you ended up forming a sandwich where you have more than one metal coordinated to the pi system. So, multi-decker sandwiches have also been isolated and characterized, but this one was only isolated in the mass spectrometer which was the first multi-decker complex to be isolated or was multi-decker sandwich complex to be characterized. Here there are two nickel atoms and three cyclopentadienyl units and got a net charge of plus 1. Now, inverse sandwiches are also possible. Here I have a cyclo octatetraene which is like a inverse sandwich synthon because of its tub type structure, the two double bonds on the cyclo octatetraene can coordinate to one metal atom on one side and the other side coordinates to another metal atom. I have shown for you on the screen structure where you have a metal atom on one side which is coordinating to two ligands on one side and the two double bonds are coordinated to the metal atom which is indicated in the center. So, you have a variety of systems. Here again I have a cyclo octatetraene which is forming multiple sandwich type structure. You can see that the cyclo octatetraene is a great synthon for forming multiple sandwiches and p5 also turns out to be an interesting system. Here I have shown for you on the screen a p5 unit which is coordinating on both sides to two different metal atoms. So, you can have very interesting structures indeed. Let me say that sky is the limit for making sandwich complexes and this is the place which is been actively pursued. Most of the time the 18 electron rule is a great guiding principle for making the right type of a complex and making choosing the right type of a synthesis. Very often it is possible to change the pie system according to your convenience so that you achieve the right electronic structure. So, with this we will go on to reactivity of these molecules.