 Welcome to this lecture on Transient Metal Organometallics in Catalysis and Biology. We have been discussing about metathesis reaction in the past two lectures and in this context we have been looking at mechanism of this metathesis reaction. What we had observed is that several probable candidates were proposed to explain the metathesis reaction and then several of them was proposed unsuccessfully till the final mechanism as proposed by Chauvin emerged to be the accurate one. Now in this regard in the last class we have spoken about pair wise mechanism. What is said that two olefins get spared up in presence of metal to give some kind of paired intermediate with the metal in it and that sort of results in obtaining the metathesis reaction with different olefins. For example, the one that is shown over here, what we saw that the olefin of this type get paired up in this pair wise mechanism through some metal templated intermediate and then finally the metathesis happens and then results in two different kinds of olefins. This is what was called a pair wise mechanism and the other mechanism which came out to be correct is this non pair wise mechanism in which one olefin reacts with a metal carbene species, this is the active species metal carbene species to give an intermediate metal cyclobutane intermediate and that sort of results into the corresponding carbene plus the olefin which is the metathesis product. What we saw over here in the pair wise mechanism is that there is one active species generating or converting to another active species as a course of metathesis reaction and simultaneously one kind of olefin giving to another kind of olefin species. This is the accepted mechanism and in this context, we have discussed the various famous names starting from Calderon, Petite as well as Grubbs who had proposed pair wise mechanisms and which unfortunately did not turn out to be correct followed by Chauvin, the French scientist who had actually proposed the correct mechanism, the one non pair wise mechanism that we have just discussed right now. Now, in this case, we have also seen that various kinds of intermediates being proposed to the intermediate through which metathesis reaction occurs and they start from being cyclobutane bound to metal to tetramethylene cyclobutane to rearranging metallacyclobutane to finally this metallacyclobutane which is shown over here and the one which is the correct one. So, this sort of provides a glimpse of the intensity of research which was undertaken in order to explain this metathesis reaction particularly to understand how this complex reaction proceeded. Another thing which is important in the mechanism non pair wise mechanism proposed by Chauvin is the fact that the active metal carbene species many of its kind are generated in the course of metathesis reaction and as a result this is more of a chain reaction where several metal carbene type species react with olefin to give the corresponding metathesis products. So, the reaction is also characterized by chain type reaction that take place during the proceedings of this metathesis reaction. Now, while discussing the mechanism, it is kind of incomplete if we do not talk about the contribution of another great scientist Professor Thomas J. Katz or Professor Tom Katz of Columbia University who actually first gave the proposition of metal carbene being the active species in the metathesis reaction. So, Professor Thomas J. Katz, Columbia University and I am happy to say that I have been fortunate enough to undertake a course offered by Professor Thomas Katz when I was a student PhD student at Columbia University and a good friend of mine he is a renowned professor now was a direct graduate student of Professor Katz when of my class when we were doing our PhD work. So, what Katz did is Katz sort of the first time explained the involvement of metal carbene interacts intermediate in metathesis reaction carbene intermediate and the Katz through this he confirmed this by proposing a correct correct formation of product predicted metathesis products while using different olefins and you know this is very nicely explained in this journal of chemical chemical society paper 1972. So, what is recognized to be of Katz's contribution is the fact that Katz further substantiated and validated with experimental evidence the mechanism which had been proposed by Chauvin that proceeded via the formation of metallocyclobutane intermediate and the product is and what Katz had proposed is the fact that metal carbene complex is the catalytically active species this is best described by the mechanism as shown over here this is the active species reacting with the olefin just by simple coordination to give the olefin coordinated carbene species MCH2 bound to CHR that then undergoes this margitory insertion or formation of metallocyclobutane ring as is shown over here leads to the formation of the metathesis product coordinated to metal with a different carbene moiety. So, with that I mean that as the formation of another olefin is produced from this are different metal carbene active species is also formed as a result of that now in presence of the second olefin coming and replacing the metal coordinate olefin giving out RCH1 CH2 and metal carbene species bound to CH2 CH2 that then undergoes this metathesis reaction to form the metallocyclobutane species as is shown over here which finally gives the metal carbene with the formation of the metathesis olefin. So, what we see is that the different kind of carbene are formed as is shown over here and similarly different olefins are formed as is seen over here and they start from two reacting olefins which is this one as well as this one. So, this is what cats had proposed as opposed to be the intermediate for metathesis reactions with metal carbene being the active species for metathesis reaction. So, the name of cats discussion on cats is done in the same breadth with that of Chauvin, Grubbs, Pettit and Calderon because it is he who validated the first mechanism through more experiments of Chauvin and that is why cats name is also taken in the same breadth as all of the previous three researchers. Now in this context it is important to mention a bit about carbene complexes mainly with regard to fissure carbene and shock carbene because those complexes containing metal carbene multiple bonds, double bonds in general were synthesized at that point of time and was an important breakthrough in terms of stabilizing metal ligand double bond or multiple bond. We have some discussion on this metal carbene complexes to see how that the development or discovery of this at that point of time how it helped elucidate metathesis reaction as done rightfully by Chauvin. So, these carbene complexes are compounds with metal carbon double bonds and usually it is donated by AMC carbene with the first bond being a sigma bond and the second one being a pi and usually that is a possibility where one is a sigma bond another is a pi bond and now there is a discussion and debate as to to what extent is the pi bond occurring in order to stabilize this metal carbene moiety. Now this is a open debate and also the other important thing is that how what kind of bonds are these sigma and pi bonds are and in terms in the types in terms of discussion on the types of bonds that sigma and pi are the discussion is around whether they are dative bond or they are covalent bond and what we will see that both types of carbene carbene where metal and carbon are bound by a dative bonds one dative sigma bond another dative pi bond is also possible this is covalent bond involving sheriff of electron between one sigma covalent bond and one sigma pi bond between metal and carbene moiety that is also possible. So, what is interesting to note is that two types of carbene mainly fissure carbene and this is sort of the carbene which consist of two types of dative bonds which is ligand to carbene to metal sigma bond and metal to carbene pi dative bonds. So, they are both dative bonds they are characteristics of fissure carbene and in such case the central metal is in a low oxidation state and have another characteristics of fissure carbene to attain that state is that they have hetero atoms at the carbene carbon atom and as just mentioned earlier that there are also metal carbene moieties where both the bonds between metal and carbene both the sigma and the pi bond existing between metal and carbene can be covalent bond x type covalent bond and these are more common in stroke carbene and these are more x type covalent bond and not a dative bond and involves sharing of electrons from the metal and the carbene sides and these are more common for complexes where central metal is in a high oxidation state and have carbon or hydrogen substituents the carbene carbon atom and another thing to note here is that if there is no hetero atom biological carbene it is also called alkylidin complex. So, these are two types of carbene which are synthesized in which the first one is fissure carbene that involves dative bond existing between metal carbon sigma bond as well as that metal carbon pi bond and then comes stroke carbene which is involves covalent covalent metal carbon sigma bond and metal carbon pi bond. Furthermore, these two carbene are characterized by the state of the carbene itself. So, fissure carbene the carbene moiety is a singlet state whereas for stroke carbene the carbene moiety is in a triplet state and that explains the formation of covalent bond in stroke carbene whereas singlet state explains the formation of dative bond in the fissure carbene. So, with these what we saw is a nice convergence of several discoveries particularly the isolation of metal carbene moiety is containing metal carbene bonds and they being put in the picture as active species for the metathesis reaction as initially proposed by Chauvin and later on substantiated by beautiful work of cats through which who really showed and predicted correct product formation using the metal carbene intermediate in explaining the olefin metathesis reaction. So, with these we come to conclusion of today's lecture where we sort of looked into the postulates made by professor Tom Katz in help corroborating and validating the original mechanism proposed by Chauvin and this Katz did that by correctly predicted the metathesis product by judiciously using different carbene and these all were done using metal carbene intermediates and also supported the fact that during the course of metathesis reaction many active species carbene type active species are formed which also take up another olefin and proceed to carry on metathesis in a chain fashion. In the course of this today's lecture, we have also looked at these active species metal carbene bonds and what we saw that there are two types of carbene that have been stabilized particularly the fissure carbene which has carbon to metal sigma dative bond as well as metal to carbene sigma pi dative bond whereas shock carbene which contains both metal carbon sigma and pi covalent bonds in fissure carbene what we saw the central metal is in low oxidation state as a result of binding of singlet carbene and have a heteroatom sterilization of the carbene carbon whereas shock carbene has a central metal atom in a higher oxidation state and there is no heteroatom on the carbene carbon it is usually carbon and hydrogen. So, with these I conclude today's discussion on the role of cats in explaining the olefin metathesis reaction. We are going to be taking up olefin metathesis in bit more detail when we take up this course apart from mechanism in trying to look at different types of reactions that are possible when we meet for the next lecture. Till then, goodbye and thank you.