 Hello everyone once again I welcome you all to MSP lecture series on trans-traumatic chemistry. This lecture is 32nd in the series in the last few lectures we are discussing about the classification of ligands by donor atoms. And let us continue discussing on various ligands and because ligands are integral part of coordination chemistry if you understand the nature of coordination modes the nature of the donor atom and how they can bind and what are the binding modes it comes very handy later when we want to use them in some applications especially in material science or in homogeneous catalysis or even in drug discovery or in biological studies. So from that point of view understanding systematically the nature of the ligands and their binding properties is very very important with this let me continue from where I had stopped in my previous lecture I had initiated discussion on synthesis of isocyanide complexes and the reactivity of bound isocyanide complexes with few examples I showed you one can make coupling products from Sino complexes isocyanide complexes at least in order to see coupling reactions happening we have to have minimum of two isocyanides on a metal center and in case if we have more probably apart from one or two others will remain as terminal isocyanides and depending upon what stoichiometry we are using two or more set of isocyanide ligands can participate in coupling reactions. So after completing the discussion on coupling reactions let us look into electrophilic attack. So usually electrophilic attack can give alkalidine complexes. Let us start with one simple general example to make you familiar with this kind of electrophilic reactions that happens with bound isocyanide ligands. Just for understanding I have taken only one CNR it does not mean that it is a monoligated metal complex or something let us assume we have several other ligands on metal this intermediate species will be in equilibrium with. So this is the end product you can see this is alkalidine moiety here. Let us take a specific example now to understand this reaction perhaps you know what is DPP, 1, 2 bis-diphenyl phosphinoethane bridging isocyanides have very high nucleophilicity and are readily attacked. For example if you take a complex where isocyanide is acting as a bridging ligand let us consider this example here treat this one with methyl iodide. So with these examples let us move on to nucleophilic reactions. So nucleophilic attacks either at carbon or at nitrogen that can give alkalidine complexes unlike electrophilic reactions where we got alkalidine complexes when nucleophilic attack occurs at either carbon or at nitrogen that leads to the formation of alkalidine complexes. Let us look into a few examples in this case. So this moiety is referred to as alkalidine that means if you have metal to this is alkalidine and if you have metal to carbon triple bond this is called alkalidine complex it is like acetylene alkyne and alkene type. Let us look into one more example here when you treat this one with ethanol this is another interesting example for the formation of alkalidine complex. So unless otherwise no specification is given when CP is written assume it is cyclopentadiene ligand having hapticity of eta 5. So in general one should remember in nothing is mentioned about this one and if something is shown this indicates it is eta 5 C5H5. So iron is in plus 2 state then this is treated with sodium borohydrate. I have given three different type of examples to show how nucleophilic reaction occurs at either carbon or at nitrogen that leads to the formation of alkalidine complexes. With this I stopped discussion on ligands having carbon as donor atom. Of course as I mentioned carbon monoxide is the most common ligand with respect to carbon donor atoms and since you are all familiar with carbon monoxide I did not discuss in an elaborated manner about those compounds. Nevertheless we come across now while discussing lot of other reactions CO compounds or metal complexes having CO. Now let me move on to nitrogen donor ligands. Nitrogen donor ligands are plenty one can see the list here it is never ending. The simplest but hard ligand with very weak sigma donor and very weak pi acceptor abilities dinitrogen in neutral ligand it can be a two electron donor or it can be four electron donor and then ammonia the most common classical ligand we come across and then all primary amines both alkyl and aryl and also secondary amines and also tertiary amines and of course from NH3 if you eliminate one hatch it leads to the formation of NH2 minus this can also act as a ligand or if you remove two hydrogen then we get amide and then if we remove all the hydrogen atoms then we get nitride all these are very good ligands and numerous examples are known in each case and when we comes to cyclic heterocycles pyridine ligands are very prominent we have plenty of pyridine ligands and also N macro cycles and also skip bases and apart from this we also have EDTA ethylene diamine tetra acetic acid where we have two nitrogen atoms along with four carob oxalate oxygens and TMEDA another very useful ligand tetramethyl ethylene diamine and ethylene diamine itself is an excellent bident ethylene that forms very stable 5 membered rings and also apart from ethylene diamine we also have diamine triamine tetramine and also polyamines to make you familiar with nitrogen donor ligands I have listed some important nitrogen heterocycles here you can see sort of this can be a bidentate ligand this can also bidentate ligand this triazole can be tridentate ligand and pyrrole can access an anionic ligand besides acting as anionic ligand it can also act as a neutral ligand for electron donor when it is N minus and imidazole we have and we have something like this immuno system we have and when it comes to bipyridines we have 2-2 dash bipyridine very useful for collating into a metal center and this is very useful for bridging especially this ligand is widely used in making metal organic frameworks and then in photophysical studies 1-10 phenanthaline is very important and terpyridine and also 1-8 naphthyridine purine adenine and also we have hexapyridine torrent apart from those we also have of course this one is ethylene derived phosphine here nitrogen can also acts as a ligand many times when nitrogen is very close to phosphorus because of the establishment of multiple bond between phosphorus and nitrogen and most of the acation nitrogen lone pair is not available for bonding in this case although nitrogen is trivalent what happens phosphorus is taking electrons from nitrogen lone pair to its sigma star so that we call it as negative hyper conjugation in case of metal complexes we call it as back bonding because of these things whenever a nitrogen is next to phosphorus it loses its donor abilities because its lone pair participates in binding with phosphorus in some books they say it is d pi p pi interaction it is not d pi p pi interaction it is p pi sigma star interaction that is called negative hyper conjugation and of course ethylene diamine tetraacetic acid I told you once this will always exist in the stable form having here is a triannic form and that is the reason if you look into the pKa third and fourth pKa values are very high because it is on NH and it is not so easily it can be deprotonated and acac is there and that is of course this is an oxygen donor ligand and then we have porphyrin the simplest porphyrin I will show you at the end how to prepare this one and another one is thalocyanin. Thalocyanin is prepared from this disino benzene and one should remember that this can be prepared only in the presence of a metal so that this grows in surrounding the metal that means it is an encapsulating ligand and then it usually does not exist independently without metal ion and of course about these things I already mentioned in my one of my previous lectures so now let us look into few ligands one at a time. Let us first look into dinitrogen again dinitrogen chemistry is very very important because we have plenty of that one in the atmosphere one can make lot of useful nitrogen compounds as simple as ammonia people always look for very mild conditions to make ammonia unlike papers process that requires high temperature and high pressure for that one the better option is looking for ideal metal complexes so the first dinitrogen complex was made by Alan and Sinov in 1965 they used this method of treating rhodium trichlorate hydrate with hydrogen to get this pentamine dinitrogen ruthenium dichotinic complex of course we have several methods I shall tell you later first let us look into the possible coordination modes of dinitrogen so nitrogen if you see here this has two lone pairs are there here so these lone pairs can go to metal so it can act as a terminal ligand or it can also act as a bridging ligand where both the lone pairs on nitrogens are utilized and of course this is same as that one and of course you can also have two different metals homo bimetallic system or hetero bimetallic system you can have or this lone pair can go to two metals so you can have a trimetallic system or we can cleave one of the bond in this fashion you can also form some sort of immunotype compound here where we have this bent structure or one can also see this kind of binding or one can also see this kind of binding also once if we break two bonds between nitrogen and autumn the end on end on is when we have one so eta one type is the most common mode in most of the metal complexes we see this one and whereas other lone pair remains intact this is the most common one this is called end on and this also called as eta one mode this is the most common one and just if you look into free nitrogen in free nitrogen the distance between n or nn bond length is 1.097 and strength units whereas in case of nitrogen compound it is increases and it can range anywhere between 1.10 to 1.16 most common bridging mode is linear this is the one so that means you can assume that the triple bond is still intact that indicates that the bridging nitrogen is linear it is almost close to 180 degrees in that case what happens nn distance will be around 1.1 to 1.36 handstand units so now let us look into the synthesis of nitrogen complex if I want to make for a specific application how to make dinitrogen complexes in the laboratory first one is a reduction method of course reaction has to be carried out under positive pressure of nitrogen because we are inserting nitrogen so this is one of the method of course here DEP is bisethyl phosphinoethane very similar to DPP so 1 2 bisethyl phosphinoethane and this is a chelating ligand so 2 chelating ligands are in this fashion and it is a square pyramidal molecule so one is using you know this reducing agent you can do substitution reaction that means replacing one neutral ligand with another one for example if we take treat this complex with N2 in methanol it can just simply substitute hydrogen molecule to form dinitrogen complex as I mentioned DEP is nothing but bis you can see 1 2 bisethyl phosphinoethane and similarly this is also 1 2 bis dimethyl so these two are widely used methods in the preparation of dinitrogen complexes of course if you want to look into more always you can go to advanced yarn chemistry by FAA cotton and others you can see lot of examples and also you can see some literature that one can use and also there are reviews are there about dinitrogen complexes one can get more details so before I proceed to look into reactivity of coordinated nitrogen let me show you some similarity between various other ligands for example when we see ligands such as isocyanide carbon monoxide dinitrogen or nitrocyl ligand we can perform at least two type of reactions if not three so they are nucleophilic substitution reactions are nucleophilic or electrophilic reactions that means how the electrophilic reactions and nucleophilic reactions can be started okay in this let us look into some similarities so m and 2 can be conveniently written in this fashion so that means this one we can show in a format like this due to the movement of electrons so something like this so now the moment I write like this delocalization of electrons you should be able to know if the where nucleophilic attack happens and where electrophilic attack happens one should remember about this one so later when you are using electrophilic sub electrophilic attack or performing nucleophilic attack you should be able to tell where this nucleophile or electrophile are going to attack the bound ligands this is very similar to what we come across in case of carbon monoxide carbon monoxide if I take like this to start with has a positive charge and it has negative charge that means electron reach and because now carbon monoxide is donated a pair of electrons to sigma bonding as a result what happens there will be net positive charge and then depending upon how much back bonding occurs next stage is something like this almost we have ketonic and bond strength is weakening as far as CO is concerned and then the extreme case what happens one can see something like this this one so more and more electrons are donated from metal to pi star and as a result what happens metal to carbon bond in strength increases and CO becomes weaker and now if you consider this one something like this we can see here something like this so now we can see how nucleophilic attack happens where electrophilic attack happens and same strategy one can use to understand the reactivity of bound cyanide as well isocyanide as well now you can see the similarities between some of these ligands so since similarities we are seeing that is a reason we can see a variety of reactions that happens and bound one so in some cases coupling can happen readily but invariably you can come across nucleophilic as well as electrophilic reactions that is look into more such reactions with bound nitrogen that is very very important from from synthetic point of view until then have an excellent time reading chemistry especially a coordination chemistry and especially getting more information from textbooks about the ligands and their behavior when they are brought very close to the metal center.