 carbon monoxide is a fairly unique ligand and in the previous lectures we have seen how carbon monoxide is occupying a very special place in organometallic chemistry. In fact, one might say that the R group which is used in organic chemistry as representation for all alkyl moieties is there for the organic chemist. So, the carbon monoxide is there for the organometallic chemist to plug in at any place and use as a ligand. It turns out that carbon monoxide however has some very unique properties and some of the unique properties are what I want to emphasize first before we look at alternatives to carbon monoxide. Any chemist wants to have some variation in the molecules that he is making. So, that new properties can be studied new reactivities can be studied. So, people have been looking for alternatives to carbon monoxide which is such a unique ligand and occupying a special place in organometallic chemistry. So, let us look first at the uniqueness of carbon monoxide. Let us just revise that the aspect that is emphasized most often in organometallic chemistry is a fact that carbon monoxide is a pi acceptor ligand. Carbon monoxide has got a pi acceptor orbital and we can see this pi acceptor orbital which is denoted here. It has got a very large lobe on the carbon side. This is the carbon side and this is the oxygen side. So, you have a very large lobe on the carbon side and a smaller lobe on the oxygen side. So, the pi acceptor orbital can overlap very effectively with the filled orbital of the metal. So, this is the metal and the metal is interacting with the carbon monoxide and this interaction becomes very effective. Carbon monoxide occupies this unique place because of its pi acceptor property. However, what we tend to ignore more often than not is a fact that carbon monoxide is also a good donor and this donation comes from the sigma orbital of carbon monoxide. The sigma orbital is also located primarily on the carbon. So, the carbon end of carbon monoxide turns out to be the negative end of the dipole in carbon monoxide. Carbon monoxide occupies a special place because of this unique property that carbon end is the negative end and it has a very small dipole moment 0.1 d by units. So, 0.1 d by units of the dipole is concentrated on the carbon end of the carbon monoxide and this carbon end is now attached to the metal center. So, there are some very unique features that result from this sigma donation from the carbon monoxide to the metal and then from the metal the electron density flows back into the carbon monoxide into the pi star orbital of the carbon monoxide. So, you can see that these two aspects of carbon monoxide make it a very unique ligand and make it a synergistic ligand. This synergism is what is called as a unique property of carbon monoxide. What is not emphasized in most of the text books is the fact that carbon monoxide is not a pi donor is not an effective pi donor and that happens because the pi orbital on this ligand which is a filled orbital. This is a filled orbital on carbon monoxide and this filled orbital on carbon monoxide can also interact with the filled metal d orbital. So, this interaction turns out to be a repulsive interaction. This interaction is a repulsive interaction and this repulsive interaction has to be minimized and this stabilizing interaction has to be emphasized or it has to be better than the repulsive interaction. So, the repulsive interaction is minimized because the overlap between the carbon monoxide and the filled metal d orbital is less because of the small size of the lobe on the carbon in the pi orbital of carbon monoxide. So, it is the combination of these three factors. The fact that you have a good pi acceptor, the fact that you have sigma donation from the carbon end and the fact that you have pi accepting property and pi donation property which is minimized. The pi accepting property is maximized and the pi donating property is minimized and this three factors tend to make carbon monoxide a very unique ligand. So, now if you want to make an alternative for carbon monoxide, how do we go about doing it? The chemist usually ends up going back to the periodic table and if you go to the periodic table you realize that the electronegativity difference between carbon and oxygen is one of the reasons why you have all these unique features for carbon monoxide. The sigma donation from the highest occupied molecule orbital of carbon monoxide is happening from this orbital which is having an ionization potential of 13.02 electron volts. This is the sigma orbital and the pi accepting orbitals are located here. They are virtual orbitals and denoted in green and these virtual orbitals are the ones which are going to accept electron density from the metal. So, you have sigma donation from here and pi accepting property of carbon monoxide arising from this orbital. This is the orbital which is the filled pi orbital on carbon monoxide and that is the one which is going to be a repulsive interaction but because of poor overlap it does not repel too much and that makes carbon monoxide very unique and these three factors are all the result of the electronegativity difference between carbon and oxygen. So, if you go to the periodic table we see that the electronegativity difference has to be maintained as much as possible. So, here is the periodic table, carbon is right here, carbon is here and oxygen is here and you can see that the electronegativity difference will keep increasing in this direction as we go from left to right in the periodic table the electronegativity difference will keep increasing. So, if we substitute as chemists often do by an element which is lower in the periodic table with respect to the element that we are substituting. So, one would replace oxygen by sulphur then what would happen is that we would lose out on the electronegativity difference between carbon and oxygen carbon and sulphur would have lesser electronegativity difference. If we go further below if we replace oxygen with a selenium that electronegativity difference will even be less. So, if we make carbon monoxide it is a good ligand if you make carbon monosulphide it is expected to be a poorer ligand carbon monosilionide should even be a poorer ligand based on the electronegativity difference. However, we will now see what happens when we make it C S and C S E. So, these are trivial alternatives that we have we can make carbon monosulphide carbon monosilionide and carbon monoteleride. It turns out that carbon monosulphide is not a stable molecule by itself. So, if one has to make carbon monosilionide one has to start with C S E 2 and similarly if we have to make C S one has to start from C S 2 and C T 2 is available, but it is still not an extremely stable molecule and it is even more difficult to convert C S 2 to C S and C S E 2 to C S E and C T 2 to C T E. So, these are transformations which have to be carried out in the presence of the metal complex which has to form the M C X complex. So, if you want to make C X combined with metal, then you have to use the C S 2. You have to start with carbon disulphide reacted with metal in the presence of this molecule. So, usually this is not a great difficulty for the chemist because there are reagents which will remove one of the X atoms from C X 2 and generate C X in the coordination sphere of the metal. So, what we have just seen is that we can in fact make an alternative to carbon monoxide. We can make an alternative molecule for carbon monoxide, but the electronegativity difference between carbon and oxygen is something which is lost. Carbon has an electronegativity value of 2.55 and oxygen has a value of 3.44. So, you can see that this is a big difference which based on the Pauling scale. This big difference is advantages for making carbon monoxide are very uniquely. When we go down the group the oxygen group we lose this advantage C S C S E and C T are not great molecules. They are not very stable, but in spite of the lower electronegativity difference you can still make these molecules in the coordination sphere of the metal. Sulfur has in fact an electronegativity value of 2.58 and selenium has a electronegativity value which is almost identical to that of carbon which is also 2.55. So, you can see that this is not going to be a great source of help, but still C S C S E and C T E are the best alternatives that we have. If you try to make an alternative molecule starting with silicon and you can make silicon sulfide you would expect that the electronegativity difference would again be good, but that is not the case. Moreover, it is not possible to make stable molecules because the tendency of silicon to form double bonds with sulfur is very small. It does not form stable molecules with the oxygen group. So, germanium is even worse and tin is even worse than germanium. So, you cannot make alternatives to carbon monosulfide or carbon monoselenide using this substitution. However, we can substitute oxygen by sulfur selenium and tellurium. So, what we have seen is that changing from carbon to silicon is not going to solve the problem of the electronegativity difference because pi bonds are not readily formed by silicon. A second complication is a fact that you have the inert pair effect. The inert pair effect which sets in at tin makes the valency of tin plus 2 rather than plus 4. So, tin likes to be in a stable oxidation state of plus 2 rather than plus 4. So, the tendency of this molecule to form a ligand is very poor. In reality, then to summarize the section what we find is that C S, C S E and C T E are good ligands for a transition metal, but the other possibilities which arise from combining silicon or tin with sulfur selenium and tellurium are not good possibilities. So, let us move on to the synthesis of some of these molecules. As I told you earlier in this lecture, it is possible to use a molecule which will remove one of the sulfur atoms in the carbon disulfide. So, what we do is to start with a rhodium complex which has got a triphenyl phosphine which is coordinated to the rhodium. This is a rhodium one species. You will notice that this is a rhodium one species. It has got one uninegative ligand and three phosphine molecules which are coordinating a pair of electrons to the rhodium. Now, when this molecule is reacted with carbon disulfide, it likes to form a carbon disulfide coordinated rhodium complex which is an intermediate in which the chloride has been ejected from the coordination sphere. So, C L minus is a species which leaves the coordination sphere along with a P P H 3. This might occur in two steps, but the intermediate that is formed has got a carbon disulfide coordinated to rhodium and this is a rhodium one complex which has got three neutral ligands. Now, in this intermediate the P P H 3 molecule that was ejected from the coordination sphere of rhodium reacts with the P P H 3 and eliminates one sulfur atom one sulfur atom from this coordinated molecule leaves with this P P H 3 to form P P H 3 double bond S. This is a phosphorous 5 species. So, phosphorous has got oxidized from plus 3 to plus 5 here. So, you have an oxidation of the phosphorous to P 5 compound and a carbon monosulfide complex of rhodium one has been formed. Now, this rhodium one complex that has been formed here has got C S bonded just like carbon monoxide, but it is in a linear fashion and this C S molecule is quite stable now in the coordination sphere of rhodium. It would not have been possible to isolate carbon monosulfide as a neutral molecule except in a matrix which is kept at a very cold temperature. So, the metal is in fact stabilizing an very unstable molecule carbon monosulfide in the coordination sphere of the metal. It is stabilizing the unstable molecule carbon monosulfide. So, what are the disadvantages of carbon monosulfide? In spite of the fact that it is possible to get carbon disulfide very readily, one should note that carbon disulfide is in fact a very poisonous chemical. So, it was difficult to generate carbon monosulfide in the lab very easily without taking adequate precautions. There are alternatives for carbon disulfide, but they are even equally dangerous. Consider for example, C S Cl 2. This is thiophosphate gene. Phosphate gene as you know is a very dangerous chemical itself and thiophosphate gene is also very unstable and it is also dangerous. It decomposes readily in the presence of moisture, but thiophosphate gene can be used in fact if you are careful enough you can use it as an alternative for carbon disulfide. So, instead of C S 2, we can in fact substitute it with thiophosphate gene or ethoxy thiophosphate and thiocloroformate is another option. It is also dangerous, but it is easier to handle than thiophosphate gene. So, we will look at some of the molecules that can be generated using these two alternatives to carbon disulfide. As I told you, these two molecules have to be used very carefully. Let us now look at how one can use thiophosphate gene as a source for a carbon monosulfide complexes. Chromium hexacarbonyl is a molecule that we have seen extensively in previous lectures. Now, this molecule which is a stable 18 electron species can be reduced in the presence of sodium and mercury. So, if you take sodium which is a metal dissolved in mercury it forms an amalgam and this amalgam very readily gives out electrons. These electrons will reduce the chromium hexacarbonyl to a dianionic species which has got two electrons more than CrCO6 and it has got one carbon monoxide less. So, this species CrCO5 2 minus can be reacted with thiophosphate gene and in that case you have the formation of CrCO5 CS. This of course will result in the formation of two Cl minus molecules ions as a result of displacement of Cl minus from the Cl2 and the electrons are coming from the chromium di anion that we generated using sodium amalgam. Now, I told you that ethyl chlorothiaformate can also be used as a source and in this case also I am going to start making this molecule from an anionic carbonyl complex. Anionic carbonyl complexes are reasonably stable and the reason for the stability comes from the fact that the negative charge can be delocalized on the carbon monoxide. It turns out that the carbon monoxide can accept this electron density into the pi star orbitals on carbon monoxide very readily and form extremely stable complexes in the presence of anionic metal ions. So, here is a system where you have a negatively charged species on the ion, ion has got a negative charge and that charge is delocalized on this two carbon monoxide. We react that now with this ethyl chlorothiaformate. Ethyl chlorothiaformate reacts with this anion and ejects one C L minus and that gives us an ion thio carbonyl complex, ethoxy thio carbonyl complex which is having an ion carbon bond and it is also having this fragment C S attached to it. Now, how do we get rid of we want to make C S complexes. So, how do we get rid of this ethoxy group? Surprisingly, if you treat this system with hydrochloric acid one normally thinks that acids and organometallic chemistry will not go together, but here is a very surprising example. You can treat this ethoxy thio carbonyl complex with hydrochloric acid and what happens is that it protonates this ethoxy group and ethanol is removed from the system. So, ethanol is removed from the system as a result of protonating this ethoxy group with this proton here and because we are adding a proton and removing a neutral molecule this complex which is formed must be charged and it is positively charged and this positively charged molecule is now having a carbon monosulphide coordinated to the ion. So, this is the carbon monosulphide that has been formed and this carbon monosulphide is coordinated to 18 electron species which is a ion cyclopentadienyl moiety with two other carbonyls. Now, this is a very good example because it allows us to compare the efficiency of carbon monosulphide and carbon monoxide attached to the same metal atom. How will these two species compare in terms of bonding? So, these are examples which are very useful for us to look at the bonding efficiency of carbon monosulphide vis-a-vis carbon monoxide. So, here is a molecule that I have drawn for you from using a structure drawing program called mercury and this gives us another example where you have both carbon monosulphide that is right here C s and carbon monoxide which is bonded here. So, both of them are attached to a single metal atom which is chromium. Now, this molecule of chromium is again a chromium 0 complex. So, it contributes 6 electrons 6 valence electrons and then you have 3 phosphite molecules. So, you have 3 phosphite molecules 3 of them attached to the chromium each one of them will give 2 electrons each and so that accounts for 6 electrons and then we have also 2 carbon monoxide units. So, 2 carbon monoxide units give 4 electrons and 1 carbon monosulphide molecule and so we have 2 electrons coming from here. So, this molecule turns out to be a nice 18 electron species where you have a chromium 0 species donating 6 electrons to trimethyl phosphite units giving 6 electrons and 3 of them are there. So, 3 of them giving 6 electrons 2 electrons each 2 carbon monoxide giving 4 electrons and 1 carbon monosulphide giving 2 electrons making a total of 18 which is a very stable system. So, in this molecule we have the advantage of comparing the carbon monoxide with the carbon monosulphide in the same species. It turns out that the chromium carbon bond this is the distance that we are talking about the chromium carbon bond. The chromium carbon bond in carbon monoxide that is here is 1.836 angstroms. So, 1.836 angstroms is the distance between the chromium and the carbon and in the carbon monosulphide this distance turns out to be 1.782. So, we have 1.782 which is a shorter distance between the chromium and this carbon in carbon monosulphide compared to the chromium and the carbon in carbon monoxide which is 1.836. So, what does this tell us? If you remember the distance between the metal and the carbon is shortened from what is expected for a single bond distance between the chromium and the carbon because of pi interactions. These pi interactions are these back bonding interactions as they are called in some text books or pi accepting character of carbon monoxide makes the carbon monoxide carbon chromium bond shorter than what you would expect. So, when it comes to carbon monosulphide there also the distance appears to be shorter than what you expect for a chromium carbon single bond. So, clearly there is a very strong interaction between carbon monosulphide and the chromium. So, the back donation from the chromium to the carbon monosulphide must be very strong indeed. This is probably one indication that you have a very strong interaction between the chromium and carbon monosulphide. So, carbon monosulphide is in fact a very good pi accepting ligand. It has got very strong interactions between the metal and the ligand both in terms of sigma donation from the carbon monosulphide and also the pi accepting character from the chromium to the carbon monosulphide. So, let us just proceed a little further look at another complex. In this complex we have iridium complex which has got again two triphenyl phosphines. Here we have two triphenyl phosphines and these triphenyl phosphines are coordinated to the iridium and in the same way you have the carbon monoxide ligand and the carbon monosulphide ligand. Once again you see that the carbon iridium distance, the carbon iridium distance is in fact much longer than the iridium carbon distance which is there for carbon monosulphide. So, you can see that this distance is 1.863 angstroms and this distance which is the iridium carbon monoxide carbon distance that is 1.945 angstroms. What is interesting is a fact that you now have two examples where the carbon metal distance is always shorter than the carbon metal distance in carbon monoxide. This is clearly indicative of the fact that it is a general phenomenon carbon monosulphide is in fact a much better ligand compared to carbon monoxide when it comes to pi accepting character. So, are there any differences between carbon monoxide and carbon monosulphide? Carbon monosulphide in fact has this tendency to have a to form a bridge between the carbon monosulphide which is bonded to the chromium. So, here is a system where you have chromium carbon sulfide and so it is a chromium C S complex where you have the X-pattern extracted 180 degree angle between the chromium carbon and the sulfur. But then this sulfur is using its lone pairs to interact with another chromium atom. This chromium atom is also C R C O 5. So, this is C R C O 5 and although we have not indicated the atoms here there is a carbon here and there is an oxygen here. So, to make this structure fairly easy to understand we have not put all the atoms in place, but each one of these lines represent a carbon monoxide molecule. So, you have a C S coordinated to a chromium and that is in fact forming a bridge to the other chromium through the lone pair on the sulfur. So, this turns out to be a fairly interesting way in which carbon monosulphide can act. Carbon monoxide is not known to act in this particular fashion. In fact bridging carbon monoxide complexes are very rare and when they do bridge they do bridge through the carbon end. So, you have systems where the metal is interacting or bridging a carbon monoxide is present in this fashion and you have a bridging carbon monoxide between two metal atoms. Here the bridge seems to be formed between M C triple bond S and then there is a second metal here. So, you can see that there is a significant difference between the way carbon monoxide behaves and the carbon monosulphide behaves. This is probably one unique difference between carbon monoxide and thio carbonyl. In fact it would be interesting for us to see this molecule in three dimensions. So, what I am going to do is to show you this molecule in three dimensions. I will take this example where you have a chromium which is interacting with the sulphur which is in fact coming from a carbonyl sulphide. This is a carbonyl atom which is present. This carbonyl atom is thio carbonyl atom is present on this chromium atom and this first chromium atom what is labeled as C R 1 is having a C S which is used as a ligand and the C S ligand is in fact bridging a second chromium atom which has got five carbon monoxides attached to it. So, this is different from the complex that I showed you little earlier, but the system is similar. You have a carbon monosulphide which is capable of bridging which is coordinated to one metal atom and it is capable of bridging the other metal atom. So, you can see this molecule as a rotated. You can see that it is in fact species where the sulphur is a very nice bridge between the two chromium atoms. The first chromium atom has got a benzene ring in which you have six carbon atoms of the benzene ring coordinated to the chromium. Then it has got a C S bonded to it and this C S is bridging the second chromium atom. Whereas the carbon monoxides which are also coordinated to the chromium one are independent. They are not interacting with any other metal neither or these carbon monoxides bonded to any other species. So, C S has a tendency to bridge which is rather unique to the metal atom. So, let us proceed further. We have now seen an example where you can have bridging carbon monosulphide and this is different from the example where you have carbon monoxide which is not a bridging molecule. So, I have here ruthenium complex now which is a carbene complex. I have shown this example only to indicate the fact that you have a carbene which is capable of pi accepting electrons from the metal on to the carbene. So, this is an example where you have a pi accepting going on between the metal. Here it is ruthenium and a carbene and in the same complex you have a C S complex. So, you have a C S coordinated to the ruthenium and again you can see that the carbene has a carbon ruthenium distance of 2.11 angstroms. That is almost like a single bond distance between ruthenium and carbon and the distance between the carbon on carbon monosulphide and ruthenium that is almost 1.737 angstroms. So, the distance between carbon monosulphide and ruthenium is much much smaller than the distance between ruthenium and carbene which is a very poor pi acceptor, but a very good sigma donor. So, you can see from all these examples that carbon monosulphide is in fact an excellent ligand. It has a great tendency to form pi bonds with the metal and it consistently forms very strong bonds with the metal because of its pi accepting character. So, even compared to examples like carbon monoxide or in fact carbene here you can see the in the same molecule carbon monosulphide competes with carbon monoxide for pi accepting. Normally apart from the bond distance between the metal and carbon monoxide it is also possible to use the stretching frequency between the two atoms which are bonded to the metal. In this case carbon and sulphur we can look at the stretching frequency C A stretching frequency how it is affected on coordination to the metal. It is surprising that once you coordinate carbon monoxide the stretching frequency between the carbon and the oxygen goes down by 100 to 200 centimeter minus 1. In the case of carbon monosulphide the reduction in the stretching frequency is not very significant, but it is also about 70 centimeter minus 1. So, the C S stretching frequency in metal complexes ranges from 1200 to 1300 centimeter minus 1 and the frequency of carbon monosulphide has been measured only in the matrix which is a matrix very cold matrix of liquid nitrogen. In that the carbon monosulphide has been trapped and that stretching frequency is around 1270 centimeter minus 1. So, you can see that the stretching frequency just like in carbon monoxide it ranges from 1200 centimeter minus 1 to 1300 centimeter minus 1, but it is lower than what is observed for the free molecule. As I mentioned before the free molecule is not stable and so it has to be this spectroscopic features have to be measured under special conditions called matrix isolation spectroscopy. So, the reduction in the stretching frequency is clearly indicative of the fact that you have electron donation from the metal on to the carbon monosulphide and this transfer of electron density into the pi star orbitals of carbon monosulphide. So, there are pi star orbitals of carbon monosulphide and these pi star orbitals are accepting electron density from the metal and as a result the carbon sulfur bond order this bond order this bond order is reduced. So, the reduction in bond order is responsible for the stretching frequency going down from 1200 centimeter minus 1 to 1200 centimeter minus 1. In a few cases the stretching frequency in fact increases from 1200 and 70 centimeter minus 1 to a slightly higher value and this increase is normally present only in positively charged systems or when you have a slight delta plus on the metal atom. If you have a slight positive charge on the metal atom then the frequency of the carbon monosulphide is not reduced from the 1278 centimeter minus 1 that you have it is in fact increased to a slightly higher value and that is around 1300 centimeter minus 1. So, both from spectroscopic and from a x ray evidence we can see very clearly that there is a strong pi bond which is formed between the metal and the carbon monosulphide and not only that the strong pi bond is weakened spectroscopically it is observable that the pi bond is weakened because of the electron density coming from metal. So, let us now go to carbon monosalinate. Carbon monosalinate can be made in a very similar fashion compared to carbon monosulphide. What one has to do is to start with CSE 2 now. CSE 2 is less stable than carbon disulphide but nevertheless it is accessible one can make carbon disalinate and that can be treated with a carbon monoxide complex. In order to make these complexes what one normally does is the following you start with the complex which has got a labile ligand. So, here is an example where you have a metal T H F complex T H F which is tetrahydrofuran. This is tetrahydrofuran is a weak ligand and that weak ligand if it is coordinated to the manganese then one can use that as a precursor for making a CSE 2 complex. This CSE 2 complex is not a stable system and it has not been isolated and characterized but if you treat this intermediate after treating this molecule with carbon disalinate if you treat it with triphenyl phosphine what happens is the following. Once again there is a elimination of one of the selenium atoms one of the selenium atoms is captured by triphenyl phosphine and you form a P 5 molecule which is phosphorous in the plus 5 oxidation state with a P double bond S E formed in the process. So, this molecule is formed as a result of interacting with this carbon disalinate which is coordinated to the manganese and now you have a carbon monosalinate which is coordinated to the manganese. So, once again we have a system where you have carbon monoxide coordinated to the metal and carbon monosalinate which is coordinated to the metal. So, we can examine both in terms of spectroscopy and in terms of bond distances and in this particular instance we will also look at the carbon monosalinate complex in three dimensions using this program. We can look at these molecules in three dimensions and we can see how the carbon monosalinate is also interacting with manganese just like carbon monosulfide. There is a linear metal manganese CSE bond and the CSE bond distances are in fact, indicative of a pi back donation from the manganese to the CSE and you can see that this is now weaker as we had expected. This is now weaker and we have poorer pi accepting character compared to carbon monosulfide, but nevertheless it still seems to be much better than carbon monoxide because if you look at the bond distance that you have in this chromium complex. Here is a chromium complex which is coordinated to carbon monosalinate and you find that the bond distance between chromium and carbon in this complex is 1.78 angstroms and this is shorter than the distance that you observe in the same complex that is 1.89 angstroms between the chromium and the carbon monoxide. So, chromium and carbon monoxide seem to form longer chromium carbon bonds compared to chromium and carbon monosalinate which is much shorter 1.78 about 0.1 angstrom shorter, but nevertheless and since we are comparing the two systems in a same environment that means the remaining ligands are the same all of them have got the same type of ligands attached to the chromium CSE and CO attached to the same chromium and we can now compare the two bond distances and it is clear that CSE is a good pi accepting ligand just like carbon monoxide. So, this is in fact a surprising phenomenon what we have learnt is that carbon monosulfide and carbon monosalinate can form equally good metal carbon pi bonds although we did not expect this in the first place based on electron activity differences. So, the pi accepting character of CSE and CSE are reasonably good. The disadvantage however is a fact that you do not have easy access to CSE and CSE molecules carbon monoxide is a very stable molecule by itself it is known as a gas that can be bottled and stored and kept under high pressure as long as you do not keep it in a nickel cylinder which it will react with you can stay you can keep carbon monoxide safely in the laboratory. Whereas, carbon monosalinate and carbon monosulfide are molecules that you cannot store you have to generate it in the coordination sphere of the metal. Now, let us take a look at what are some other tricks that a chemist can use in order to generate alternative molecules to carbon monoxide. Because of the difficulty that we have in identifying or making carbon monosulfide one can think of an alternative way of making a ligand which will be just as good as carbon monoxide. Instead of changing the oxygen to sulphur let us change oxygen to nitrogen. Now, if you change oxygen to nitrogen there is one electron less in this atom compared to carbon monoxide. So, C n has got one electron less it is got 13 electrons instead of 14 electrons which carbon monoxide have. We can add this electron to form a ion. So, let us just add this electron we add this electron and form the cyanide ion. The cyanide ion as you might already be familiar with is quite a stable molecule. It forms a variety of complexes and it is isoelectronic with carbon monoxide. So, can it not be a good alternative to carbon monoxide? The answer is yes. In fact, carbon alkyl isocyanides are good ligands as well where you have protonated the C n minus with H plus. That is you react C n minus with a proton and then you can have R n C. So, this molecule an alkyl isocyanide isocyanide can be a good replacement for carbon monoxide. So, there are alternative ways to make a ligand which is as good as carbon monoxide by converting the oxygen to a nitrogen adding an electron and then protonating it. These are tricks which a chemist uses little bit of alchemy in order to generate new ligands. So, what are the type of ligand what are the type of complexes that the cyanide ligand forms? Metal cyanides are not considered as organometallics based on tradition because although they have a metal carbon bond. It is very often found that the cyanide is in fact coordinated to high oxidation state metals. So, here is an example where you have iron coordinated to cyanide, but iron is present in the plus three oxidation state. So, if iron is plus three then you find that other organometallic molecules or ligands which are found in organometallic chemistry are not coordinated to or not stabilized by iron three. So, cyanide has the ability to stabilize high oxidation states. So, this is considered as classical behavior or cyanide is grouped under classical ligands. So, although you have a molecule which has a metal carbon bond, you have a metal carbon bond and that qualifies for calling it an organometallic species. You normally do not call it an organometallic molecule. So, here is a system where you have iron coordinated to six cyanide ligands just like you have six carbon monoxide ligands coordinated to chromium, but this molecule is normally considered under coordination chemistry. Neutral metals are rarely stable with C n minus and the reason for this is that as you add a cyanide you also add a negative charge and when you add a negative charge then the excess electron density has to be pushed into the ligand and cyanide is not as good a pi accepting ligand as carbon monoxide. So, one normally does not have many organometallic examples where carbon monoxide, cyanide and other organic ligands are coordinated to the metal. Here is a unique example this is in fact iron two species that means iron is in the plus two oxidation state, but you have both carbon monoxide and cyanide coordinated to the metal. I have chosen all examples in such a way that we can compare the ligands the new ligands that we are talking about with an old ligand carbon monoxide that we have studied earlier. So, in this particular case you can see that the distance between cyanide is in fact longer it is 1.93 angstroms compared to 1.76 angstroms which is the distance between carbon and iron in this species. So, this bond distance is shorter this bond distance is shorter and the cyanide bond distance the carbon iron distance is much longer. So, you can immediately guess that the pi accepting character of cyanide. So, the pi bond between the cyanide ligand and the iron is much poorer than the pi bond between carbon monoxide and the iron atom. So, you can understand how the negative charge on the cyanide is not a great help. So, one has to replace the cyanide ligand negative charge with a proton. So, that you can form a neutral molecule which will be a better pi accepting molecule. Since R the R group or the proton can be replaced with an any R group we can have alkyl isocyanides. These alkyl isocyanides can in fact be changed to represent a wide variety of molecules it can be R can be in a real group it can be methyl ethyl or propyl and this changes the steric effect. It can also affect the electronic influence the ligand influence ligand places on the metal. So, in principle R and C or an alkyl isocyanide can be a more useful ligand than carbon monoxide. But in practice R and C is not a great molecule for the reasons that we will study in the following lectures. But suffice it to say that chromium ion cobalt nickel can all form homo elliptic complexes with R and C which means R and C is the only ligand which is coordinated to the metal. So, a metal is coordinated to M units of R and C and this molecule is stable. For preparing the nickel complex where nickel will have 4 units of R and C attached to it. So, you will have 4 R and C molecules with nickel one has to just start with nickel tetracarbonyl. You treat nickel tetracarbonyl with R and C and you will form this isoelectronic species which has got 4 alkyl isocyanides coordinated to the nickel. So, let us just quickly take a look at these structural properties because R and C has got this valence bond structure where you have a negative charge on the carbon. Here is the valence bond structure written on the top of the screen you have R n plus triple bond C minus as one of the resonance structures. So, you have a very large negative dipole on the carbon and it becomes a much better donor. And so it is in fact a better sigma donor than carbon monoxide and it will replace carbon monoxide in nickel tetracarbonyl and form a complex. And this can be done because you have a negative charge. You can also form complexes with a positively charged metal system just like cyanide, but this time the molecule is exactly similar to the carbon monoxide complex. You have a neutral ligand not a negatively charged ligand a neutral ligand which is coordinated to the metal center and that also is quite stable. So, you have you can have complexes with palladium. You can have complexes with platinum and both of these are positively charged and they form nice complexes with alkyl isocyanides. And it is this resonance structures which make it very good ligand. You have a negative end on the carbon and that makes it a much better donor. And the metal can pump electron density back into the pi star orbitals of the cyanide alkyl isocyanide and that can make this metal carbon pi bond a very stable system. So, what we have seen today is that there are in fact alternative ligands for carbon monoxide. What one uses is a little bit of alchemy and the periodic table to generate a new set of ligands. And the ligands that we have seen are C s, C s e and C t. Although we have not seen examples of this ligand this can also be made and then C n minus and C n minus can also be protonated. So, that you have C n r that is alkyl isocyanide and that can also be a good ligand for the metal atom. So, with this we conclude this lecture on alkyl isocyanides, cyanides, carbon monotelerides, carbon monosilinides and carbon monosulphides. And these are good alternatives for carbon monoxide in organometallic chemistry.