 Once again, welcome you all to MSP lecture series on interpretive spectroscopy. In my last lecture, I was discussing about the influence of various factors on stretching frequency of carbon monoxide in a homolyptic as well as mixed ligand complexes. And if the metal is positively charged, what would happen is metals are reluctant pi donors as a result stretching frequency would be larger on the other hand, if the metal is negatively charged, it is electrons rich to minimize internal repulsion. What happens? It becomes a good pi donor as a result stretching frequency drops. So now, let us consider analysis of more such examples here. I have listed here a series of complexes having D10, D6 electronic configuration. And this table includes both early as well as late transitive metals. One fact you should remember is early metals despite having electron deficiency are very good pi donors in contrast late metals which are rich in electrons, but they are reluctant pi donors that can be clearly seen here in these complexes of both early and late metals. If you look into D10 system having silver having a carbon monoxide group, here it is a CO compound A G is in plus state and here it is 22 O 4 centimeter minus 1 and this is much larger than the free gaseous CO it is 21 43. And that indicates again how late metals are reluctant pi donors. And if you look into nickel tetracharbonyl, again when the four carbon monoxide are there and D10 system despite that stretching frequency is quite high here 20, 60 centimeter minus 1. On the other hand in case of tetracharbonyl cobalt anion shows much less stretching frequency new CO around the 1890. On the other hand MN with positive charge MN CO 6 plus it shows 2090 and then CR CO 6 is 2000 and vanadium hexa carbonyl anion shows 1860 here. So that means here since it is magnesium positively charged it is a reluctant pi donor as a result what happens less electron density is going to the pi star of CO and hence stretching frequency is more here. And again here because of anion it is electron rich vanadium now 3D to 4S 2, 5 and 6 electrons are there now. And as a result what happens it is a very good pi donor and as a result what happens stretching frequency drops. That means increase in electron density on a metal center resulting in more back bonding to the CO ligands and hence lower the stretching frequency. More electron density would then enter into the pi star orbital and weaken the CO bond. Therefore it makes the MCO bond strength increasing and more double bond like character with stretching frequency coming around almost ketone or aldehyde carbonyls what we see in case of organic chemistry. This is what exactly happens here and also I showed you in my previous lecture it can also have something like this. Now I have shown here chromium hexa carbonyl YAMO diagram and here you can see this one is representing 6 sigma orbitals sigma bonds between chromium and CO this one and then this is pi bonding. So pi bonding you can see here pi star would combine with T2G to generate a set of bonding and anti-bonding orbitals and the bonding orbitals would occupy electrons from this T2G set this is pi symmetry whereas here we are considering 6 ligand group orbitals symmetry adopted linear combination of atomic orbitals having symmetry of A1G T1U and EG and now this 12 electrons would be occupying 6 orbitals and this also you can consider as like D2SP3 D2SP3 can be seen D2S and P3 again VLS bond here you can bring it here and here that means you have a defined sigma bond between chromium to carbon monoxide and also we have pi bond between chromium and CO because of this one what happens we have something like this happens or I would say something like this so this result in dropping of teaching frequency of carbon monoxide from when compared to free CO. Now let us look into nickel tetracharmonyl here this is the nickel tetracharmonyl nickel having detailed electronic configuration and then four COs are there and four COs I have shown so many electrons here of course these electrons are from four COs four COs if we consider we have this is the so this one is responsible so we should have four such lone pairs on carbon that makes them sigma donor or neutral donor towards the metal centers if there is a metal to carbon monoxide bond is there that is held by these two electrons present on carbon they are represented here and then these six electrons six into four 24 electrons would be accommodated here you can see here this is what I have shown here and this one is deeply buried so I am not showing now consider from wellness bond theory we say that nickel tetracharmonyl has sp3 hybridization and tetrahedral no doubt it is tetrahedral but sp3 really involves you can see here and if you consider these four coming from here they remain non-bonding that means basically and they are supposed to combine with T2 and A1 to make four sp3 hybrid orbitals to which these four electrons should go and to establish nickel to CO sigma bonds four of them but that is missing and they remain as non-bonding these electrons remain as non-bonding that means in nickel tetracharmonyl we don't have nickel to carbon monoxide sigma bond at all then how it is surviving it is surviving because of back bonding this pi star of CO combines with T2 and E to generate set of bonding and anti-bonding orbitals and now the electrons from D10 are smoothly transferred to this one through back bonding so that means NICO4 is just getting stabilized or surviving because of only back bonding that explains why nickel tetracharmonyl compounds are unstable and highly volatile and also you can see stretching frequency is also much higher for the same reason here we do not have a formal sigma bond between nickel and carbon monoxide but we have pi bond that pi bond holds them not as strong as we see in case of chromium iron carbonyl complexes but nevertheless NICO4 exists but it's highly volatile compound and readily you can dissociate CO and this is how they made nickel tetracharmonyl to generate two nickel through decomposition now I have given a list of complexes having different type of phosphines along with nitrogen donor ligands such as astronitrile and pyridine here you can see all are derivatives of MOCO3 molybdenum tricharmonyl three carbon monoxide are replaced by trifluorophosphine here trimethylphosphide and triphenylphosphine astronitrile and pyridine focus your attention towards stretching frequencies here in case of trifluorophosphine stretching frequencies are quite high 2090 2055 so this indicates not much electron density from the metal molybdenum is going towards the pi star of CO why it's not happening because there is a computation for back bonding from PF3 as well PF3 is as good as CO in terms of its pi acceptor capability as a result what happens not much electrons it goes to CO pi star and C triple bond is not much affected and hence we see highest stretching frequency but when we move from trifluorophosphine to trimethylphosphine is relatively less pi acceptor in nature compared to trifluorophosphine as a result what happens the stretching frequency drops here and now since they are not competing well with carbon monoxide more electron are going to the pi star of carbon monoxide in case of this compound here and hence stretching frequency drops that's even more pronounced in case of trifluorophosphine trifluorophosphine is a good sigma donor but not really a very good pi acceptor that's again reflected in the stretching frequencies of CO here and then when you go to austral nitrile austral nitrile is only a sigma donor now as a result what happens all electron density whatever is there from the metal it's zero valent that should go to only remaining three CO and hence it drops further in case of pyridine again it drops further to 1746 and 1888 so this gives a measure of the influence of other ligands present along with carbon monoxide on their stretching frequencies if they are competing well if they are good pi acceptors the stretching frequency doesn't drop considerably but if they are poor pi acceptor or no pi acceptors and only good sigma donors then stretching frequency drops considerably because more and more electron density would go to pi star up on the remaining carbon monoxide so this is a nice analogy this can also give you some information about the position of these ligands in the spectrochemical series as well so now what we should remember is when CO bridges two or more metals apart from carbon monoxide acting as a terminal ligand whatever we saw now stretching frequencies all are of carbon monoxide acting as terminal ligands so when CO bridges two or more metal atoms as you've seen FE to CO 9 CO bridging stretching frequency will be less when CO's are substituted by other ligands which are only sigma donors new CO value drops further due to more intake of metal pi electrons to pi star CO group that's what I showed you in my table in the last slide in case of FE to CO 7 dipyridine 2 to dash dipyridine for example FE to CO 9 if you take replace two carbon monoxide by a bidentate ligand such as 2 to dash bipyridine CO can act as a bridging ligand evidence for a bridging mode of coordination can be easily obtained through aiaspotoscopy when they are bridging they are more or less they are similar to ketonic carbon monoxide we see in case of organic compounds that means they will be much less in the specific frequency it will be around 18 to 1700 or even less so that clearly indicates that we have bridging carbon monoxide in a complex so all metal atoms bridged by a carbonyl can donate electron density to the pi star of the CO and we can CO bond in case of FE to CO 7 dipyridine CO stretching frequency is 2080 for terminal whereas for the bridging one this comes around 1850 so this also indicates how we can distinguish between terminal carbon monoxide and bridging carbon monoxide and further drop is there whether it's if we have poly nuclear or poly metallic centers whether CO is bridging two metals or three metals could also be gauged simply by looking into the stretching frequencies of carbon monoxide in the eye spectrum so pi acceptor the ability of CO in MCO 6 place it at 0.1 to 1.2 electron per CO that means if you consider any metal carbonyl and carbon monoxide have a capacity to take anywhere between 0.1 to 1.2 electron density to their pi star orbitals anti-burning orbitals that means stretching frequency decreases as more and more carbon monoxide groups are substituted because you'll be left with only few carbon monoxide to take care of electron density present in the pi orbitals so complete substitution of CO from MCO 6 has been achieved only by poly dented ligands or ligands with electronegative substituents on donor atoms having empty pi orbitals for accepting electrons the best competitors for CO or phosphines as I had a re-mentioned also I showed you how it varies from PF 3 to trimethylphosphide PPH 3 like this or PME 3 so here back bonding decreases it comes in this fashion so advantages with phosphine is coordination properties can be readily altered so why phosphines are more versatile compared to carbon monoxide is no matter what happens in order to call elegant CO carbon monoxide there should be carbon there should be oxygen and the CO bond order can vary apart from that one we cannot much do with the structure of CO on the other hand when you consider P PR 3 say PR 3 by putting more electron withdrawing group on phosphorus we can make it poor sigma donor but excellent pi acceptors on the other end if you put more electron donating groups on phosphorus we can make it very good sigma donor but poor pi acceptor on the other hand by a combination of these things we can have moderate donor and acceptor properties so that we can put them into desired metal complexes to use in some applications particularly in case of homogeneous catalysis for organic transformations this is where the importance of phosphine comes into pictures in their ability to control the coordination and say electron saturation at the matter center in various oxygen states this kind of unusual valence and all those things observed in case of metal complexes is because of the versatility of phosphines among no pi acceptor ligands examples are diagram NH3 H2O H2O they are not pi acceptor ligands so mixed metal coronals with one or more diagram like ligands can also show some trends in their new CO that means when you have only sigma donor ligands or with hard donor atoms they can also show some trends in their stretching frequency in the stretching frequency of carbon monoxide for example if you consider the stretching frequency of CO in mCO3 diagram which are higher than those of mCO3 diene twice so greater electrolyte of donor there's donor ability of oxygen so if you consider phosphorus arsenic antimony and bismuth have sigma star rubbers for back bonding all of them have sigma star for back bonding CO has pi star whereas phosphines are I would say ER3 where E equals phosphorus arsenic antimony and bismuth they have sigma star orbitals for back bonding so relative sigma donating ability of your both donor atoms may be estimated from the stability of their addition complexes with ELCL3 so ELCL3 is a very good Lewis acid so it can form readily adducts with those things to what extent these adducts are stabilized it would give some information about their relative sigma donor ability but pi acceptor ability can be compared by making mixed ligand complexes of both carmel and phosphines are arsenic, bismuth compounds the order of donor abilities follows this order here and also in case of chalcogens it follows this order of course we call them as nitrogen also group 15 elements isomers and point groups of substituted OH carmel groups I have shown here this is just to identify based on point group how many active CO stretching bands are observed for a given geometry for example octahedral complexes we can have MCO5L one ligand is there in that case what happens your point group will be C4V square pyramidal geometry you can assume the relationship of 5 caromerox it will be square pyramidal geometry and you can anticipate three bands 2A1 plus E and when we have MCO4L2C we can have C2V point group in that case we can observe four stretching frequencies and when it is a trans it has a D4H and we can get only one stretching frequency and then when we consider MOCO3L3 here we can have cis and trans that is facial and meridional facial we will be having C3V so we will see two stretching frequencies for CO whereas meridional has C2V we can see three stretching frequencies similarly if we consider MCO2L4 that means the ligands are very good by acceptor we can have in case of hexa carbonyls such as chromium and tungsten we can have when the two caromerox and four other ligands are two bidentic ligands in that case also we can have cis and trans isomers and here it cannot be meridional or it only cis and trans so we can have in case of C3V and in case of trans C2V and we can see here too and we can see here one stretching frequencies so this gives some idea about octahedral complexes with substitution up to four caromeroxide what would happen and what are the corresponding point groups depending upon geometric isomerism they show cis and trans facial meridional in case of trigonal bipyramidal geometry we can have if we replace one CO we can have MCO4L and it can be axial it will be C3V three bands if it is radial C2V there will be four bands we can see and again MCO3L2 two axial and two radial we can have D3H and one and when you have two radial C2V and three bands we can observe and when we have one axial and one radial we can have cis and we can have this kind of three bands we can see here and similarly we can have MCO3L2 three radial and two radial radial and one axial one radial and we can see the corresponding point groups here so with this table by comparing the complexes we have on hand we should be able to identify IR active new CO modes here what you should do is you should write the corresponding actual structures with geometry and try to identify the active modes then it is easy to remember them see I have shown here if we replace one CO what we get is this one here pentacarmonyl and here it has C4V symmetry you can see three bands and then if we replace three of them we can have either facial or you can have meridional and the corresponding active IR modes for CO are shown here and similarly when you go for MCO4L2 we can have trans or we can have cis whether you have C2V symmetry we have here here we have so you should be able to tell here yes we can see one the one whereas here we can see four bands sometime we may see three bands where two are merged or overlapped so when we look into tigral bipyramidal geometry you can have all five are there you can see two bands when we replace one axial one we can get three bands when we put equatorial one we can see four bands here and then when we have D3H symmetry we can see only one because all are in the plane and if we have C2V symmetry we can see two bands and when you do not have any in this case what happens we will see two bands again and similarly in case of tetrahedral complexes we have MCO3L we can see two and then this tetrahedral will be showing you one and then if you replace them with two ligands ML2CO we can see two bands so this is all about mixed carbonyl complexes having other ligands how many bands you can see that can be determined using the point group and then we can really identify the point group in those molecules and we should be able to predict number of bands here so let me stop here and continue in my next lecture few more problems before I proceed to discuss on mass spectrometry until then have an excellent time thank you