 once again welcome you all to MSP lecture series on interpretive spectroscopy. In my last lecture, I was telling you about relationship between the stretching force constant of a bond with respect to its stretching frequency. And as I mentioned, there are three simple equations to find out unknown entities such as stretching frequency or stretching force constant. The simplest one being nu bar equals 130.3 into square root of f by mu. Here f is in newtons per meter and then mu is in atomic mass unit. If you want to use the standard equation derived from Hooke's law that is nu bar equals 1 over 2 pi c into square root of f by mu. Here mu has to be multiplied by 1.67377 into 10 raise to minus 27 to convert a mu into k g and rest is alright you can calculate and find out. So, in another equation nu bar equals 4.12 into square root of f over mu. So, in this we have to convert stretching force constant from newtons per meter to dynes per centimeter. So, if it is 1 digit or 2 digit you multiply it by 10 raise to 5 and if it is 4 digit multiplied by 10 raise to 3 you are automatically converting that into dynes per centimeter and then if you calculate at the end there will be little bit difference will be there, but all the three equations are equally good. The simplest one being nu bar equals 130.3 into square root of f by mu very easy to remember and you can practice with the table I gave where I listed a reduced mass also there is no need to worry about that one and also I gave you force constant for all the bonds I showed you different bonds in newtons per meter and also corresponding stretching frequency in centimeter minus 1 in wave number was also given. So, you verify and make yourself comfortable in calculating those parameters. Let me continue discussing about now the important one as far as coordination compounds and argumentary compounds are concerned it is carbonyl complexes and in front of respect of carbonyl complex is very very important to know the donor and acceptor properties of other ligands present along with carbon monoxide in mixed ligand complexes though structure determination number of bands in the CV stretching region all those things are very important if the number is consistent with that provided by the selection rule of a particular point group may be assigned to the molecule again if you look into the point group and character table you can identify infrared infrared active bands and also how many bands are expected for a particular point group also you can know that I would be showing you later and then what is important is the stretching frequency in the region of 2000 centimeter minus 1 is very very important for compounds containing carbonyl groups. What are the limitations of predicting the structure from the number of CV stretching modes? For example, the number of stretching frequencies what we observed in the spectrum may not really tell the fact that how many CO groups are present. So that means we should try to see some relationship between the molecular structure of a compound and activity of the CV stretching modes for a given point group or for a given geometry for substituted carbonyl complexes when we consider substituted non-homolyptic carbonyl complexes in that case point group may be assigned considering the local symmetry of the metal and the carbonyl groups provided the ligands have the spherical symmetry is very important you can assign point group considering the local symmetry of the metal and the carbonyl groups provided the ligands have the spherical symmetry if not considering the symmetry of the molecule as a whole so either way we can correlate the geometry with respect to number of carbonyl groups present and also number of stretching frequencies are observed. For example, you consider here cyclopentatri carbonyl manganese and also another example is tetracharbonyl vanadium with CP group cyclopentadienyl vanadium tetracharbonyl or you have here tricharbonyl benzene compound of chromium or molybdenum and if you try to do the electron count also you can do it here manganese in plus one state if it is manganese in plus one state we have six electrons are there and six electrons are there and six electrons are there this innate electron complex this is another method to make yourself about electron counting and here again six are there and then here four and plus eight because vanadium is in plus one state so this also a electron complex and here it is six plus six plus six this also a electron complex so all are a electron complex so we also learned about counting the electrons this is by you can use ionic method or covalent method doesn't matter so all these compounds show two stretching frequencies when you look into the air spectra if you look into the first one it has C3V symmetry and the second one has C4V symmetry and third one has C3V symmetry again so that means both cyclopentadienyl group and also eta 6 are in are axially symmetrical with respect to these point groups symmetry is lowered if aniline or thiophene replaces are in benzene or cyclopentadienyl group then the symmetry of the whole molecule has to be considered for example if you see here you can see the symmetry is here in case of tricarbonyl cyclopentadienyl manganese and you have a C3V is there you can also have C3V and here you can have C4 axis of rotation so that means basically you can consider the symmetry of the molecule as a whole because the other ligand is also symmetrical with respect to the point group that is identified and then if you consider here if you put a hydro atom to make a thiophene or adding nitrogen to make it pyridine then what happens the symmetry of the molecule is lost and then the symmetry of whole molecule has to be considered here as a result what happens you will see 3 in case of this one as well as 3 in case of this one stretching frequencies so 3 COs will be observed in both the cases the trends in stretching frequency of compounds belonging to a series having related structures can be interpreted using simple bonding scheme for example how to make this compounds for example we take homolyptic carbonyl example iron pentacarmonyl or chromium molybdenum tungsten hexacarmonyl or manganese mn to co10 or co2 co8 in case of cobalt if we chain uv light or thermally also we can activate in presence of ligands to have a series of compounds of this formula then after making this compound just if you subject them to infrared spectroscopy that will give you some idea about the number of stretching frequencies present and also probably local symmetry and then this is the M O diagram for carbon monoxide and I am sure you are all familiar with carbon monoxide and if you consider simple Lewis dot structure so here we have 4 and here we have 6 10 valence electrons are there here and then initially what we can do is we can put a bond here and then one bond is there and then put something like this and then the remaining two electrons will be put like this so now Lewis dot structure okay octet is not satisfied for this one so this electrons will come here and this electrons would come here as a result what happens triple bond is established and this lone pair remains and this lone pair is there and you know that this lone pair on carbon is responsible for carbon monoxide to act as a neutral ligand and this lone pair is what shown here and then these three sets of are shown here these three represents and then this one is shown here this is deeply buried as a result what happens when C O is acting as a ligand it can never use this lone pair for coordination it only uses this and many times students often get confused that when it's bridging two metal centers it acts as a four electron donor very similar to halides that is not true when it is bridging with the two metal centers or three metal centers what we are doing is we are generating electron deficient compound so here if you make it it is a four center two electron bond four center two electron bond and if you have something like this it is three center two electron bond see whatever the electrons are present here this will be shared between two metals so these things I would tell you what would happen when it's acting as a terminal ligand bridging ligand try bridging leg and what would happen stretching frequency we can consider so this is what we should focus on and then what happens we have the pi star is also there the pi star energy of C O is quite comparable to T 2 G orbitals or DX at the YZ and the XY orbits of metal complexes which essentially are non-bonding in the absence of back bonding or pi bonding so they interact with pi star to generate a set of bonding and anti-bonding orbitals and this bonding orbits would be taking electrons from metal that we call it as back bonding and one also should remember the fact that one carbon monoxide can take anywhere between 0.2 to 1.6 electrons to its pi star orbital through back bonding this is d pi I would say pi star bonding or I would also say it is it's a d pi means here it is a dx y dx z or dy z okay now you can see again so this is this represents sigma bond formation two electrons are there and this is C O bond formation is there this is sigma and then this interaction it can be with any of those orbitals such as dx y or dx z or dy z so this pi star would interact and this is called pi bonding this is pi bonding and this is sigma bonding so when sigma bonding would make carbon monoxide electron deficient as a result what happens pi bonding will be initiated and when the pi bonding is there carbon monoxide is electron rich and sigma bonding will be more strengthened so this is called synesthetic effect because of this one what happens metal to carbon bond is stabilized when more and more metal to carbon bond is stabilized C O bond will be weakened and it will be elongated and the stretching force constant decreases and also C O stretching frequency also decreases so these two modes of bonding are mutually reinforcing and is called synergic effect charge removal through pi bonding leads to more extensive sigma bonding while charge donated through sigma bonding thus facilitates further back bonding so this mutual give and take benefits that metal to carbon bond and it will be more strengthened and becomes more and more stable let us look into some reactivity here and when we consider metal hexacarbonyl such as chromium albinatangsten hexacarbonyl and when we react them with only nitrogen donor ligands which are sigma donor in nature for example austenonitrile benzonitrile or if you take triethylamine only alkylamine or arylamines in this case what happens maximum replacement of only three carbon monoxide is observed here because since nitrogen donor ligands are good sigma donors to minimize interact on repulsion and also to stabilize the metal in its zero valent state more and more back bonding has to be considered so minimum of three carbon monoxide are needed in such cases to minimize interact on repulsion so that zero valent metal is stabilized so in this context when we are considering only sigma donor ligands in case of hexacarbonyl we can replace only three of them with these things on the other hand if we consider sigma donor and pi acceptor ligands such as phosphines it's possible to replace up to four carbon monoxide and if the phosphine is much stronger like trifluorophosphine it's very easy to knock off all carbon monoxide to form homo elliptic phosphine complex for example if we take thia co6 and if we use six pf3 it is possible to get rid of all six carbon monoxides to get a homo elliptic pf3 six complex or one can also see some some other compounds like bidentate ligands like this even this ligand is comparable to almost its pi acceptor ability to carbon monoxide it can also if we use three equivalents of them it can also replace our carbon monoxide to form something like this so such compounds have been well established and reported in 1980s and 1990s so what we should remember is as far as infinite stretching frequency is concerned free carbon monoxide shows around 2133 or in some books say 2147 so one can consider 2140 or something and in case of metal hexacarbonyl the range is around 2000 centimeter minus one so that means there is a considerable drop in the stretching frequency this is because of the population of electrons into the pi star of CO decreasing the bond order so increase in negative charge on metal is observed by new CO changes for example when we consider isoelectronic series such as mnCO6 plus chromium hexacarbonyl and vanadium hexacarbonyl anion we can see what would happen to the stretching frequencies when we go from cationic to neutral to anionic stretching frequency drops because this is electron rich when it is electron rich metal more and more electrons density goes to the pi star as a result metal to carbon bond is strengthened and CO bond is weakened and whereas here it is here it is in positive charge as a result it reluctantly donates electrons to the pi star of carbon monoxide as a result what happens stretching frequency and would be a little higher compared to this one so we can consider the overall equilibrium of metal to carbon bond with two extreme cases in this fashion when moderate back bonding is there something like this and the back bonding increases becomes almost ketonic and here if the back bonding is quite extensive then we can have a situation like this these two are the extremes and here this is the with good donor and acceptor properties of the other ligands it can exhibit something like this so stretching frequencies are given here you can see 2096 quite high and it is 2000 and 1859 so as the electron density increases on the metal center stretching frequency also decreases because more and more electrons will be promoted to pi star this also we can call it as charge transfer metal to ligand charge transfer in spite of its isoelectronic series because of the positive charge and negative charge this observation can be made easily so now I have given for detain complexes of different metal ions here silver karma maroxide plus and n-s-c-o4 nickel tetracharbonyl and cobalt tetracharbonyl anion manganese hexacharbonyl cation neutral chromium hexacharbonyl verinium c-o6 for just comparison I have given here values free c-o is 21 43 you can consider this as the standard value and then in case of ag plus it is 2204 a little higher than the free gaseous and then nickel tetracharbonyl considerably higher so that means in nickel tetracharbonyl it appears that the back bonding is not extensive and of course if we consider overall 3d 4d and 5d metals early metals despite having electron deficiency they are excellent pi donors on the other hand despite having large electron density among the late metal late rosin metals that means I'm talking about iron afterwards and having more electron density still they are reluctant pi donors that can be seen here by simply looking into the stretching frequency of karma maroxide that is bound to these late metal ions silver it is 2204 and then nickel tetracharbonyl 2060 and cobalt tetracharbonyl anion 1890 here it is more because metal is anionic and more electron density is there and stretching frequency will be very less compared to other one and then on the other hand here manganese plus so positive doesn't donate very easily so it comes around 2090 chromium hexacarbonyl moderate 2000 and again here anionic stretching frequency further drops to 1860 compared to 1890 in case of cobalt this also indicates early metals are very good pi donors so increase in the electron density on a metal center results in more back bonding to the carbon monoxide ligates more electron density would enter into the carbonyl pi star orbital and weaker co part therefore it makes metal to karma maroxide bond strength increasing and more double bond like character here what I had shown in my previous slide so now let's look into chromium hexacarbonyl to see how back bonding happens so here we have six karma maroxides are there and we have taken ligand group orbitals are there symmetry our adopted linear combination of atomic orbits are concerned here in polyatomic molecules and here six ligand six karma maroxide will be having a symmetry of a1g t1u eg to match with metal 3d 4s and 4p orbitals so 3d we have eg and t2g because octahedral splitting and then of course 4s is a1g and 4p triplet degenerate t1u is there now they combine in this fashion to generate six co bonding orbitals in which 12 electrons would be accommodated here and then they these six electrons whatever is there on zero valent chromium will be sitting here and then these electrons as I mentioned would interact with this orbital t2g will would interact with pi star having t2g symmetry to generate bonding and anti-bonding orbitals of pi symmetry and then these electrons would come here so this would explain sigma donation as well as pi acceptor properties of karma maroxide this is for nickel tetracharbonyl so nickel tetracharbonyl we know that nickel is in zero valent state and it is tetrahedral and then valence bond theory suggests sp3 hybridization having nickel something like this and then co will be binding something like this actually the molecule is tetrahedral in nature but when we look into a mo diagram valence bond theory says without any hesitation that it is sp3 but when you look into molecular orbital diagram it gives a different hint about the geometry and if you see nd here if you consider here 3d and 4s and 4p are much higher in energy and then if you just look into the sigma and pi bonding orbitals of karma maroxide for karma maroxide these are not at all interacting these two are supposed to interact with this one and this one a1g at t1u from 4s and 4p to establish 4 nickel to karma maroxide bonds but that is not seen and here they remain as non-bonding so here as non-bonding and there's no interaction whatsoever with this with this that means basically sp3 hybridization predicted from valence bond theory doesn't explain bonding in nickel tetracharbonyl and then but what you can see is here we have pi star orbitals of 4s and they are interacting with t2 here not t2g t2 I would say to have back bonding that means nsvo4 survives only on back bonding but there is no sigma bonding that means these electrons are more or less confined to karma maroxide itself they are not forming nickel sigma bonding at all you can see here all are here I would show you in the next slide you can clear here well you may be surprised why I have shown so many electrons here of course if you consider here here one pair two pair three pair and four pairs are there four pairs per karma maroxide and three for triple bond one for lone pair and similarly we have 12 electrons four pairs are there four into four sixteen pairs of electrons should be there and this is deeply buried this can be ignored those sixteen pairs should be shown here all the sixteen pairs are shown here that says that it looks complicated but our attention should be towards these four electrons here so they are not at all involved in binding so you can see here so these these are supposed to interact with this as well as this one to make four nickel to karma maroxide so these ones and these are nothing but these electrons on karma maroxide so this is not they remain almost like non-bonding so that means how the karma maroxide are held it is because of back bonding here e and t2 here and it splits here into e and t2 and then these 10 electrons from d10 system are occupying here so that indicates why nsco4 is highly volatile highly unstable because it doesn't have any nickel to karma maroxide sigma bonding unlike chromium hexa carbonyl where we saw there is chromium to carbonyl sigma bond in all six carbonyl groups so here we have both whereas here we have only this one not this one is missing because there is they're non-bonding here this electrons remains non-bonding they're not interacting and many textbooks are not showing this one and of course if you want to look into more you can look into these references that have shown here so now let's look into the effect of different types of ligands on new co that means stretching frequencies in mixed ligand complexes so one system I have taken here tricarbonyl marbonyl complex having different type of phosphines and other nitrogen donor ligands and if you consider here tris trifluorophosphine tricarbonyl marbonyl complex is there three karma monoxide groups are replaced by pf3 and as I mentioned pf3 is an excellent pi acceptor it's a poor sigma donor but an excellent pi acceptor as a result you can see stretching frequencies are much higher very close to free co but when you replace pf3 trimethyl phosphide it is relatively weak pi acceptor compared to trifluorophosphine here considerable drop is there in the stretching frequencies that means more back bonding is observed and the other end when you go to trifluorophosphine and trifluorophosphine is a good sigma donor but moderate pi acceptor and it is less weak pi acceptor compared to trimethyl phosphide further it drops here then if you consider tris carbonyl complex with astronitrile astronitrile is only sigma donor and now only three karma monoxide are there it further drops here because more back bonding happens to remaining three karma monoxide groups and then in case of pyridine three also it is much relatively lower this indicates how the ligands present along with karma monoxide can influence the stretching frequency of karma monoxide if they are competing equally well for back bonding their stretching frequency increases on the other hand if they are weak the ligands that are incoming are weak pi acceptor their stretching frequency drops considerably and c o bond becomes weakens okay so let me stop here and continue more discussion on metallocarbonyls and their stretching frequencies in my next lecture until then have an excellent time thank you