 Hello everyone, welcome you once again to MSP lecture series on transformative chemistry. In my previous two lectures I gave the method of preparation of important compounds useful in doing substitution reactions in coordination chemistry and also their utility in organic transformation as homogeneous catalysts or precursors. So let me now try to do the classification of ligands. As you know ligands can be classified in several ways. The simplest way of classifying all ligands into two categories is classical or simple donor ligands and non-classical ligands or pi bonding and pi acid ligands. So what are the difference between classical or simple donor ligands and non-classical ligands which besides having sigma donor capability they also can be pi donors or pi acceptors. So these classical or simple donor ligands act as electron pair donors to acceptor ions or molecules and form complexes of all types of Lewis acids, metal ions or molecules including main group elements. How good non-classical ligands? They are also called as pi bonding or pi acid ligands. They form largely with the transfer metal atoms and we will come across special interaction between the metal and the ligands under non-classical ligands and these ligands act as both sigma donors and pi acceptors due to the availability of empty orbitals of suitable symmetry and energies comparable to with those of metal non-bonding orbitals. For example, if you consider octahedral complexes here T2G remains as non-bonding and T2G orbitals you know already that they are DXY, DXZ and DYZ and they have pi symmetry and they can interact with suitable pi acceptor orbitals of ligands and back bonding takes place. For example, consider a tertiary phosphine and ammonia both can act as bases towards H plus but phosphorus differs from N because in tertiary phosphines we come across low energy sigma star orbitals essentially of anti-bonding in nature whereas N does not have such orbitals for back bonding. So that means sigma star orbitals of tertiary phosphines can interact with T2G or non-bonding orbitals of a metal complex and accept electrons which are not utilized in metal to ligand bond formation and hence it can reinforce the metal to phosphorus bond that is we call it as back bonding. Let us consider carbon monoxide that do not have measurable basicity to protons yet readily reacts with metals like nickel that have high heats of formation or high heats of atomization to give compounds such as nickel tetra carbonate. Ligands may also be classified electronically depending upon how many electrons that they contribute to a central atom that means apart from classifying them into classical non-classical one can also classify ligands depending upon how many electrons they are in a position to donate or contribute to the central atom in form of pairs. For example, if a ligand is capable of giving only two electrons that is called monoidentate ligand or if it has two donor atoms with each one a pair of electrons it is called bidentate ligand or it can be tridentate ligand tetradentate or it can be polydentate. So atoms are groups that can form a single covalent bond or one electron donors. For example, if you take F minus SH minus CH3 minus consider there are one electron donors a CL also for that matter okay of course I had already discussed and classified them in my previous lectures any compound with an electron pair are two electron donors for example ammonia, water, tertiary, phosphine, carbon monoxide etc and groups that can form a single bond at the same time donate an electron pair are called three electron donors. We also come across many an ideal and classical example is acetate here acetate you can see if you take acetic acid and remove one hydrogen it forms anionic and now it forms a a covalent bond here and then this carbon monoxide this oxygen also have a pair of electrons two pairs of electrons are there one of that one can you know rotate in this fashion and it can give. Since here it shows two resonance structures we represent acetate in this fashion having electron distribution or this pair of electrons are delocalized and it becomes a mono anionic and this is a three electron donor in neutral method okay whereas in this one it is a four electron donor because I minus is there and CO so this is essentially four electron donor according to ionic method in neutral method it is three electron donor just dot will represent here and thus it takes a negative charge it is not a four electron donor. So now we also come across two donor atoms in a molecule they are called bidentate ligands bidentate ligands when they bound entirely to one atom one metal atom are termed as chelate and if they are bound into two different metal atoms they are called bridging of course we are also using the terms kappa for chelate and nu we are using for bridging that all the day I told you in the beginning. Bidentate ligands on chelation can form three four five or even six membered rings according to the size of the chelate ring and further classification is according to the nature of donor atoms of the ligand. So that means if you take any species and if you identify the donor atom then we can classify according to the donor atom that means we may classify them as carbon nitrogen oxygen phosphorus sulfur or hydrogen donor atoms. So under oxygen we come across several the simplest one being water but also if you take triphenyl phosphine oxide here phosphorus no longer has a pair of electron because phosphorus in plus five state oxygen has a pair of electrons. So that triphenyl phosphine oxide is also an oxygen donor and of course when we go for hard acids and hard base concept we come across sulphate, acetate, carboxyrate all those things come and under carbon we come across a variety of ligands a few I have shown here cyclopentadienyl as you know like allyl group or carbon monoxide and that and also neutral ligands such as benzene etc. For nitrogen you know nitrate is there nitrosil is there primary secondary and tertiary amines are there and we have plenty of fredel ligands are also there and besides that we have several macro cycles having nitrogen as donor atoms. So here I have listed some important compounds with their abbreviation and formula to make you familiar I must have shown this one in my previous lecture however let me continue telling something about these things as well before I take up the classification of ligands by donor atoms. So hydro we call it CH- of course carbonyl CO, cyanome we call CN- methyl CH3- and cyclopentadienyl Cp- or C5H5- carbonate is there CO3 2- and amine we call ammonia NH3 and pyridine is there and then bipyridine is there of course in case of bipyridine we come across two type of bipyridines one is 2, 2 dash bipyridine and another one is 4, 4 dash bipyridine 4, 4 dash bipyridine can only act as a bridging ligand whereas 2, 2 dash bipyridine can comfortably chelate to a metal center and triphenyl phosphine is there and of course under here we come across several phosphorus compounds shall elaborate more when we go to phosphorus as donor atom and aqua we call water and acetyl acetonato is also there in soma and anionic complex similar to acetate and thiocyanato and chloro and EDTA is a hexadentate ligand and then let us compare the donor and acceptor capability of several ligands with respect to carbon monoxide many of these ligands isoelectronic with carbon monoxide of course we all know that carbon monoxide is a better sigma donor and also a better pi acceptor thiocarbonyl is does not have an independent existence however this can be generated in situ starting from appropriate precursors are starting materials to establish a metal to CS bond and when you make this one this is a better sigma donor and it is a better acceptor than even CO and N2 dinitrogen we come across many dinitrogen complexes I shall discuss more about that one under nitrogen and of course we all know that this is very inert molecule as a result it is a versus sigma donor and it is a versus pi acceptor and it is very challenging to coordinate N2 to metal complexes and cyanide very good sigma donor but is it is a poor pi acceptor I shall tell you why it is a poor pi acceptor it is already negatively charged as a result what happens it is a poor pi acceptor CNR it is a good sigma donor and also good pi acceptor and nitrosil good sigma donor and good pi acceptor and then as I mentioned carbon monoxide is good sigma donor and a very strong pi acceptor then how to distinguish to what extent a carbon monoxide has taken electrons from metal to its anti bonding orbital so that can be readily monitored using IR spectroscopy by looking into the CO stretching frequencies if you have terminal carbonyl groups in that case what happens depending upon how many carbon monoxides are there and what is the geometry the stretching frequency can range from 1850 to 2100 centimeter minus 1 and when they are bridging what happens it will be having double bond character as a result stretching frequency drops significantly to be in the range of 1700 to 1850 and when it is bridging three metal centers it further drops and one can observe stretching frequency between 1600 to 1700 and of course one should know that free CO has the stretching frequency of 21 43 centimeter minus 1 and as I mentioned carbon thio monoxide is a better sigma donor and also a better pi acceptor and free carbon thio monoxide is very unstable but at low temperature if you want to monitor it shows a stretching frequency of 273 and terminal CS will be having stretching frequency between 1160 to 1410 and when it is bridging the same stretching frequency will drops to 1100 to 1160 and then when it is bridging three metal centers it further drops and appears in the range of 1040 to 1080 centimeter minus 1 and NMO it is a worse sigma donor and it is a better pi acceptor and of course again it can show two different types of binding one is linear one is bent when it is linear it is a three electron donor when it is a bent it is one electron donor so here when it is linear and whether it is linear or bent one can also analyze by simply doing electron count on the other hand experimental one can also know by simply taking IR spectrum of that complex for linear NMO stretching frequency for NMO appears around 1600 to 1850 for bent it comes around 1500 to 1700 that means characterizing these compounds and identifying the nature of coordination should not be a problem because IR comes very handy in these cases N2 as I mentioned it is a worse sigma donor and worse pi acceptor and it is very challenging to make these complexes nevertheless I will show you preparation of these compounds and what kind of metals one should look for to make dinitrogen complexes and free N2 has Raman active band at 2331 and of course when it is binding terminally with one nitrogen donating its lone pair to metal the stretching frequency will be 700 to 2200 and Cn minus is a good to moderates and of course here also one can look at stretching frequency that should be a problem and CnR is a stronger sigma donor isocyanate but weak pi acceptor and in this case the stretching frequency comes around 200 to 2200 here HOMO highest occupied molecular orbital is much more anti-bending than that of CO one should remember when we look into the activity and tertiary phosphine is a good sigma donor and stronger pi acceptor and of course it is very interesting to compare and the donor and acceptor ability of carbon monoxide and phosphine let me do it in our appropriate juncture as we progress with this one it is very interesting and we can always say why a tertiary phosphine is a better ligand compared to carbon monoxide we can discuss those things in length let me do it at later stage so now let me show you a few examples of all type of ligands we come across before I take up the classification of ligands by donor atom starting with hydrogen ligands this ACAC you can see here because of tautomerism what happens it becomes a double bond is shifted here and it becomes O minus and then this acts as again it can O minus can be here or here so again the negative charge is delocalized and it acts as a mono anionic ligand and also it is a 4 electron donor in this fashion similarly oxalate di anionic and then one can also go for tithyl dicharbomethion here and salicilaldehyde anion again it is very similar to acetate and 8 hydroxyquinol and O minus anionic and neutral it can be and then dimethylglohexane also can be anionic and phenylene bis dimethyl arsine this is 1 2 substituted one is a very nice bident ethyl ligand and also we have several bident ethyl ligands phosphines this is DPPM and diphenyl phosphino methane it is 1 2 bis diphenyl phosphinoethane and this is diphenyl phosphinoamine and of course here R can be H or any other alkyl aryl group and then this is also another interesting oxygen bridge ligand and of course as I mentioned we have plenty of nitrogen based cyclic and acyclic ligands so there are numerous examples of cyclic ligands of nitrogen the simplest one being pyridine and pyridine pyrazine triazole pyrrole of course it can be anionic and pyrazole ok imadazole is there and then 2 2 dash bipyridine a chelating ligand 4 4 dash pyridine ideal for bridging and also we have 1 10 phenanthraline this is another interesting ligand terpyridine and this is naphthyridine and this is purine adenine and also this is hexopyridine and also it is a torrent so we have plenty of nitrogen donor ligands are there apart from this we also have classical macrocyclic ligands which are widely seen among biologically active molecules for example this one is also a ethylene diamine derived bisphosphine and this is the famous EDTA ethylene diamine tetra acidic acid and then it can form readily anionic as a result it can be tetra anionic and then this will be hexadentate ligand and then we have this acac we have and then as I mentioned several nitrogen macrocycles are there this is one such compound this is thalocyanin and this one can be prepared only in situ and this also called in encapsulation one can make something like this here and then porphyrin if you want to know the difference between several macrocyclic ligands if you simply look into the porphyrin it has 2N and 2NH are there opposite side that means it can be di anionic and it can stabilize metal in plus 2 state and carol will be having one nitrogen and 3NH will be there tri anionic and homo porphyrin if you take one CH2G2 bridge will be there instead of one CH2 here that is called homo porphyrin and porphyrin seen opposite ends direct pyrol links will be there I have not given here just I have given the information about those one can look into these structures of some of these nitrogen macrocycles in standard textbooks. So in this porphyrin opposite ends have direct pyrol links and two CH2G2 links on other two ends will be there and also we have carphyrin seen in that one one direct and three CH2G2 links will be there and in case of hemiporphyrin we have two adjacent CH2 and one direct CH2G2 links will be there the links I am talking about is this thing that separates pyrol groups and this I have done it again I shall just tell you about pure sigma donors this is sigma donors are these ligands pure sigma donors they do not have donor acceptor capabilities pi acceptor or pi donor capabilities and then we have sigma donors and pi donors all these halogens and NCO, NCS, N3 minus, OH minus, OR, ACAC etcetera they are all sigma donors as well as pi donors and we have sigma donors and pi acceptors very important ligand system they are also called as non-classical unfortunately they are called carbon monoxide, Sino, NO, N2, RNC, PR3 and one can also write a spectrochemical series for these things by looking into their donor and acceptor capabilities and similarly among pi donors and pi acceptors we have several carbon donor ligands such as Cp minus C6H6, C7H7 plus and ethylene and acetylene they are called pi donors and pi acceptors because this pi electrons are going to the metal as sigma that is the reason they are called pi donors one of the bond in a double bond will be donated to the metal in the form of two electrons that is the reason they are called pi donors and pi acceptors but however this pi bond whatever is that is established is sigma in nature and let me give a extended spectrochemical series also before I begin the discussion it is very difficult to remember the relative positions nevertheless by looking into the nature of the ligand we should be able to tell whether a given ligand which is a strong field ligand or a weak field ligand relatively we should be able to tell them to the extent we should be familiar with the nature of the ligands. So now let us start the classification of ligands by donor atoms to begin with let us consider hydrogen the simplest donor atom typical ligands of hydrogen donor are hydrides, dihydrogen and also several main group hydrides okay for example BH4, AlH3 etc. First hydride complex was iron tetracharbonyl hydride and then this was reported by Walter Heiber and Kovarkas in 1931. He also reported this cobalt hydride having 4 carbon monoxide. So German documentation has indicated that this complex was used in hydro formulation much earlier in mid 50s hydrides such as rhenium hydride here by Wilkinson and also this was made by Fischer and also this one platinum hydride was made by Chart. So all these hydrides of rhenium, marlinum and platinum were made in 1950s by these respective chemists. First dihydrogen complex is MOH2 CO3 times and tricyclohexyl phosphine 2 times was reported by Kubas in 1984. So let us look into the coordination modes of hydrogen there can be terminal hydrogen also will be there so something like this. So this is terminal apart from this terminal and standard we can also see hydrogen bridging two metal centers and of course you can recall the structure of borane where we form three centered two electron bond in the same way here also it establishes a three centered two electron bond this one bind with suitable orbitals of both the metal centers and of course you can also have two bridging hydrides in this fashion without metal metal bond or with metal metal bond and three hydride linking or bridging two metal centers are also known and also we can have mixed bridging ligands such as halides and also hydrogen and also one can also have something like this hydrogen bridging three metal centers. These are the some common coordination modes or binding modes of hydrogen among metal hydride complexes. The affinity for hydrogen is very small or 0 for D block elements with an exception of palladium and also palladium silver alloy and copper complex. So the rate of diffusion of hydrogen is very high through a metal membrane of this metal compared to any other metals in the among transform element series studies have shown the presence of palladium hydride phases. That is the reason palladium on carbon is widely used in hydrogenation reactions. So now let us look into the binding modes of hydrogen and of course when you look into H2 molecule we have a pair of electron that establishes HH bond that pair of electron can be readily donated to M through sigma donation very similar to olefins and this is pi donation. On the other hand this sigma star is empty this sigma star of H2 can be comparable to the sigma star of PR3. So that means in that case the sigma star can also interact with suitable metal by symmetry orbital such as DXY DX or DYZ to establish back bonding that is from metal to hydrogen anti bonding that means hydrogen can act as a sigma donor as well as a pi acceptor. More details will be provided while considering oxidative addition of H2 to S2 and other complex when I go to oxidative addition reductive elimination reaction I shall more elaborate about you know how one can manipulate hydrogen coordination to metal center for the purpose of hydrogenation of olefins and other unsaturated organic compounds that you cannot do conventionally even at very high temperature. So let me stop at this juncture and continue discussing more about hydrogen as donor atom with the examples and method of preparation and properties and bonding etc in my next lecture until then have an excellent time reading chemistry.