 Hello everyone, once again I welcome you all to MSP lecture series on Transmittal Chemistry. We are almost completing the classification of ligands, now we are in the last series of ligand system that is called halogens group 17 elements. Of course if you want to know more about the properties of halogens always you can revisit my course on main group chemistry and look into the details about halogens and also when I was discussing about oxygen donor atoms, I did not give more emphasis for sulphur, selenium and tellurium and they have some chemistry very similar to oxygen. But however again visit Chalcogen series or group 16 elements in my previous course on main group elements and then try to familiarize yourself about the donor and acceptor properties of Chalcogens with this let me talk about the halogens. In my previous lecture I just initiated discussion on halogens, I did mention about the nature of donor properties it can establish first as an anion a covalent bond and then due to the presence of these pair of electrons it can establish another sigma bond and two more pi bonds and for example I have shown here this let us say this pair goes to establish a covalent bond and now among these three pairs one can form a sigma bond and other two can form pi bonds. Due to the pi acidity and having low energy field sigma orbitals and low energy field pi orbitals it can show up to four coordination number that means it can act as a eight electron donor or four pairs of electrons we have examples in our own research I shall show you all those things as you progress with this lecture and of course one can generate a series of homolyptic halogen series and when you consider coordination number six and having six halogen atoms are halides they adopt preferably octahedral geometry and when we have four early metals prefer tetrahedral geometry whereas late metals like palladium platinum they prefer square pillar geometries and I have shown some homolyptic halide complexes here and if you just look into titanium tetrachloride colorless once when we say colorless you should be able to correlate that one with the spectroscopic properties here of course here DD transition is not possible because D0 as a result it is colorless of course four are there it is tetrahedral and then when you go to Ti2Cl10 di anionic it is also colorless and it has three electrons and this is octahedral. So then if you go for hexachlorotitanate this is yellow in color again it is T2G0 but here yellow color comes because of some charge transfer transition occurs this is where the back bonding can be seen here and in this case it is octahedral hexachlorochromate and we have this one with manganese pale pink pale pink because here it splits into E and T2 because it is tetrahedral G should not be there and E and T2. So here what happens we have three electrons in T2 system that is higher in energy and E is lower in energy so all are completely if it is spin forbidden as a result pale pink of course you will understand more when I go to spectroscopic properties of metal complexes and FECL4 2 minus again tetrahedral colorless and tetrachlorochromate you can see blue in color it is tetrahedral nickel tetrachlorochompound is blue in color we have tetrahedral geometry CuCl4 is green again tetrahedral palladium tetrachloropaladate or tetrachloropalatinate they are respectively brown and pink in color of course they have D8 electronic configuration and they adopt square feather geometries and also you know after going through crystal field theory and ligand field theory why these molecules adopt square feather geometries. So then what are the methods we have to prepare halogen complexes or halide complexes so you can directly interact metals with halogens at higher temperature to generate homolyptic metal halides and metal halides also can be prepared by the neutralization of a metal oxide or a metal hydroxide or carbonate with appropriate halogen acid this is true only in case of alkali metals and alkaline earth metals for example sodium chloride and of course this reaction is not feasible when we talk about transfer metals and metal halides may also be prepared by reacting metal oxides with the halogen at high temperature using carbon. And then when we look into the molecular orbital diagrams for explaining metal to halogen bond formation whether the terminal bond or bridging bond you can see here each metal atom presents an empty sigma orbital directed more or less towards the bridging halide ligand X minus generally denoted which are you know wave functions are pi 1 and pi 2 and halides have four field valential orbitals. So the metal orbitals combine in symmetric and anti-symmetric fashion to form both bonding and anti-bonding orbitals and these four electrons then occupy two bonding metal orbitals to form one covalent bond and one coordinate bond that result in a halogen bridging two metal centers and of course the angle can vary it can be anywhere between 60 to 180 degree depending upon type of halogen under consideration and also to an extent what kind of metal we are considering and here charge to size ratio or charge everything matters let me elaborate after couple of slides. So here one can write a typical amount diagram to show the bridging nature of halogen where it bridges two metals with one covalent bond and with one coordinate bond both are sigma in nature. Let us look into the binding modes of halides. The simplest one is simple terminal eta 1 is more appropriate or k 1 and now mu 2 means it is bridging. So of course when it is bridging this one this symbolizes that the ligand is bridging but it does not say in what way it is bridging whether linear bridging is there or bent bridging is there or what is the angle that information does not come that information can only come from x-ray structure analysis and of course when halogen bridges three metal centers we denote with mu 3x something like that we can write and then of course if you take a typical x minus we have octet is satisfied you can see here also one is gone and then here two are still there okay one may be something like this and now this also something like this okay usually we represent as coordinate bond there is no need to write in terms of arrow there is a old fashion nevertheless to just stress upon the nature of bonds I have shown here beside this one what happens a halide can also utilize all pair of electrons to bridge four metals very interesting we have examples in our own laboratory I will show you now the arrangement of this are like square plane it is something like this okay we have metals here and a halogen sits here to establish bond utilizing all four various electron pairs you can see some examples I have shown here metal halides for example oxy halide is there VCl4O here you can see here and then we can see the bridging one here and many metals including even main group metals adopt this kind of geometry to have bent structure something like this but when you go for fluorides most of them have linear binding and hence they are tetrameric in nature whereas in case of chlorides and to an extent with bromides and iodides they are dimeric in nature whereas with fluorines exclusively they prefer tetrameric geometry the reason is very simple when you look into the size of F minus that is much smaller in size compared to chlorides bromides and iodides so in this case for example if you bring two positively charged metals something like this and if you have a bridging mode with fluorine what would happen is something like this in order to establish a bond since it is very small they have to be brought very closer when you bring them very closer it is almost like bringing two positively charged species ions very close to each other and they will repel as a result what happened this is destabilized the best way to bind in case of fluorides is to have a linear geometry so something like this so that this are kept at further distance to minimize the interaction between these two with this reason what happens always whether you take even main group elements or even alkaline metal alkaline earth metal or any transfer metals usually most of the occasions we come across linear binding of fluorides when they are bridging to metal centers and then in that case what happens you cannot have with great difficulty you can trimeric structure with there will be some ring strain will be there as a result they adopt tetrameric structure something like this shown here and this is what happens in case of CuF2 and CrF2 also and also CrF2 so when we talk about CuF2 or CrF2 we should not think that they have only two ligands and they are linear they have octahedral arrangement that means in the lattice they will be having something like this but at the end if you look into the composition each copper has or each chromium has two fluorine atoms whereas when you look into the lattice the arrangement is like this all of them have octahedral geometry and of course here in this case it is a D9 system you can see tetragonal elongation will be there and as I mentioned here this is the most preferred one and the angle can vary from 160 or 150 to 180 depending upon other metals and their size and also their charge this is the structure of palladium chloride and platinum chloride with chlorine as X and in case of rhodium trichloride this is the structure adopted by rhodium you can see rhodium is octahedral is surrounded by 6 chlorine atoms of course composition does not change that is the reason whenever you write always we write RhCl3 in same thing is to in case of palladium and platinum also the ratio metal to halogen remains 1 is to 2 so that plus 2 can be explained oxygen state. So this is tetrahedral tetrachlorocobaltate and in this case we have rhodium is in 1 and we have here cycloctadiene is there and with olefins whether you take norboradiene and cycloactadiene or 2 ethyliens this is a preferred geometry here and of course one can break this one symmetrically and vacate this coordination site to add another stronger ligand since chloride or in general halides are weak ligands you can always hand spate the cleavage of this bond when we use strong field ligands to coordinate here to form 2 independent molecules and of course with 6 we have octahedral geometry here with copper with different halides we come across different type of arrangement in the unit cell you can see here copper 1 fluoride CuF we have this kind of arrangement is there when we have chloride this is slightly different we are more or less similar where in case of copper 1 we have this kind of arrangement and for example alpha copper has this one beta copper has this one in case of iodides and we also have gamma copper has in this kind of and of course one can also see at what temperature one can convert alpha into beta or beta back into alpha or something like this and the average copper to iodide bond distance is about 2.338 and strong units. Another interesting thing is depending upon what kind of ligands we have with copper especially copper cuprous halides have remarkable ability to undergo association to give a series of structures we will not come across we cannot see with other metal halides that is the interesting aspect of cuprous halides here. So for example you can have simple CuX or you can have 2Cl in a linear fashion or you can also have 3 in this fashion or you can have 4 and also these units can combine to form a dimeric species this is called rhombic unit where we have 2 bridging halogens are there or of course when you have bridging halogens now other ligands can come here something like this or you can have unsymmetrical coordination one side 3 one side 2 coordination or one side 4 one side 3 coordination and these units again combine to form a cuban type very rare and also you can also see once cuban is formed one of this phase can open up in this fashion one can also have open cuban that I will show you here in this cartoon how these things arrange depending upon the ligand structure you should remember all these arrangements I am going to show are dictated by the ligand structure especially with the bidentite ligands and in particular I am talking about this phosphins here for example you take this one of course if you take a Cux moiety 2 Cux moieties come in this kind of arrangement to establish bridging this is how usually happens for example if there is a lack of another ligand and then vacant site is there and this is what exactly happens this is how we come across many dimeric species for example you take AlCl3 same thing happens and it is supposed to have 4 coordination but we have 3 as a result what happens it undergoes dimerization. Now you take this one and another one can come something like this now two such units can establish a new geometry called Cubane where alternate corners are occupied by copper and halogens this is to in case of chloride bromide and iodide as well on the other hand if you just slip one of the rhombic unit and then you get some sort of stair step or if you do something like this in a different fashion you can get a ladder type or if you arrange little bit pull them apart little bit to have some distortion at copper and then bring another one to visualize centrosymmetry. Now you can see 4 copper atoms are in the plane now we have 2 type of binding here this halogen is bridging all the 4-meters that means all 4 pairs of electrons are utilized here and here it is bridging here simply 2 bridging 2 bridging this is a rare example where it shows 2 bridging as well as 4 bridging you can see here 3 bridging is there. So these assemblies are you should remember are dictated by ligand framework depending upon what type of ligands you have how much separation is there how much bulkiness is there that would decide how this individual CUX series have to assemble in one of these arrangements. You can see here these are all examples from my own laboratory with different ligands now you can see here the dark purple ones are halogens here and these are reddish ones are copper copper one system you can see here in the space filling model how this is bridging and whereas here 3 such bridges are there 2 copper units and here also here we have 4 copper atoms are there one is other side and 3 bridging tri bridging halogens are there and this is an example where you can see 1 halogen bridging all the 4 metals that means it is establishing bonds 1 covalent bond and 3 covalent bond with all the metals and these all 4 metals are copper 1 ions are in the plane this is again very interesting and they show very interesting photo physical properties photo luminescence and then you can see this one space filling model I have just expanded how this is bridging 2 metals and this is bridging 4 metals. So this shows how much versatility is there in case of halogens as ligands in our own laboratory so when we have something like this you can see especially with copper 1 system the type of coordination we come across and how some of these moieties are arranging I showed you here you can see these 4 copper atoms are in the plane and this one is bridging above and this one is bridging below and these are in the sides acting as mu 2 whereas this one are mu 4 this is some crystal structure to make you familiar with structures of molecules here is a copper 1 system here and of course we have some water here coordinated you can see the entire 2 ligands bidentate ligands bisphosphines are there binding in this fashion as bridged bidentate ligands and then we have one 4 bridging one below 4 bridging and in the sides one in front one backside acting as mu 2 halides this is 2 in case of both bromides and iodides and also chlorides and also very interesting compounds were made having like soda light topology okay you can see here and this one I have shown is like Cu 4 X 4 mighty where 4 copper atoms are there in the alternate corners and then we have 4 halogens are there and other corners and then this is a bidentate ligand you can see you can count how many so each one is a 4 copper atoms are there and 4 halides are there and you can see how many are there in monomeric unit it has this kind of space inside okay and this is how it looks like that means very interesting compounds can be made just to make you familiar with the versatile chemistry one can think of with cuprous halides with bidentate ligands of different type I am showing these things here you can see here how they interact so Cubane is formed and they act as you know monodentate ligand each towards one and that means they are essentially monodentate with respect to one unit but they are bridging two such units in this one and this propagates to form something like this we have one is Cubane is there and also one open Cubane is there here this is called diamondoid structure in the diamondoid structure you can see we have two types here this is an extended diamond diet and this is another one all are diamond diet here you can see here and here is extended because we have Cubane is there and also open Cubane is also there here. So all these compounds also behave very similar to zeolite molecules in terms of gas absorption reversible absorption properties just I am showing you all these things to show versatility this is how in the three dimensional view looks like this is a three dimensional coordination polymer we have some of these cavities you recall your visualization of zeolites they look very similar we have cavities and all those things very interesting chemistry and of course I edited a book on copper one complexes of different phosphines in 2019 if you are interested you can just look into the book with this I complete discussion on classification of ligands. So we learned starting from hydrogen and then we moved on to carbon donor atoms under carbon donor atoms we came across many interesting ligand systems and also we looked into the reactivity that can be performed on coordinated ligands and eventually that leads to some organic products coming out all those things we saw in case of nitrogen also how a coordinated nitrogen can be activated through different type of reactions that means you can perform nucleophilic substitution reaction or electrophilic substitution reaction or in some cases you can also perform coupling reactions all these things when we understand thoroughly and if you see we can regenerate these things and try to make it reversible and then their utility comes in organic transformations and of course with phosphorus they are very versatile and also I showed you how they stand out among all sigma donor and pi acceptor ligands and especially how a bidentate ligand is preferred in catalytic reaction compared to monodentate ligands and how a bulky ligand can dictate reaction and how it can stabilize coordinatively unsaturated and electronically rich metals and then it can allow dissociation of ligands for preparing that complex for oxidative addition. All those interesting aspects we discussed and wherever appropriate I also brought molecular diagrams to show the binding properties and binding modes. In my next lecture I shall begin the discussion on two important reactions when it comes to the utility of organometallic compounds and coordination compounds in homogeneous catalysis that is oxidative addition reaction and reductive elimination reactions. So let us discuss those things in my next lecture until then have good time reading chemistry.