 Next we are going to discuss heteronuclear diatomics with particular reference to HF and we will also talk about carbon monoxide. Now in HF what happens is this neglect the molecular orbitals for the moment. On the left hand side we have shown hydrogen atom 1s orbital, on the right hand side we have shown fluorine atom 2s and 2p orbitals and that should raise questions. What is going on here? Why is it that energy of 2p orbitals for fluorine is so much less than the energy of hydrogen atom 1s orbital? Well that is because you have so many more protons in the nucleus that electrons are sort of drawn in and the energy gets lower and lower as you go as you make as you go higher up the atomic number ladder. So, for fluorine you do not even consider the inner orbitals 1s orbital they are non-bonding that means they do not participate in linear combination you can think like that. And if you think of F2s and F2p and compare with H1s first thing that we want to say is that we are considering the xyz we are considering the internuclear axis to be the z axis. So, when we want to discuss sigma interaction it is the 2pz orbital of fluorine that is going to participate. 2px and 2py orbitals will remain on fluorine because on hydrogen atom there is no other orbital of sufficiently low energy that have comparable symmetries with 2px and 2py of fluorine. So, you can think of hydrogen atom like this the only orbital available is 1s others are very high energy. For fluorine we have let us say 2px 2py 2pz so 2pz is like this this is the sigma interaction that has taken place if at all there will be 2s as well we will see that. But now 2px is here 2py is there there is nothing like that at sufficiently low energy for hydrogen atom. So, this px and py orbitals do not participate in any linear combination for forming of molecular orbitals these are called non-bonding orbitals and they stay on fluorine. So, if you go back to that Lewis electron dot structure you will remember that there are lone pairs on fluorine. So, these are your lone pairs the only additional thing we are saying here is that these lone pairs reside in px and py orbitals which are collectively now called the 1py orbital. But remember 1py orbital here is actually non-bonding and localized on fluorine also. What about the sigma orbitals? Well sigma orbitals you see they do participate in a linear combination because there is a right geometry but energies are so much lower that this bonding orbitals that you get are mostly orbitals that have contribution from 2s and 2p orbitals of fluorine not so much for from hydrogen atom 1s and this 3 sigma the anti-bonding orbital here that resembles the hydrogen atom wave function closely. How do I know all this? I know this from ionization energies which is determined by something called photoelectron spectroscopy. It is established that for Hf you have an ionization energy of about 18.8 electron volt which is very close to the value of ionization energy for fluorine atom 2p. So, energetically this diagram is correct whereas for 1s orbital of hydrogen atom I hope you have not forgotten that the ionization energy is 13.6 electron volt. We get another ionization energy for Hf at 13.4 electron volt very close values that is how this energy diagram of the molecular orbitals has been constructed. So, now see this orbital is close in energy to hydrogen atom 1s orbital. So, in the linear combination that you have you have about 98 percent contribution from hydrogen atom 1s orbitals and 2 percent maybe from the orbitals of fluorine. So, it is predominantly hydrogen orbital. Similarly for bonding and non-bonding I had said exclusively F even for bonding there is a small maybe 2 percent contribution from hydrogen atom 1s orbital, but these are mainly made up by atomic orbitals of fluorine 2s and 2pz atomic orbitals of fluorine. So, what we encounter is a lopsided electron distribution. Look at the electron configuration where are the electrons? I have 1 electron from hydrogen 7 electrons from fluorine and once you start filling them in there is no memory you just fill them in in the increasing order of energy for molecular orbitals. So, all the electrons are in either bonding or non-bonding orbitals which are either mainly fluorine orbitals or exclusively fluorine orbitals. What does that mean that in Hf the electron density is very strongly lopsided. It is very strongly localized over fluorine and hydrogen gets a very small share. What is it? It is a polar covalent bond is not it and an extreme case of that would be ionized bond. So, this is something nice that we learn from our discussion of Hf that only orbitals with comparable energies and compatible symmetries was put participate in linear combination to give you the molecular orbitals. So, if any of these conditions is not fulfilled it will not happen. So, in case of Hf you have predominantly fluorine like bonding and orbitals exclusively fluorine like non-bonding orbital and predominantly hydrogen like anti-bonding orbitals. And since there is no electron in the anti-bonding MO the electron density that is there is very strongly lopsided towards fluorine we get a polar covalent bond. You can perform exactly similar discussion for HCl the only difference being that in case of HCl the orbitals that have comparable energy with hydrogen atom minus orbital are actually 3s and 3p orbitals and for HBr where you need to consider this 4p orbitals. This is how you handle this problem. Second part of our discussion of heteronuclear diatomic molecules is on carbon monoxide. Carbon monoxide I think I am not very sure if you are going to study this in the remaining part of the course but perhaps you know already that carbon monoxide is a very good complex ligand for metal ions they can form carbonyl complexes. It has also been established that this carbon atom can act as a sigma donor it forms a dative bond or coordinate bond and it can also accept as a pi acceptor that is called back bonding and that gives rise to synergistic effect this is something we learn in inorganic chemistry. Why does that happen? Why is it that it is carbon atom and not oxygen atom that likes to donate the electron in a sigma fashion and why is it that it can sustain back bonding and it is very important also because you know carbonyl complexes do have lots of application in organometallic chemistry like this okay and carbon monoxide is a poisonous gas why because this is a carbon representation of hemoglobin which transports oxygen and CO2 at the heart of hemoglobin you have this iron okay ferrous iron and what happens is that in regular hemoglobin this is a structure of hemoglobin there is this heme unit where you have porphyrin and the porphyrin ligates the metal ion from 4 sides in a square planar fashion so the axial positions are available one axial position is usually taken up by solvent molecule as is believed the other one is taken up by this amino acid from the protein side chain okay distal histidine amino acid moiety from the protein chain and the way it transports oxygen and carbon dioxide is that oxygen and carbon dioxide bind in the other axial position by driving out the solvent molecule problem with carbon monoxide is that when carbon monoxide comes and binds like this it does not want to leave it forms a very very strong complex carbonyl hemoglobin and that is why hemoglobin is blocked it cannot participate in oxygen transport and the organism died out of lack of oxygen as fiction so CO2 and O2 form reversible complexes with hemoglobin whereas carbon monoxide forms an almost irreversible complex with half life of several hours and by that time the damage is done that is why it is a poison question is why to explain this one uses the concept of hybridization proposed by Linus Pauling hybridization means you mix two orbitals to generate two new orbitals and I will tell you this model and I tell you what the problem is also so the assumption here is that in carbon there is SP mixing and you form H1 and there is another SP mixing which gives you H2 to hybrid orbitals and both are sigma kind of orbitals similarly for oxygen you get lower line H3 hybrid orbital higher line H4 hybrid orbital do you see the problem here the problem is in our earlier discussion of homonuclear diatomics we had said that in case of oxygen there is no SP mixing because the energies are too far apart in this case then why is it that we have an SP mixing there is no good answer you can try to argue that the other atom is carbon and CO is actually isoelectronic with N2 but I think there is a lot of controversy over this we use this model because it helps us understand things but please be aware that it is not a completely accepted model okay now what happens see H3 has such low energy right so even though symmetry is right it cannot form a bond it remains as a non-bonding orbital I keep forgetting about the Px and Py 4 chairs similarly H2 has too high an energy so that remains a non-bonding orbital the only difference is that the H2 orbital is a non-bonding orbital on carbon and the H3 orbital is a non-bonding orbital on oxygen next what happens H1 and H4 are both sigma kind of orbitals the energies are close enough so they can give you sigma and sigma star molecular orbitals what about your Px and Py they are going to give you the pi and pi star molecular orbitals so this is believed to be the electron the energy diagram for carbon monoxide now let us fill in the electrons 4 electrons from carbon remember we are neglecting the N equal to 1 shell all together we are only talking about the valence shell okay so 4 electrons there and 6 electrons for oxygen when we fill them again there is no memory how do we fill them lowest energy MO has 2 electrons the higher one has 2 next we have 2 degenerate pi bonding orbitals so there we are going to accommodate not 1 electron pair but actually 2 electron pairs how many are taken care of 2 4 6 8 2 more are there well we did go it will go to this sigma non-bonding so this is the electron configuration by our proposed model nothing what is the sigma non-bonding remember HF how some orbitals were lopsided on hydrogen and some orbitals were lopsided on F here also the sigma and V is actually lopsided why because sigma and V is a non-bonding orbital on the carbon atom so it is very safe to say that these 2 electrons are actually on carbon and they are highly directed so that is why they make carbon atom the good sigma donor at this point you might want to ask what is the need of invoking hybridization at all you could have done this using pz yes but you know the shape of pz is like this and shape of a hybrid orbital is like this the major lobe and minor lobe is very small the nucleus is in minor lobe so now this is your carbon monoxide this is carbon this is oxygen carbon is known to be a strong sigma donor so it is not very unreasonable to expect that it is using a hybrid orbital which is very highly localized rather than regular pz orbital that is why we invoke hybridization here and that is why carbon monoxide is a good sigma donor through the carbon atom how about this back bonding well back bonding if you remember the figure takes place like this you have a d orbital very bad diagram you have a d orbital like this remember this is z direction so if I have and let us say this is y direction or something if we have py then it can form a pi overlap okay what is this the center is the metal what is this this is carbon okay if the orbital is like this right and it is not even a pure orbital remember it is a pi star orbital so it will be bent towards this way now you can have pi kind of interaction in fact if I show you the orbitals this is what they are supposed to look like this kind of an orbital makes co a good sigma donor through the carbon atom this kind of orbitals remember they are also localized on carbon atom makes carbon atom a good pi acceptor and that is how the synergistic effect takes place so much for a monuclear and heteronuclear diatomic molecules what about polyatomic molecules to explain polyatomic molecules one needs to invoke hybridization we are not going to do it here but let me just put up one flag if I ask you why is methane tetrahedral many of you are going to say it is tetrahedral because it uses sp3 orbitals that is wrong it is VACPR that determines the shape of the molecule palantial electron pair repulsion there has to be minimum repulsion for a stable molecule right and it just turns out that if you have 4 bond pairs the most happy situation is when they form a tetrahedral not a square planar structure that is what determines the shape and to rationalize the shape or keeping the shape in mind to build a quantum mechanical description that is when we invoke hybridization it is not the cause. So, this is one parting remark I wanted to make before closing this discussion.