Limitations of Crystal Field Theory

Transcript

0:00Introduction

Hello everyone! Imagine a report card. The student is crystal field theory, the theory we have used all through this chapter. Why are complexes coloured? It has an answer. Why are some of them magnetic? It has an answer. High spin or low spin? It answers that too. Now ask it one more question. Carbon monoxide carries no charge at all, and yet it is the strongest ligand in the whole spectrochemical series. So why is that? Here, the theory has no answer. So today we find out exactly where crystal field theory fails, and why. Questions on this do come up in JEE, so let's begin.

0:40How crystal field theory sees a complex

So, first, a quick recall from the start of this chapter. Crystal field theory, written CFT on screen, treats every ligand as a point negative charge, or as the negative end of a dipole, as the brackets say, and it takes the bond between the metal and the ligand as purely ionic: only charges, nothing shared. Okay? Now let me ask you this. If a ligand is only a charge, then what should decide how strongly it splits the d orbitals? Its charge, yes, exactly! In this picture, an anion, a negative ion, should split the d orbitals more than a neutral molecule, which is only the negative end of a dipole. Keep that prediction in mind. Clear?

1:24What CFT explains well

Now, before the failures, let us give credit where it is due. On the right is the splitting picture from the first lesson: the five d orbitals of the free ion split into an upper set, e g, and a lower set, t two g. First, CFT explains colour. Light lifts an electron from t two g up to e g, a d d transition, with a dash between the two d's. Okay? Second, magnetic properties. Once we know how the electrons fill the split orbitals, we can count the unpaired electrons, and that gives the magnetism. With me? Third, the choice between high spin and low spin. Delta naught, the Greek capital delta with a small o below, is the octahedral gap, the arrow labelled delta naught in the octahedral picture, and P is the pairing energy. Comparing these two decides the spin. Right? And fourth, CFT gives real energies: delta naught, and the crystal field stabilisation energy, written CFSE on screen. It gives us numbers, not just words. Got it?

2:26Limitation 1: the order of the series

So, limitation number one. Use our prediction on two real ligands: fluoride, F minus, the small minus up top being its charge, and water, H two O, the small two below counting the hydrogens. By charge, fluoride should split the d orbitals more. Okay? Now recall the real spectrochemical series from earlier in this chapter. Which ligand actually comes out stronger, fluoride or water? It is water, yes, exactly! The less than sign says F minus is weaker than H two O, water. So the neutral molecule beats the charged ion, the exact opposite of the prediction. Right? And it gets worse. Carbon monoxide, C O, has no charge at all, and it is the strongest ligand of all. That was our report card question. With me? So a theory built only on point charges cannot explain this order. That is limitation one: CFT cannot explain the order of the spectrochemical series. Clear?

3:29Limitation 2: covalency and π bonding

Now, why does it fail? Here is limitation number two, as the heading says: covalency, and pi bonding, where pi is the Greek letter on screen. Real bonds between a metal and a ligand are not purely ionic. They have some covalent character, some sharing of electrons, and CFT ignores it completely. Okay? It also ignores pi bonding, the second part of our heading. Right? And pi bonding is no small detail. It is needed for metal carbonyls, the complexes of carbon monoxide, which you will meet in chapter six. Got it? So now, can you answer our report card question? Why is neutral carbon monoxide the strongest ligand of all? Because of pi back bonding, very good! We met this when we built the spectrochemical series. That is a kind of pi bonding, and CFT leaves it out completely. That is why the theory had no answer for our question. With me?

4:25The fix, and a trap

So who fixes these problems? Two theories that include covalency, the sharing of electrons in the bond: ligand field theory, and molecular orbital theory. They overcome these problems. Okay? Now a quick trap. If CFT fails like this, should we throw it away? Is it simply wrong? No, no, that is the trap, so be careful here! CFT is still useful: it explains colour, magnetism, and the choice between high and low spin very well. It falls short only on the points we just saw. Clear?

5:02Example 1: what CFT explains and what it cannot

Now, Example one, from the course. Which of these does CFT explain: the colour of complexes, magnetic properties, the order of the spectrochemical series, and high and low spin states? Pause the video and try it first. Done? Okay, let's check. The colour of complexes: yes, CFT explains it through d d transitions. Okay? Magnetic properties, and high and low spin states: yes again, both of them, just as we saw a minute ago. Clear? Now the order of the spectrochemical series. We use this series with CFT all the time, to judge whether a ligand is strong or weak. So CFT explains the order too, right? No, and this is exactly the trap! Using the series is not the same as explaining it. The order needs covalent bonding, which CFT ignores; remember water beating fluoride. With me? So the answer is all of them, except the order of the spectrochemical series. Three out of four on the report card. Got it?

6:09Example 2: CFT vs VBT, one advantage and a shared flaw

Now a slightly harder one, Example two. Give one advantage of CFT over VBT, valence bond theory, and one shortcoming shared by both. Hint: recall the limits of VBT from chapter four. Pause the video and try it first. Done? Okay, let's check. From chapter four: VBT does not explain colour, and it gives no energies, no splitting and no stabilisation energy. Okay? And CFT does both. It explains colour, and it gives energies, delta naught and CFSE, in numbers. So that is one advantage. Right? Now the shortcoming shared by both. Think back to limitation two. What do both theories fail to treat properly? Covalent pi bonding, very good! Neither theory treats it properly, and that is the shared shortcoming. Clear? So the full answer has two parts. The advantage is that CFT explains colour, and the shared shortcoming is that neither handles covalency or pi bonding well. Got it?

7:14Quick revision: CFT strengths, limitations and the fix

So, quick revision. CFT explains colour, magnetism, and high and low spin, and it gives real energies. But with ligands as point charges, it cannot explain the order of the spectrochemical series. It ignores covalency and pi bonding, and ligand field theory and molecular orbital theory, M O theory on screen, fix this. Now go on to the past JEE questions on crystal field theory in the next sections of this chapter. All the best!