Structural Isomerism

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Take chromium chloride with six water molecules. Its formula is C r C l three, dot, six H two O, okay? Now, this one formula comes in three different forms. One form is violet, one is blue-green, and one is green. Same atoms, same formula, but three different colours? How can that be? And how would you tell the three apart with a simple test? Today we study structural isomerism in coordination compounds. Questions on it come up often in JEE, so let's begin.

0:37Ionisation isomers: an ion swaps places

Let us start with ionisation isomers. Look at the first compound: open square bracket, C o, cobalt, then N H three in round brackets, five times, then S O four, close bracket, and B r, bromine, outside. The square brackets hold the coordination sphere, the metal with its ligands. The small numbers written below count the atoms or groups. The second compound has the bromine inside the bracket, and the sulfate, S O four, outside. Right? Now count the atoms in both. One cobalt, five ammonias, one sulfate and one bromine, in each. Same formula, so they are isomers. But then, what exactly has changed between the two? The bromide and the sulfate have changed places, yes, exactly! An ion inside the coordination sphere swaps with the counter ion, the ion outside the bracket. This is called ionisation isomerism. Clear? In water, only the ion outside the bracket comes free. So the first compound gives bromide ions, and the second gives sulfate ions: different ions in solution. Got it? Now a quick trap. Look at C o, N H three, five times, C l, in square brackets, and then C l two outside. There is a chloride inside and there are chlorides outside. So swap them, and we get an ionisation isomer, right? No, no, wait, that is exactly the trap! Swap a chloride for a chloride, and you get the same compound back. So this one cannot show ionisation isomerism. You need two different ions to swap. With me?

2:20Hydrate isomers: water inside or outside

Now, back to our puzzle from the start. Here is the violet form: C r, then H two O in round brackets, six times, all in square brackets, and then C l three. All six waters sit inside the sphere around chromium, and all three chlorides sit outside. Okay? Now suppose one water moves out of the sphere. Chromium still holds six groups around it. So what takes the empty place? Chloride takes its place, yes, very good! So we get C r, H two O, five times, C l, in square brackets, then C l two, dot, H two O, written after the bracket. The dot shows that the water which left is still in the formula, just outside the sphere. And this form is blue-green. Right? One more step: two waters out, and two chlorides in. C r, H two O, four times, C l two, in square brackets, then C l, dot, two H two O, after the bracket. And this form is green. Clear? So all three are C r C l three, dot, six H two O, the same formula. The only difference is whether water sits inside or outside the sphere. These are hydrate isomers. Hydrate isomerism is solvate isomerism where the solvent is water, which is why solvate is written in round brackets. So our three colours are three different compounds. Example two shows how to tell them apart. Got it?

3:50Linkage isomers: same ligand, another donor atom

Now let us take the next type. Do you remember ambidentate ligands? An ambidentate ligand can bond to the metal through different atoms, okay? Take the nitrite ion. It has one nitrogen atom and two oxygen atoms. So tell me, which atom bonds to cobalt? It can be either one, and that is the whole idea! Here it bonds through the nitrogen: C o, N H three, five times, then N O two in round brackets, all in square brackets. The two plus written up high is the charge on the ion. This one is yellow. Right? And here the same nitrite bonds through an oxygen, so we write it as ONO, with the bonding oxygen first. Same atoms again, but this one is red. With me? So, same formula, but a different donor atom. These are linkage isomers, and they need an ambidentate ligand. Clear?

4:48Coordination isomers: two complex ions swap

And now the last type of structural isomerism. Some salts are made of two complexes. The cation is a complex, and the anion is a complex too. Okay? Here is one: C o, N H three, six times, in square brackets, then C r, C N, six times, in square brackets. Cobalt holds six ammonias, and chromium holds six cyanides. Right? Now swap the ligands between the two ions. Give the ammonias to chromium, and the cyanides to cobalt. What compound do you get? Yes, exactly, you get C r, N H three, six times, in square brackets, then C o, C N, six times! Same atoms, same formula, but a different compound. Got it? So, ligands exchanged between the cation and the anion: these are coordination isomers. In this example, the two metals have swapped all their ligands. Clear?

5:46Example 1: BaCl2 or AgNO3 test for two ionisation isomers

Alright, example one, an easy one. How would you distinguish C o, N H three, five times, B r, in square brackets, then S O four, from C o, N H three, five times, S O four, in square brackets, then B r outside the bracket? Pause the video and try it first. Done? Okay, let's check. Both compounds contain bromine, and silver nitrate, written A g N O three, gives a precipitate with free bromide. So both will give a precipitate with silver nitrate, right? No, and that is exactly where students slip. Only the ion outside the bracket is free in solution. The ion inside stays bonded to cobalt. Okay? In the first compound, the sulfate is outside, so it is free, but the bromide is not. Add barium chloride, B a C l two, and the free sulfate gives a white precipitate. With me? In the second compound, the bromide is outside, so it is free. Add silver nitrate, and the free bromide gives a pale yellow precipitate. So barium chloride picks out the first, and silver nitrate picks out the second. Here is the trick for every such question: test for the ion outside the bracket. Clear?

7:01Example 2: moles of AgCl from the three CrCl3.6H2O isomers

Now, remember our puzzle from the start? Let's finish it. The three hydrate isomers of C r C l three, dot, six H two O are treated with excess silver nitrate. How many moles of silver chloride, written A g C l, does one mole of each give? Pause the video and try it first. Done? Okay, let's check. Every formula has three chlorines. So each one gives three moles of silver chloride, right? Oh no, careful, that is the trap! Only the chloride outside the sphere is free to give silver chloride. The chloride inside stays bonded to chromium. So we just count the chlorides outside the bracket. Okay? The violet form has all three chlorides outside. So it gives three moles of silver chloride. Right? The blue-green form has one chloride inside and two outside. So it gives two moles. With me? And the green form has two chlorides inside and only one outside, so it gives just one mole. So the answer is three, two and one. The three colours really are three different compounds, and one silver nitrate test tells them apart. That is our puzzle solved. Got it?

8:19Quick revision: ionisation, hydrate, linkage, coordination

So, quick revision. Ionisation isomers: an ion inside swaps with the counter ion. Hydrate isomers: water inside or outside the sphere. Linkage isomers: an ambidentate ligand uses another donor atom. And coordination isomers: ligands swap between the cation and the anion. Now go and try the practice problem on this page. All the best!