Werner's Theory and Basic Terms

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Werner took three compounds of cobalt chloride with ammonia, and added excess silver nitrate to one mole of each. With six ammonias, he got three moles of silver chloride. With five ammonias, two moles. And with four ammonias, just one mole. But every one of these compounds has three chlorines! So where did the missing chlorides hide? You will crack it yourself by the end. This is the starting point of the whole coordination chemistry unit, and it comes up often in JEE, so let's begin.

0:36Double salt or complex?

So, first, two compounds that look alike. Here is Mohr's salt, a double salt, written F e, S O four, dot, N H four in round brackets with a two, S O four, dot, six H two O, all as one formula. The small numbers below the letters count atoms, and the dots join the parts. Okay? In water, it dissociates completely and gives the tests of all its ions: iron two plus, ammonium, and sulphate. The raised signs are the charges: two plus on iron, plus on ammonium, two minus on sulphate. Now a complex, potassium ferrocyanide: K four, open square bracket, F e, C N six, close square bracket. It has iron in it too. So tell me, will it give the test for iron two plus? No, it will not, and that surprises many students. It gives the test for the potassium ion, K plus, only. There is no test for iron two plus, and none for cyanide, C N minus. Right? The reason is that the part in square brackets, F e C N six with a charge of four minus, stays intact in water. We call it the coordination entity. So a double salt breaks into all its ions, but a complex keeps its bracket together. Clear?

1:52Werner's theory (1893): two valencies

Now, Werner explained all this in eighteen ninety three. He said that a metal in a complex shows two kinds of valency. The first is the primary valency. It is ionisable, it equals the oxidation state of the metal, and it is satisfied by negative ions. Okay? The second is the secondary valency. This one is non-ionisable, and it equals the coordination number. With me? Now let me ask you this. Suppose a molecule is neutral, with no charge at all. Which valency can it satisfy, primary or secondary? It is the secondary valency, yes! The secondary valency is satisfied by ligands, and ligands can be neutral or negative. The primary valency needs negative ions. Got it? And one more point. The ligands are directed to fixed positions in space. The double arrow on screen means this leads to that: fixed positions in space give the complex a definite geometry.

2:52The basic terms

Okay? Alright, now the basic terms, one by one. The coordination entity is the central atom together with its ligands, written inside square brackets. Right? Ions outside the bracket are called counter ions, okay? The coordination number is the number of ligand donor atoms bonded to the central atom. The Greek letter sigma on screen tells you these are sigma bonds. So we count donor atoms. Clear? The coordination polyhedron is the arrangement of these donor atoms in space: octahedral, tetrahedral, square planar, and the three dots mean and so on. Got it? Now let us use these words on a real compound: open square bracket, C o, N H three in round brackets with a six, close bracket, C l three. The coordination entity is cobalt with six ammonias, with a charge of three plus, and the three chloride ions outside are the counter ions. So, what is the coordination number of cobalt here? It is six, exactly! Each ammonia gives one donor atom, and there are six of them. With me?

4:01Oxidation number, kinds of complexes, counting ions

Next, the oxidation number of the central atom. It is the charge the metal would carry if all the ligands were removed along with their electron pairs. Okay? Let us try potassium ferrocyanide. Let x be the oxidation state of iron. Each cyanide is minus one, so six of them give minus six, and the four potassium ions outside mean the bracket carries four minus. So x minus six equals minus four, which gives x equals plus two. Right? Now, homoleptic means only one kind of ligand, like cobalt with six ammonias, three plus. Got it? Heteroleptic means more than one kind, like C o, N H three in round brackets with a four, C l two, with a charge of plus one. Four ammonias and two chlorides. Clear? And here is the key to our puzzle. Only the chloride outside the bracket ionises, so only that chloride precipitates, as silver chloride, written A g C l on screen. Chloride inside the bracket stays put. Okay? We can also count the ions in solution, from conductance or from colligative properties. The number of ions gives the charge type of the complex. With me?

5:11Example 1: primary and secondary valency of Pt

Alright, example one, an easy one. Give the primary and secondary valency of platinum in this complex: open square bracket, P t, N H three in round brackets with a four, C l two, close bracket, C l two. Pause the video and try it first. Done? Okay, let's check. Two chloride ions sit outside the bracket, so the complex ion carries a charge of two plus. Right? Let x be the oxidation state of platinum. Ammonia is neutral, and each chloride inside is minus one. So x, plus four times zero, plus two times minus one, equals plus two, which gives x equals plus four. The primary valency is the oxidation state, so it is four. Okay? Now the secondary valency. There are four ammonia ligands, so the secondary valency is four, right? Oh no, careful, that is exactly the trap in this question! The two chlorides inside the bracket are ligands too. Four donor atoms from ammonia, plus two from chloride, gives six. So the secondary valency is six, and the answer is four and six. Clear?

6:22Example 2: formulae of Werner's cobalt ammine chlorides

Now, remember our puzzle from the start? Let's finish it. Werner found that one mole each of C o C l three with six, five and four ammonias gives three, two and one mole of silver chloride, with excess silver nitrate, A g N O three. Write their formulas. Pause the video and try it. Here is a hint: cobalt holds six donor atoms. Done? Okay, let's check. Cobalt has a coordination number of six here, and only the chloride outside the bracket gives silver chloride. Okay? With six ammonias, all six places are filled by ammonia. So all three chlorides sit outside: C o, N H three in round brackets with a six, in square brackets, then C l three. Three moles of silver chloride, just as Werner found. Right? Now five ammonias. They fill only five of the six places around cobalt. So what fills the sixth place? One chloride, yes, exactly! That chloride goes inside the bracket and cannot ionise. So only two chlorides are outside, and we get two moles of silver chloride. The formula is C o, N H three in round brackets with a five, C l, in square brackets, then C l two. Got it? And with four ammonias, two chlorides go inside, and only one is outside: C o, N H three in round brackets with a four, C l two, in square brackets, then C l, giving one mole of silver chloride. So that is where the missing chlorides were hiding: inside the bracket, as ligands! Clear?

7:58Quick revision: double salt, two valencies, AgCl test

So, quick revision. First, a double salt gives all its ions in water, but a complex keeps its bracket intact. Second, the primary valency equals the oxidation state, and the secondary valency equals the coordination number. Third, only the ions outside the square bracket ionise, as the silver chloride test shows. Now go and try the practice problems on this page. All the best!