Ligands, Denticity and Chelates

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. In one well known cobalt complex, six ammonia molecules sit around the metal ion, and each of them bonds through its nitrogen. Six ligands, six positions, okay? Now the strange part. One single ion, called EDTA, can take all six positions around a metal ion by itself! One ligand, and six bonds? How is that even possible? Keep that question in mind. Today we study ligands, denticity and chelates. Questions on these come up often in JEE, and they are quick marks once you learn to count donor atoms. So, let's begin.

0:43Donor atoms and denticity

Let us begin with two simple words. Ligands are the groups bonded to the central metal. And the atom of a ligand that actually bonds to the metal is called the donor atom, okay? Now, the second word is denticity. Denticity is the number of donor atoms through which one ligand binds at once. Keep those two words, at once, in your mind, because they will save you from a trap later. Right? Let us try one. Look at ammonia, written N H three on screen; the small three written below means three hydrogen atoms. So ammonia has one nitrogen and three hydrogens. How many donor atoms does it use? Yes, exactly, just one, the nitrogen! So ammonia is monodentate, with one donor atom. The same is true for water, H two O, for chloride, C l minus, for cyanide, C N minus, and for C O, carbon monoxide. Also for pyridine, written p y, and for triphenylphosphine, written P P h three. The small raised minus signs are the charges on those ions. Clear?

1:52Bidentate and polydentate ligands

Now, what if one ligand has two donor atoms? Take the ligand called e n, which has two nitrogen atoms, one at each end. Can both these nitrogens bind to the same metal at once? Yes, both of them can! So e n is bidentate, with two donor atoms. In the round brackets is its formula: H two N, C H two, C H two, N H two, a nitrogen at each end of a short chain. Next to it is oxalate, C two O four, with a two minus charge written raised. It also binds through two atoms. With me? More bidentate ligands in the course list: glycinate, then acetylacetonate, written a c a c with a raised minus sign, then bipy, phen and dimethylglyoximate. Keep this list ready, because questions asking you to pick the bidentate ligand come up often. Okay? Now, let us add even more donor atoms. The ligand dien binds through three donor atoms, so it is tridentate. And trien binds through four, so it is tetradentate. Got it? And now the big one, EDTA, with its four minus charge written raised. It binds through six donor atoms, so it is hexadentate. So tell me, is EDTA always hexadentate? Not always, so be careful here! EDTA can also bind through fewer atoms. That is exactly why denticity counts only the donor atoms that bind at once. Clear?

3:24Ambidentate ligands: one of two atoms

Now, a different kind of ligand. An ambidentate ligand can bind through either of two atoms. Either one of them, right? Look at N O two minus. Written this way, it binds through the nitrogen, so its name is nitrito N, with N for nitrogen. Written as ONO minus, it binds through an oxygen: nitrito O, with O for oxygen. Same ion, but a different donor atom, okay? There are two more pairs to remember. SCN minus and NCS minus, and then C N minus and N C minus. The slash between each pair means or. In each pair, the atom written first is the one that binds to the metal. Got it? Now, a trap question. The ligand e n also has two donor atoms, its two nitrogens. So is e n an ambidentate ligand? It is not, and this is a classic trap. An ambidentate ligand has two different donor atoms, but only one is used at a time. The ligand e n uses both its nitrogens at once, so it is bidentate, not ambidentate. With me?

4:31Chelates and the chelate effect

Now the most beautiful idea of this lesson. When a polydentate ligand binds through two or more atoms, it forms rings. Okay? Picture e n holding the metal with both its nitrogens. The metal, the two nitrogens and the chain between them close up into one ring. Right? Any complex with such rings is called a chelate. Remember this word, chelate, because the next idea depends on it. Clear? Now compare two complexes of the same metal. In one, two ammonia molecules give two nitrogen donor atoms. In the other, one e n gives two nitrogen donor atoms. Which one holds on more tightly? The e n complex, the chelate, yes! Chelates are more stable than comparable complexes of monodentate ligands, and this is called the chelate effect. Remember that name; it comes up often. Got it?

5:25Example 1: denticity of EDTA, dien, oxalate, glycinate, en

Alright, let us do Example one. Give the denticity of EDTA four minus, dien, oxalate, glycinate, and finally e n, okay? Remember to count only the donor atoms that bind at once. Pause the video, and try it first. Done? Okay, let's check. EDTA four minus binds through six donor atoms, so its denticity is six. It is hexadentate. Right? Next, dien binds through three donor atoms, so three. And oxalate binds through two, so two. Clear? Now glycinate. It has two different donor atoms, a nitrogen and an oxygen. Two different donor atoms, so glycinate is ambidentate, right? That is the trap, so be careful! Glycinate uses the nitrogen and the oxygen together, both at once. So it is bidentate, and its denticity is two. Got it? And e n binds through both its nitrogens at once, so its denticity is also two. So the answers, in order, are six, three, two, two and two. Easy marks, once you count only the atoms that bind at once. With me?

6:36Example 2: how many chelate rings EDTA forms

Now Example two, a little harder. How many chelate rings does EDTA four minus form when it wraps round a metal ion as a hexadentate ligand? Pause the video and try it first. Done? Okay, let's check. Hexadentate means six donor atoms, all bound to one metal. So six donor atoms means six rings, right? No, no, wait, let us test that idea on e n first. The ligand e n has two donor atoms, but it closes only one ring with the metal. Two minus one equals one. So the rings are one less than the donor atoms. Clear? Here is the topper's trick. The first donor atom to bind makes no ring, and every donor atom that binds after it closes one more ring with the metal. So for an open chain ligand, even a branched one like EDTA, the number of rings equals the number of donor atoms, minus one. Okay? Now apply it to EDTA four minus. Six donor atoms, so six minus one gives five rings. Right? And each of these rings has five atoms, counting the metal, so the answer is five rings, each of them five membered. Back to our puzzle: one EDTA takes all six positions and locks the metal inside five rings. That is why it holds on so firmly. Got it?

7:59Quick revision: denticity, ambidentate ligands, chelates

So, let us do a quick revision. First, denticity: count the donor atoms that bind at once. Second, an ambidentate ligand can bind through either of two atoms, but only one at a time. And third, a chelate has rings and extra stability, and for an open chain ligand, the rings are one less than the donor atoms. Now go and try the practice problem on this page. You will find it easy. All the best!