Coordinate Bonds

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. You know a covalent bond is a shared pair of electrons, usually one electron from each atom, right? Now take the hydrogen ion, H plus. It has lost its only electron, so it has zero electrons to share. And yet, ammonia picks up this H plus and forms the ammonium ion, with four nitrogen hydrogen bonds. How can an ion with no electrons at all form a bond? Strange, isn't it? And there is a second surprise. Once the ion has formed, nobody can tell which of the four bonds is the new one. Today one simple idea explains both: the coordinate bond. And in JEE, counting coordinate bonds in a molecule or an ion comes up often. So, let's begin.

0:48Who gives the electrons?

So, first, the bond you already know. In an ordinary covalent bond, two atoms share a pair of electrons, and each atom puts in one electron. Okay? Now think. Must it always be one from each? What if one atom puts in both electrons, and the other puts in nothing at all? Can that still be a bond? Yes, it can! That is a coordinate bond, also called a dative bond: a covalent bond in which both shared electrons come from one atom, and none from the other. It is still a covalent bond, a pair shared between two atoms. Only the giving is one-sided. Right?

1:24Donor and acceptor

Now, the atom that gives both electrons is called the donor. Tell me, what must the donor have, to give away two electrons together? Yes, a lone pair, very good! That is a pair of electrons not yet used in any bond. So, the donor gives both electrons, and for that it needs a lone pair. With me? And the atom that gives nothing is called the acceptor. It receives a whole pair. So, what must the acceptor have, to hold it? An empty orbital, correct! The acceptor needs a vacant orbital, one with no electrons in it, to hold the incoming pair. So both conditions together: a lone pair on the donor, and a vacant orbital on the acceptor. Okay? And how do we write it? With an arrow. On screen, the letter A comes first, then an arrow, then the letter B on the right. The arrow points from the donor, A, to the acceptor, B, the direction in which the pair is given. Got it?

2:23Puzzle one solved: the ammonium ion

Now, back to our puzzle. Ammonia, N H three, meets the hydrogen ion, H plus. The small three below is a subscript, for three hydrogen atoms, and the small plus on top is the charge. So, which one is the acceptor here? H plus, yes, exactly! It has no electrons at all, but it has an empty orbital: the one s orbital you see on screen. Empty, and ready to receive a pair. Right? And the donor? The nitrogen of ammonia. Nitrogen has three bonds to hydrogen, and it still has one lone pair left over. Perfect for giving. Clear? So the nitrogen gives its lone pair to H plus. On screen: H three N, arrow, H plus. The arrow points from nitrogen to the hydrogen ion. The product is the ammonium ion, N H four plus. Puzzle one solved! H plus gave nothing. It only received. With me? And water works the same way. The oxygen of H two O donates a lone pair to H plus. H two O, arrow, H plus, gives the hydronium ion, H three O plus.

3:32The trap: which bond is the coordinate one?

Okay? Now, puzzle two. The ammonium ion has four nitrogen hydrogen bonds, written N, dash, H, where the dash is the bond. Three were already there in ammonia, and one is the new coordinate bond. So the new one must be different, and you can point to it, right? Which one is it? No, no, careful! That is the trap. You cannot point to it. In N H four plus, all four N H bonds are identical. Got it? They have the same length and the same energy. So the ion is a regular tetrahedron, with all four hydrogens exactly alike. Clear? The arrow only records who gave the electrons: just bookkeeping. Once formed, a coordinate bond is like any other covalent bond, and only that bookkeeping differs. Puzzle two solved!

4:20Coordinate bonds hide in many places

Right? Alright, where else do we meet coordinate bonds? Inside the square brackets: a copper ion, with four ammonia molecules, N H three in round brackets, with a small four outside. Then S O four, the sulphate ion. How many coordinate bonds are there? Four, yes! Each ammonia gives its lone pair to the copper ion, C u two plus, where the two plus on top is its charge. Four ammonias, four coordinate bonds. With me? Now a surprise: C O, carbon monoxide. There are three bonds between carbon and oxygen, and in one of these three, both electrons come from oxygen. So even this tiny molecule has a coordinate bond. Okay? And in ozone, O three, and in nitric acid, HNO three, one oxygen receives a pair from the central atom. So both of them contain one too. Clear?

5:16Example 1: why BF3 can accept a lone pair but CF4 cannot

Now, the first example. Why can B F three, boron trifluoride, form a coordinate bond, but C F four, carbon tetrafluoride, cannot? Hint: think donor and acceptor. Pause the video and try it. Done? Okay, let's check. Start with boron. It makes three bonds with fluorine, and each bond is a shared pair. So, how many electrons sit around boron? Six, correct! Three bonds, two electrons each: only six. Boron is short of an octet. Right? So boron has a vacant orbital, and it can accept a lone pair from a donor. Boron trifluoride is a ready acceptor. Got it? Now carbon, in C F four. Four bonds, so eight electrons: a complete octet. No vacant orbital, so it cannot accept. And all four of carbon's valence electrons are used in bonds, so carbon has no lone pair either. Carbon cannot donate. Okay? So the answer: only boron has an empty orbital. See, those two conditions decide everything. No lone pair, no donor. No vacant orbital, no acceptor. With me?

6:28Example 2: coordinate bonds in the Al2Cl6 dimer

Now a slightly harder one. Aluminium chloride, A l C l three, joins up in pairs to give A l two C l six. Two units joining like this is called dimerising. How many coordinate bonds are there in A l two C l six, and why does A l C l three dimerise? Pause and try it. Hint: count the electrons around aluminium first. Done? Okay, let's check. Aluminium in A l C l three makes three bonds, just like boron did. So, how many electrons around it? Six, yes! Same as boron. So each aluminium is electron deficient, with room for one more pair. That is why A l C l three dimerises. Right? Now, a quick doubt. Two units join together, so one coordinate bond holds them, and the answer is one, right? Oh no, careful! Both aluminiums are electron deficient, and each one needs a pair. Chlorine has lone pairs, so one chlorine of each unit donates a lone pair to the aluminium of the other unit. Clear? So the chlorine of the first unit gives to the second aluminium, and the chlorine of the second unit gives to the first. On screen: two chlorine, arrow, aluminium bridges, so two coordinate bonds. Two, not one.

7:48Quick revision: donor, acceptor and the dative bond

Got it? So, quick revision. One: in a coordinate bond, both shared electrons come from one atom. Two: the donor needs a lone pair, and the acceptor needs a vacant orbital. Three: once formed, it is identical to an ordinary covalent bond. Now go and try the practice problems on this page. All the best!