Colour of Transition Metal Compounds

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Blue vitriol, the crystals of copper sulphate with water inside them, is blue. But anhydrous copper sulphate, with no water at all, is white. Same metal, the very same copper two plus ion. So where did the blue colour go? Interesting, right? Keep that question in your mind. Today we learn why transition metal compounds are coloured, and why some of them are not. Questions on colour come up often in JEE, so let's begin.

0:35Why transition metal ions are coloured

So, let us start with the basic picture. In a complex, or a hydrated ion, the metal ion is surrounded by ligands; in a hydrated ion, they are water molecules. These ligands split the d orbitals, so some d levels sit higher in energy than others. Okay? Now let me ask you something. An electron sits in a lower d level, and a higher d level just above it still has a vacant place. What can that electron do when light falls on it? Yes, exactly, it can jump up! The electron absorbs visible light, and jumps to a higher d level. We call this a d d transition; the short dash between the two d's means from one d level to another. Clear? Now, here is where many students slip. We see the complementary colour of the light absorbed, not the absorbed colour itself. Right? So here is a quick check for you. Suppose a solution absorbs orange red light. What colour does it appear to us? It looks blue, very good! Absorbs orange red, looks blue; the double arrow on screen is read as, so. Copper two plus is the example, and the small raised two plus is its charge. Got it?

1:50No d–d transition: colourless ions

Now the other side of the story. Take scandium three plus, with the three plus written small and raised as its charge. It has no d electrons at all. So can it be coloured? It cannot, because there is no d electron to jump. We call such ions d zero, with the zero written small and raised. Scandium three plus and titanium four plus are d zero, so they are colourless. Okay? Now the other extreme. What if the d subshell is completely full, with all ten d electrons? Can an electron jump to a higher d level then? It cannot, because there is no empty d place left to jump into! So d ten ions are also colourless, like copper one plus, zinc two plus and silver one plus; a plus sign alone means a charge of one plus. With me? So here is the rule. Colour from a d d transition needs a partly filled d subshell, not empty and not full. Right? But keep one puzzle in mind. K Mn O four, potassium permanganate, is deep purple; the small four written low means four oxygen atoms. Yet its manganese is in the plus seven state, written M n with the Roman numeral seven in brackets, and that is d zero. We will crack this one in Example two, okay?

3:06Colours of the hydrated ions

Now, the actual colours of the hydrated ions, the ions in water. Titanium three plus is purple. Vanadium three plus is green, and vanadium two plus is violet. And the ion written V O two plus, vanadium with one oxygen, is blue. Next, chromium three plus is violet, and chromium two plus is blue. Manganese three plus is violet too, and manganese two plus is pale pink. Then iron three plus is yellow, iron two plus green, cobalt two plus pink, nickel two plus green, and copper two plus blue, as we just saw. Right? Now look at iron. Same metal, different oxidation state, different colour: iron two plus is green, and iron three plus is yellow. Vanadium does the same, green at three plus and violet at two plus. So learn these pairs together. Clear?

3:56Water matters: copper sulphate

Now, back to our puzzle from the start. Anhydrous copper sulphate, C u S O four, without water, is white. Blue vitriol is C u S O four, dot, five H two O, and it is blue; the raised dot joins the five water molecules held in the crystal. Right? Both contain the same copper two plus ion. So what is the water doing here? Yes, exactly, the water molecules act as ligands! Water ligands around copper two plus split its d orbitals, so that red light is absorbed, and we see blue. With me? Now take the water away. Without the water ligands, the splitting is not in the visible range, so no visible light is absorbed, and it looks white. Puzzle solved! The ligands matter, not just the metal ion.

4:42Example 1: which of Ti3+, V2+, Cu+, Sc3+, Fe2+ are coloured

Got it? Now, Example one. Which of these give coloured aqueous solutions: titanium three plus, vanadium two plus, copper one plus, scandium three plus and iron two plus? Write the d count of each ion. Pause the video, and try it first. Done? Okay, let's check. Titanium three plus is three d one, vanadium two plus is three d three, and iron two plus is three d six. All three are partly filled. Okay? Now copper one plus. Copper salts are blue, we just saw that. So copper one plus is coloured too, right? Oh no, careful, that is exactly the trap! The blue ion is copper two plus, which is three d nine. Copper one plus is three d ten, completely full. With me? And scandium three plus is three d zero, completely empty. So neither copper one plus nor scandium three plus has a d d transition. Clear? So the coloured ones are titanium three plus, vanadium two plus and iron two plus. Just look for a partly filled d subshell, and it is done in seconds. Got it?

5:49Example 2: why KMnO4 is purple though Mn(VII) is d0

Now, Example two, our permanganate puzzle. Manganese in the plus seven state, in K Mn O four, has no d electrons. Why is the salt deep purple? Think about which electron could move. Pause the video, and try it first. Done? Okay, let's check. Manganese plus seven is three d zero, so no d d transition is possible. Then whose electron is jumping? Look at what surrounds the manganese. Yes, it is the oxygen atoms around it, the ligands! An electron moves from an oxygen orbital, a ligand orbital, to an empty manganese orbital. With me? This is called ligand to metal charge transfer; L arrow M on screen means from the ligand to the metal. It absorbs strongly in the visible, so the purple is deep. Clear? So the answer is charge transfer, not d d. Writing d d transition for permanganate is a common mistake, so do not fall for it. Okay? And the same cause explains two more colours. C r O four two minus, the chromate ion, is yellow, and C r two O seven two minus, the dichromate ion, is orange; the raised two minus is the charge. Chromium here is plus six, written with the Roman numeral six, so it is d zero, and this is charge transfer again. Got it?

7:12Quick revision: d-d colour, d0 and d10 ions, charge transfer

So, quick revision. First, colour comes from a d d transition, and we see the complementary colour of the light absorbed. Second, d zero and d ten ions have no d d transition, so ions like scandium three plus and zinc two plus are colourless, and the ligands, such as water, matter. And third, the d zero oxoanions, permanganate, M n O four minus, chromate and dichromate, get their intense colour from charge transfer. Now go and try the two practice problems on this page. You will find them easy. All the best!