Rules for Formulae and Names

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Two students named the same cobalt complex. One wrote chlorido penta ammine cobalt three chloride. The other wrote penta ammine chlorido cobalt three chloride. Same words, different order, and only one of them gets the mark. So which one is right? Naming questions come up often in JEE, and the options usually differ by one small word or one small ending. So, let's begin.

0:32Writing the formula of a complex

So, first, writing the formula. The central metal atom is written first. Then come the ligands, in alphabetical order of their symbols. And as the note in brackets says, a ligand's charge does not matter here. Okay? Now look at this complex ion: open square bracket, C o, then N H three in round brackets with a small five, then N O two in round brackets, close square bracket, and two plus at the top right. The round brackets, the parentheses, hold each polyatomic ligand together, the small number at the lower right counts it, and the two plus is the charge of the whole entity. So why is N H three written before N O two? It is just alphabetical order, yes, exactly! N H comes before N O, as in a dictionary. Abbreviations go in parentheses too: here, cobalt with three e n ligands, and the whole ion is three plus. The letters e n are a short form, and we will expand them in a minute. And notice, there is no space between the ligands and the metal. Right? So, the whole entity sits inside square brackets, and its charge goes outside the bracket. And for a salt, the cation is written first. Clear?

1:49Naming: the order of the parts

Now, naming. The cation is named first, then the anion, just as in simple salts. With me? Inside the entity, we name the ligands first, in alphabetical order, then the metal, then its oxidation state, in Roman numerals inside round brackets. So capital III after cobalt is read as cobalt three. Okay? Now, back to our puzzle. Penta ammine starts with the letter p, while chlorido starts with the letter c, so chlorido should come first, right? No, no, careful, that is exactly the trap! When we put the ligands in order, we ignore the multiplying prefixes, di, tri, penta, each written with a little dash after it, and the three dots mean and so on. So we compare ammine with chlorido, and the letter a comes before the letter c, so ammine goes first. Got it? So the second student gets the mark. This one, open square bracket, C o, N H three in round brackets with a small five, then C l, close square bracket, and C l two outside, is penta ammine chlorido cobalt three chloride. Ligands, then the metal, then its Roman numeral, then the counter ions. And one small note: this formula is usually printed with N H three before C l, as here, even though strict alphabetical order would put C l first. Either way, the name stays the same, because the name always follows the naming rules. Right?

3:20Names of the ligands

Next, the ligand names. Anionic ligands end in ido, ato, or ito, like chlorido, cyanido, hydroxido and oxalato. The little dash in front marks an ending. Clear? Now, the nitrite ion, N O two with a small minus sign at the top right, can bond to the metal through nitrogen, or through oxygen. So how should the name show which atom is bonded? We write the donor atom after the name, exactly! Bonded through nitrogen, it is nitrito N, with nitrogen as the donor. Bonded through oxygen, written ONO minus, it is nitrito O, with oxygen as the donor. With me? Older names, like chloro, cyano and nitro, still appear in exam options, so do not be confused by them. Okay? Now, neutral ligands. Four of them get special names. Water, H two O, is aqua. Ammonia, N H three, is ammine, spelt with a double m. C O is carbonyl, and N O is nitrosyl. Got it? And e n is the short form of ethane one two diamine, which is neutral too. Every other neutral ligand simply keeps its own name. Right?

4:38How many? The counting prefixes

Now, how do we say how many of each ligand? For simple ligands, we use di, tri, tetra, penta and hexa, for two, three, four, five and six. Okay? But when the ligand's name already has a prefix, or is complicated, we use bis, tris and tetrakis instead, and we put the ligand's name in round brackets. Clear? Here is the classic case: open square bracket, C o, e n in round brackets with a small three, close bracket, then C l three. Ethane one two diamine already contains di in its name, so three of them is tris, not tri. Tris, ethane one two diamine in brackets, cobalt three chloride. Got it? And one more prefix. When a ligand bridges two metal atoms, it carries the prefix mu, the Greek letter you see on screen. With me?

5:31Negative complexes: the -ate ending

Alright, now the negative complexes. When the coordination entity is an anion, the name of the metal ends in ate. Okay? For example, cobalt becomes cobaltate, nickel becomes nickelate, and in the same way we get zincate, chromate, manganate and platinate. So, iron in an anion becomes ironate, right? No, no, wait, there is no ironate! Some metals use their Latin roots. Iron, F e, becomes ferrate. Copper, C u, becomes cuprate, and silver, A g, becomes argentate. Gold, A u, becomes aurate, lead, P b, becomes plumbate, and tin, S n, becomes stannate. Clear?

6:19Example 1: name K4[Fe(CN)6] and K3[Fe(CN)6], ferrate II, III

Alright, example one, an easy one. Name these two: K four, open square bracket, F e, then C N in round brackets with a small six, close square bracket, and the same with K three. Pause the video and try it first. Done? Okay, let's check. The potassium ions, K plus, sit outside the bracket, so the coordination entity must be an anion. And an anionic entity means iron becomes ferrate. Right? Now the first one. Let x be the oxidation state of iron. Each cyanide is minus one, and four potassium ions outside mean the bracket carries four minus. So x, plus six times minus one, equals minus four. The double arrow means therefore, so x equals plus two. Okay? So the first one is potassium hexa cyanido ferrate two. Now, the second compound has only three potassium ions. So what is the oxidation state of iron there? Three potassium ions mean the bracket carries three minus. So x, plus six times minus one, equals minus three, and x equals plus three. Yes, it is iron three! So the second one is potassium hexa cyanido ferrate three. Only the Roman numeral changes. Got it?

7:41Example 2: name [CoCl2(en)2]Cl, bis not di

Now a slightly harder one. Name this complex: open square bracket, C o, C l two, then e n in round brackets with a small two, close square bracket, and then C l outside. Pause the video and try it first. Here is a hint: watch the prefix for e n. Done? Okay, let's check. One chloride ion, C l minus, sits outside, so the entity carries one plus. Ethane one two diamine is neutral, and each chloride inside is minus one. So x, plus two times minus one, plus two times zero, equals plus one. So x is plus three: cobalt three. Right? Now the order. Chlorido starts with the letter c, and ethane one two diamine starts with the letter e, so chlorido comes first. And there are two e n ligands. So, is it di, or bis? It is bis, yes, absolutely! Ethane one two diamine already contains di in its name, so we use bis, not di, with the name in round brackets. Clear? So the full name is di chlorido bis, ethane one two diamine in brackets, cobalt three chloride. Ligands in order, the right prefixes, the metal with its Roman numeral, and then the counter ion. With me?

9:05Quick revision: formula order, ligand order, -ate ending

So, quick revision. First, in a formula, the metal comes first, then the ligands in alphabetical order, all inside square brackets. Second, in a name, the ligands go in alphabetical order, ignoring di and bis, then the metal, then its oxidation state. Third, in an anionic entity, the metal's name ends in ate, with Latin roots like ferrate and argentate. Now go and try the practice problems on this page. All the best!