Ligands (alphabetical), metal, oxidation state. Name [Co(NH₃)₆]Cl₃.

IUPAC Naming of Coordination Compounds — Rules and Examples

Question

Name the following coordination compound using IUPAC rules:

[\text{Co(NH}_3)_6]\text{Cl}_3

Also state the oxidation state of the central metal ion.

Solution — Step by Step

Split the compound into cation and anion first. Here, $[\text{Co(NH}_3)_6]^{3+}$ is the complex cation and $\text{Cl}^-$ is the counter ion (outside the bracket).

We name the cation before the anion — same as in ionic compounds like sodium chloride.

Each $\text{NH}_3$ is neutral (charge = 0). The overall complex ion carries a $3+$ charge.

So: $x + 6(0) = +3$ , which gives $x = +3$ . Cobalt is in the +3 oxidation state.

We have 6 ammonia ligands. The IUPAC name for $\text{NH}_3$ as a ligand is ammine (double ‘m’ — don’t confuse with amine).

Since there are 6, we use the prefix hexa: hexaammine.

The metal is cobalt. Since it’s in a cation, we keep the English name: cobalt(III).

The Roman numeral in parentheses shows the oxidation state — this is mandatory in IUPAC nomenclature.

$\text{Cl}^-$ is chloride. It goes at the end, as a separate word.

Putting it all together:

Hexaamminecobalt(III) chloride

Why This Works

IUPAC rules for coordination compounds follow a strict hierarchy. Ligands come before the metal (within the complex name), and they must be listed alphabetically — ignoring the numerical prefixes. So if you had both ammine and bromo ligands, you’d write bromo before ammine because ‘b’ comes before ‘a’, regardless of how many of each there are.

The Roman numeral oxidation state is non-negotiable in IUPAC naming. Cobalt forms both $\text{Co}^{2+}$ and $\text{Co}^{3+}$ complexes, and the name alone wouldn’t tell you which — hence cobalt(II) vs cobalt(III).

For anionic complexes (when the metal is in the anion), the metal name gets an -ate suffix: cobalt becomes cobaltate, iron becomes ferrate, and so on. This question has a cationic complex, so no suffix needed.

Alternative Method

You can use the charge balance method to find the oxidation state before naming:

Total positive charge on cation = Total negative charge on anions

[\text{Co(NH}_3)_6]^{x+} = 3 \times \text{Cl}^-

x = +3

This approach is faster when you have multiple counter ions. For $[\text{Co(NH}_3)_6]\text{Cl}_3$ , three chlorides each carry $-1$ , so the complex must be $+3$ . Works every time.

Memorise these special ligand names — they appear in every CBSE paper: $\text{NH}_3$ → ammine, $\text{H}_2\text{O}$ → aqua, $\text{CO}$ → carbonyl, $\text{CN}^-$ → cyano, $\text{NO}^+$ → nitrosonium, $\text{NO}_2^-$ → nitrito/nitro.

Common Mistake

Writing “hexamine” instead of “hexaammine” is the most common error — and CBSE examiners specifically watch for it. The ligand name ammine has double ‘m’. When you add the prefix hexa, you get hexaammine (with double ‘a’ in the middle). Students often write hexamine, which is actually a completely different compound (methenamine). You lose the full mark for this slip.

A second trap: many students write “cobalt III chloride” without the brackets or proper ligand naming, treating it like a simple ionic compound. The complex ion must be named as a unit.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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