Naming coordination compounds

One name for one complex

A pharmacist, a chemist in Japan and a student in Kenya all need to know that they mean the same compound. So IUPAC, the international union of chemists, has fixed rules. The rules look long at first, but each one is short. Let us take them in order.

The picture shows the pattern. The name of $\text{K}_{4}[\text{Fe}(\text{CN})_{6}]$ is potassium hexacyanidoferrate(II). Read it from left to right: the cation, the number and name of the ligands, the metal, and the oxidation state of the metal.

The rules, one by one

1. The cation is named first and the anion second. This is the rule for $\text{NaCl}$, sodium chloride. The complex ion can be the cation or the anion. The cation and the anion are two words.
2. Inside the complex ion, the ligands come first and the metal last. The complex ion is written as one word, with no spaces.
3. The ligands are named in alphabetical order. Count the first letter of the ligand name. The prefixes di, tri and tetra do not count. So tetraammine comes before dichlorido, because a comes before c.
4. Negative ligands end in -o. Chloride is chlorido, cyanide is cyanido, hydroxide is hydroxido, oxalate is oxalato, sulphate is sulphato. Older books write chloro, cyano and hydroxo. You will still see them in some papers.
5. Neutral ligands keep the name of the molecule. Four have special names: $\text{H}_{2}\text{O}$ is aqua, $\text{NH}_{3}$ is ammine, $\text{CO}$ is carbonyl, $\text{NO}$ is nitrosyl. Note the double m in ammine. Without it, the word means an organic amine.
6. The number of identical ligands is shown by a prefix. Use di, tri, tetra, penta and hexa. For a ligand whose own name already has a prefix, or is long, use bis, tris or tetrakis with the name in brackets: bis(ethane-1,2-diamine).
7. The oxidation state of the metal is written in Roman numerals in brackets straight after the metal name. A state of zero is written (0).
8. A cationic or neutral complex has the ordinary name of the metal. An anionic complex gets the ending -ate. The metal name often changes.

For rule 8, take the metals one at a time:

• Most metals only add -ate: cobaltate, nickelate, zincate, chromate, platinate, aluminate.
• Some use the old Latin name: iron gives ferrate, copper gives cuprate, silver gives argentate, gold gives aurate, lead gives plumbate, tin gives stannate.

9. Ambidentate ligands show the donor atom. Write nitrito-N when the nitrite ion bonds through nitrogen and nitrito-O when it bonds through oxygen. Write thiocyanato-S and thiocyanato-N in the same way.

The names of common ligands

Keep this list at hand. Each line gives the formula, the charge and the name:

• $\text{NH}_{3}$, neutral: ammine
• $\text{H}_{2}\text{O}$, neutral: aqua
• $\text{CO}$, neutral: carbonyl
• $\text{NO}$, neutral: nitrosyl
• en, neutral: ethane-1,2-diamine
• $\text{Cl}^{-}$: chlorido
• $\text{Br}^{-}$: bromido
• $\text{CN}^{-}$: cyanido
• $\text{OH}^{-}$: hydroxido
• $\text{C}_{2}\text{O}_{4}^{2-}$: oxalato
• $\text{SO}_{4}^{2-}$: sulphato
• $\text{NO}_{2}^{-}$: nitrito-N (through nitrogen) or nitrito-O (through oxygen)
• $\text{SCN}^{-}$: thiocyanato-S or thiocyanato-N

Naming, step by step

Example 1. $[\text{Co}(\text{NH}_{3})_{6}]\text{Cl}_{3}$. The cation is the complex. Its ligands are six ammonia molecules: hexaammine. The metal is cobalt, and the ion is a cation, so we keep cobalt. The three chloride ions give a charge of $-3$, so the complex is $+3$, and cobalt is $+3$ because ammonia is neutral. The anion is chloride. The name is hexaamminecobalt(III) chloride.

Example 2. $[\text{Cu}(\text{NH}_{3})_{4}]\text{SO}_{4}$. Four ammonia molecules: tetraammine. The sulphate gives $-2$, so the complex is $+2$ and copper is $+2$. The name is tetraamminecopper(II) sulphate.

Example 3. $\text{K}_{4}[\text{Fe}(\text{CN})_{6}]$. The cation is potassium. The complex is the anion, so the metal ends in -ate, and iron becomes ferrate. Six cyanido ligands: hexacyanido. The complex has a charge of $-4$ and six cyanide ions give $-6$, so iron is $+2$. The name is potassium hexacyanidoferrate(II).

Example 4. $[\text{Pt}(\text{NH}_{3})_{2}\text{Cl}_{2}]$. Two ligands are different. Alphabetical order puts ammine before chlorido, so the name starts with diammine and goes on with dichlorido. The complex has no charge. Platinum is $+2$. The name is diamminedichloridoplatinum(II). The isomer with the two chlorine ligands on the same side is the drug cisplatin.

Example 5. $[\text{Ni}(\text{en})_{3}]\text{Cl}_{2}$. The ligand name has a locant and a prefix inside it, so we use tris and brackets: tris(ethane-1,2-diamine). Nickel is $+2$. The name is tris(ethane-1,2-diamine)nickel(II) chloride.

Choose ammine and six ligands with cobalt at +3. The name is hexaamminecobalt(III). Now change the ligand to cyanido. The charge on the complex becomes $-3$, and the metal name changes to cobaltate. The ending depends on the charge on the complex, not on the metal.

Practise with new numbers

From a name to a formula

Going back is just as common. Follow these steps:

1. Write the metal symbol first, inside square brackets.
2. Write the ligands in alphabetical order of their symbols, with brackets round any ligand that has more than one atom and a number. Write the number of each ligand as a subscript.
3. Find the charge on the complex: the metal state plus the ligand charges.
4. Write the counter ions outside the brackets, enough to make the whole compound neutral.

Many books write the neutral ligands before the charged ones, as in $[\text{Co}(\text{NH}_{3})_{4}\text{Cl}_{2}]^{+}$. The strict IUPAC order is alphabetical, which gives $[\text{CoCl}_{2}(\text{NH}_{3})_{4}]^{+}$. Both mean the same ion. The name is the part where the alphabetical rule must be followed exactly.

Example. Sodium dicyanidoargentate(I). Two cyanide ions are the ligands, each $-1$. Silver is $+1$. The complex has a charge of $-1$, so one $\text{Na}^{+}$ is needed. The formula is $\text{Na}[\text{Ag}(\text{CN})_{2}]$. This ion is how silver is taken out of its ore.

Example. Potassium tetrachloridopalladate(II). Palladium is $+2$ and four chloride ions are $-4$. The complex has a charge of $-2$, so two $\text{K}^{+}$ ions are needed. The formula is $\text{K}_{2}[\text{PdCl}_{4}]$.

Try it in Python

The program names a complex from its metal, its ligands and its charge. It follows the rules in order: the oxidation state first, then the ligands in alphabetical order, then the metal.

Common mistakes

• Writing amine for ammine. The ligand $\text{NH}_{3}$ is ammine, with two m.
• Putting the ligands in the order of the formula. The name uses alphabetical order. Ignore di, tri and tetra when you sort.
• Using the cation name of the metal in an anion. An anionic complex of iron is ferrate, not iron.
• Writing the charge instead of the oxidation state. The number in brackets is the oxidation state of the metal, in Roman numerals.
• Leaving a space between the ligands and the metal. The complex ion is one word.
• Naming the anion first. The cation always comes first, whichever part is the complex.

Remember these

• Cation first, then anion. Inside the complex: ligands in alphabetical order, then the metal.
• Anionic ligands end in -o. Aqua, ammine, carbonyl and nitrosyl are the special neutral names.
• Prefixes: di, tri, tetra, penta, hexa. Use bis, tris and tetrakis with brackets for long ligand names.
• An anionic complex gets -ate. Iron, copper, silver and gold use ferrate, cuprate, argentate and aurate.
• The oxidation state goes in Roman numerals in brackets, with (0) for zero.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name $[\text{Ag}(\text{NH}_{3})_{2}]\text{Cl}$.

2. Write the formula of potassium hexacyanidochromate(III).

3. Which name is right: tetraaminecopper(II) sulphate or tetraamminecopper(II) sulphate?

4. Why is $\text{Na}_{2}[\text{Zn}(\text{OH})_{4}]$ called sodium tetrahydroxidozincate(II)?