How to Name Coordination Compounds (IUPAC Method)

K₃[Fe(C₂O₄)₃], [CoCl(NH₃)₅]Cl₂, [Pt(NH₃)₂Cl₂]: formulas like these look intimidating, yet every one of them can be turned into a name by following the same short procedure. The IUPAC system for coordination compounds is really a set of small decisions made in a fixed order. Once you have the order in your head, naming a complex takes less than a minute, and the name tells a reader exactly what is bonded to the metal.

This guide assumes you already know what a complex, a ligand and a coordination number are. If not, start with our introduction to coordination compounds. The names used here follow the current IUPAC recommendations (the 2005 “Red Book”), so anionic ligands end in -ido and -ato. Older textbooks write “chloro” and “cyano”; you will still see them, and they are not wrong so much as out of date.

Before you start: split the formula

A coordination compound usually has two parts: the complex inside the square brackets and any counter-ions outside them. In [Co(NH₃)₆]Cl₃, the complex is the cation [Co(NH₃)₆]³⁺ and the three Cl⁻ ions are counter-ions. They are not bonded to the cobalt at all.

Everything inside the brackets is part of the coordination sphere. Everything outside is named separately, just as in an ordinary salt.

The method, step by step

Step 1. Name the cation first, then the anion

As with sodium chloride, the cation comes first and the anion second, with a space between them. This is true whether the complex is the cation, the anion, or both.

• [Co(NH₃)₆]Cl₃: complex is the cation → “…cobalt(III) chloride”
• K₄[Fe(CN)₆]: complex is the anion → “potassium …ferrate(II)”
• A neutral complex such as [Ni(CO)₄] has no counter-ion and is named as a single word.

Step 2. Identify every ligand and its charge

List each ligand, its formula and whether it is neutral or anionic. You need the charges later to work out the oxidation state.

Ligand	Charge	Ligand name
H₂O	0	aqua
NH₃	0	ammine
CO	0	carbonyl
NO	0	nitrosyl
H₂NCH₂CH₂NH₂ (en)	0	ethane-1,2-diamine
F⁻, Cl⁻, Br⁻, I⁻	−1	fluorido, chlorido, bromido, iodido
OH⁻	−1	hydroxido
CN⁻	−1	cyanido
O²⁻	−2	oxido
NO₂⁻	−1	nitrito-κN (bonded through N) or nitrito-κO (through O)
SCN⁻	−1	thiocyanato-κS or thiocyanato-κN
SO₄²⁻	−2	sulfato
CO₃²⁻	−2	carbonato
C₂O₄²⁻	−2	oxalato (systematic: ethanedioato)

The rule behind the anion names is simple: take the anion’s own name and change the ending. -ide becomes -ido, -ate becomes -ato, -ite becomes -ito. Neutral ligands generally keep their molecule name, with four special exceptions worth memorising: aqua, ammine, carbonyl and nitrosyl.

Notice the double m in ammine. With one m, “amine” means an organic compound containing nitrogen; “ammine” means NH₃ bonded to a metal.

Step 3. Add multiplying prefixes

For simple ligands, use di-, tri-, tetra-, penta-, hexa-. For ligands whose names already contain a number or are long and complex, such as ethane-1,2-diamine, use bis-, tris-, tetrakis- and put the ligand name in brackets: tris(ethane-1,2-diamine).

IUPAC does not drop vowels when prefixes meet, so write hexaaqua and tetraammine, not “hexaqua” or “tetrammine”.

Step 4. Put the ligands in alphabetical order

Arrange the ligands alphabetically by their names, ignoring the multiplying prefixes. “Tetraammine” is filed under a, and “dichlorido” is filed under c. So tetraammine comes before dichlorido, even though “d” would come before “t”.

A prefix that is part of a ligand’s actual name does count. Dimethylamine is filed under d, because “dimethyl” is part of the molecule’s name rather than a count of ligands.

Older rules put anionic ligands before neutral ones. The current rule is purely alphabetical, which is less work.

Step 5. Name the metal

Write the metal’s name straight after the last ligand, with no space.

• If the complex is a cation or neutral, use the normal element name: cobalt, copper, platinum, nickel.
• If the complex is an anion, add -ate. Several metals use their Latin root:

Metal	In an anionic complex
Iron	ferrate
Copper	cuprate
Silver	argentate
Gold	aurate
Lead	plumbate
Tin	stannate
Cobalt	cobaltate
Nickel	nickelate
Zinc	zincate
Platinum	platinate
Chromium	chromate

Step 6. Give the oxidation state

Work out the metal’s oxidation state from the charges (see how to find oxidation numbers):

oxidation state of metal = charge on complex − sum of ligand charges

Write it in Roman numerals in round brackets straight after the metal name: cobalt(III), ferrate(II). For a zero oxidation state, write (0).

IUPAC also allows the charge number of the whole complex instead, written as an Arabic numeral with its sign, for example hexacyanidoferrate(4−). You will meet both styles; the Roman-numeral oxidation state is the one most exam boards expect.

Step 7. Add the counter-ion, and “ion” if it’s a lone complex ion

Finish with the other ion’s name (chloride, sulfate, potassium and so on). If you are naming a charged complex on its own, add the word ion: hexaaquacopper(II) ion. Neutral complexes never take “ion”.

Worked demonstration

Example 1: [Co(NH₃)₆]Cl₃

• Step 1: Cation = [Co(NH₃)₆]³⁺ (charge from three Cl⁻ outside). Anion = chloride.
• Step 2: Ligands: six NH₃, neutral → ammine.
• Step 3: Prefix: hexaammine.
• Step 4: Only one ligand type, so no ordering needed.
• Step 5: Complex is a cation → cobalt.
• Step 6: Oxidation state: +3 − 0 = +3.

Name: hexaamminecobalt(III) chloride. Note that there is no “tri” before chloride. In salt names the number of counter-ions is usually left out, because the charges already fix it.

Example 2: [CoCl(NH₃)₅]Cl₂

• Step 1: Two Cl⁻ outside, so the complex is [CoCl(NH₃)₅]²⁺.
• Step 2: Ligands: one Cl⁻ (chlorido) and five NH₃ (ammine).
• Step 3: Prefixes: chlorido, pentaammine.
• Step 4: Alphabetical: ammine before chlorido.
• Step 5: Cation → cobalt.
• Step 6: Oxidation state: +2 − (−1) = +3.

Name: pentaamminechloridocobalt(III) chloride. The same formula shows why the brackets matter: one chloride is a ligand and two are counter-ions, and a precipitation test with silver nitrate would find only the two outside the brackets.

Example 3: K₄[Fe(CN)₆]

• Step 1: Four K⁺ outside, so the complex is [Fe(CN)₆]⁴⁻, an anion.
• Step 2: Ligands: six CN⁻ → cyanido.
• Step 3: hexacyanido.
• Step 5: Anion → ferrate.
• Step 6: Oxidation state: −4 − (6 × −1) = +2.

Name: potassium hexacyanidoferrate(II). Its partner K₃[Fe(CN)₆] is potassium hexacyanidoferrate(III). The iron is the only thing that changes.

Example 4: [CrCl₂(H₂O)₄]Cl

• Step 1: One Cl⁻ outside → complex [CrCl₂(H₂O)₄]⁺.
• Step 2: Ligands: two Cl⁻ (chlorido), four H₂O (aqua).
• Step 4: Alphabetical: aqua before chlorido → tetraaquadichlorido.
• Step 6: Oxidation state: +1 − (2 × −1) = +3.

Name: tetraaquadichloridochromium(III) chloride. If you know the geometry, add an isomer prefix: trans-tetraaquadichloridochromium(III) chloride has the two chlorides opposite each other.

Example 5: K₃[Fe(C₂O₄)₃]

• Step 1: Three K⁺ → complex [Fe(C₂O₄)₃]³⁻, an anion.
• Step 2: Ligands: three oxalate ions, C₂O₄²⁻ → oxalato. Oxalate is bidentate (see our list of common ligands), but denticity doesn’t change the name.
• Step 3: Because each ligand is a polyatomic ion with a longer name, many chemists use tris(oxalato); “trioxalato” is also seen.
• Step 5: Anion → ferrate.
• Step 6: Oxidation state: −3 − (3 × −2) = +3.

Name: potassium tris(oxalato)ferrate(III).

Example 6: [Pt(NH₃)₂Cl₂] (cisplatin)

A neutral complex, so it’s one word with no counter-ion. Ligands: two ammine, two chlorido. Oxidation state: 0 − (2 × −1) = +2.

Name: cis-diamminedichloridoplatinum(II). The cis prefix matters here: the cis isomer is the anticancer drug, while the trans isomer is not used as one. Platinum is not given a Latin root because the complex is not an anion.

Example 7: working backwards from a name

“Tetraamminecopper(II) sulfate”:

• tetraammine → four NH₃ ligands, charge 0;
• copper(II) → Cu²⁺, and “copper” (not cuprate) means the complex is a cation;
• complex charge = +2 + 0 = +2, so [Cu(NH₃)₄]²⁺;
• sulfate is SO₄²⁻, so one of each balances the charge.

Formula: [Cu(NH₃)₄]SO₄.

Writing the formula: a note on order

In names, ligands go alphabetically by name. In formulas, IUPAC places the metal symbol first and then lists the ligands alphabetically by the first symbol of their formula or abbreviation. That is why you will see [CoCl(NH₃)₅]²⁺ (C before N) and [PtCl₂(NH₃)₂] in newer sources. Many textbooks still write neutral ligands first, for example [Pt(NH₃)₂Cl₂]. Both are understood; follow your syllabus.

Two extra situations

Ambidentate ligands. Some ligands can bond through either of two atoms. Nitrite can bind through nitrogen or oxygen, and thiocyanate through sulfur or nitrogen. The kappa (κ) notation names the donor atom: nitrito-κN versus nitrito-κO. Older books call these “nitro” and “nitrito”, and the two isomers really do differ in colour.

Bridging ligands. When one ligand bonds to two metal centres at once, its name gets the Greek prefix μ (mu), for example μ-hydroxido for an OH⁻ holding two metal atoms together. You mostly meet these in university courses.

Pitfalls to avoid

• Counting prefixes when alphabetising. “Dichlorido” goes under c, not d.
• Spelling ammine with one m. It changes the meaning.
• Forgetting -ate on anionic complexes. Look at the charge before you write the metal name. If the complex bracket is negative, the metal name must end in -ate.
• Using Latin roots where none exist. Cobalt, nickel and zinc become cobaltate, nickelate and zincate, not something Latin-sounding.
• Writing the complex charge instead of the oxidation state. In [Fe(CN)₆]⁴⁻ the charge is 4− but iron is +2. Always subtract the ligand charges.
• Naming the counter-ions as ligands. Only atoms inside the square brackets are bonded to the metal.
• Adding “ion” to a neutral complex. [Ni(CO)₄] is tetracarbonylnickel(0), full stop.
• Leaving spaces inside the complex name. The ligands and metal form one continuous word; the only space separates cation from anion.
• Using the outdated -o endings in an exam that expects the 2005 rules. Check which your specification uses; if unsure, chlorido and cyanido are the current IUPAC names.

Key takeaways

• Name the cation first, then the anion, exactly as for simple salts.
• Inside the complex: ligands alphabetically (prefixes ignored), then the metal, then its oxidation state in Roman numerals.
• Anionic ligands end in -ido, -ato or -ito; the special neutral names are aqua, ammine, carbonyl and nitrosyl.
• A negatively charged complex takes -ate, with Latin roots for iron, copper, silver, gold, lead and tin.
• Use bis-, tris-, tetrakis- with brackets for complicated ligand names, and add cis/trans, κ or μ when the structure needs it.
• Practise both directions: formula to name, and name back to formula. The oxidation number calculator is a handy check on your arithmetic.