Nomenclature

Coordination compounds are substances in which a central metal atom or ion is bonded to a surrounding set of molecules or ions called ligands. Examples such as [Cu(NH₃)₄]SO₄ (tetraamminecopper(II) sulfate) and [Fe(CN)₆]³⁻ (hexacyanoferrate(III)) play vital roles in industrial chemistry, biochemistry, and analytical chemistry. To communicate unambiguously about these complex species, chemists worldwide follow a standardised set of naming rules established by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC nomenclature system for coordination compounds was developed and refined throughout the 20th century, with major contributions from Alfred Werner, whose landmark work on the structure of coordination compounds earned him the 1913 Nobel Prize in Chemistry. The naming rules encode four key pieces of information: the identity and number of each ligand, the identity of the central metal, its oxidation state (written as a Roman numeral), and the identity of any counter ions that balance the overall charge of the complex. Mastering coordination compound nomenclature requires learning a specific word order, a set of ligand name prefixes, and the rules for determining oxidation states. Once these rules are internalised, any coordination compound can be named or its formula decoded systematically. This article walks through each rule step by step, using real examples including cisplatin, EDTA complexes, and common transition metal complexes encountered in A-level and IB chemistry courses.

Key takeaways
  • Name the cation before the anion
  • Name ligands before the central metal
  • Use multiplying prefixes for the number of each ligand
  • State the oxidation state of the central metal
  • Use the suffix -ate for anionic complexes
  • Indicate geometric isomers with cis- and trans- prefixes

Name the cation before the anion

Just like for simple ionic compounds, when a coordination compound is ionic the cation is named first and the anion second, with a space between them. If the complex ion itself is the cation, it is named first. Example: in [Co(NH₃)₆]Cl₃, the complex cation [Co(NH₃)₆]³⁺ is named first (hexaamminecobalt(III)) followed by the anion chloride, giving hexaamminecobalt(III) chloride.

Name ligands before the central metal

Within the complex ion, ligands are named before the central metal atom or ion. Ligands are listed in alphabetical order (ignoring multiplying prefixes such as di-, tri-, tetra-). Common ligand names: NH₃ → ammine; H₂O → aqua; Cl⁻ → chlorido; CN⁻ → cyanido; OH⁻ → hydroxido; CO → carbonyl; NO⁺ → nitrosyl; en (ethane-1,2-diamine) → ethane-1,2-diamine; ox²⁻ (oxalate) → oxalato; EDTA⁴⁻ → ethylenediaminetetraacetato. Anionic ligands that originally ended in -ide have that suffix changed to -o (e.g., chloride → chlorido, cyanide → cyanido). Neutral ligands keep their molecular names except for the special cases: NH₃ (ammine), H₂O (aqua), CO (carbonyl), and NO (nitrosyl).

Use multiplying prefixes for the number of each ligand

The number of each type of ligand is indicated by a Greek prefix placed directly before the ligand name. For simple ligands use: 2 = di, 3 = tri, 4 = tetra, 5 = penta, 6 = hexa. When the ligand name itself contains a multiplying prefix (e.g., ethane-1,2-diamine) or is complex, use the alternative series: 2 = bis, 3 = tris, 4 = tetrakis, 5 = pentakis, 6 = hexakis, and enclose the ligand name in parentheses. Example: [Co(en)₃]³⁺ is named tris(ethane-1,2-diamine)cobalt(III) ion.

State the oxidation state of the central metal

The oxidation state of the central metal is written immediately after its name as a Roman numeral in parentheses, with no space. The oxidation state is calculated using the known charges of the ligands and the overall charge of the complex. For example, in [Fe(CN)₆]³⁻: CN⁻ carries a charge of −1, so 6 × (−1) = −6. The overall charge is −3. Therefore: x + (−6) = −3, so x = +3. The metal is iron(III). If the complex is neutral and all ligands are neutral (like in [Ni(CO)₄]), the metal is in oxidation state zero: nickel(0).

Use the suffix -ate for anionic complexes

If the overall complex ion is an anion, the suffix -ate is appended to the metal name (using the Latin root for some metals). Important examples: iron → ferrate, copper → cuprate, gold → aurate, silver → argentate, lead → plumbate, tin → stannate. Metals that retain their English names simply add -ate: cobalt → cobaltate, chromium → chromate (note: chromate is also the name for CrO₄²⁻, so context matters), nickel → nickelate, platinum → platinate. Example: K₃[Fe(CN)₆] contains the anion [Fe(CN)₆]³⁻, named hexacyanidoferrate(III), giving the full name potassium hexacyanidoferrate(III).

Indicate geometric isomers with cis- and trans- prefixes

When square-planar or octahedral complexes can exist as geometric isomers, the prefixes cis- (same side) and trans- (opposite sides) are placed at the very beginning of the name. The chemotherapy drug cisplatin has the formula [PtCl₂(NH₃)₂] with both Cl⁻ ligands on the same side: cis-diamminedichloridoplatinum(II). The inactive isomer is trans-diamminedichloridoplatinum(II). For optical isomers, the descriptors Δ (delta, right-handed propeller) and Λ (lambda, left-handed propeller) are used for octahedral complexes.

The Key Equation

\ce[Co(NH3)4Cl2]++2Cl−−>[Co(NH3)4Cl2]Cl2\ce{[Co(NH3)4Cl2]^+ + 2Cl^- -> [Co(NH3)4Cl2]Cl2}
  • \ce{[Co(NH3)4Cl2]^+} — Complex cation (none): The coordination sphere: central Co³⁺ ion surrounded by four ammine ligands and two chlorido ligands, net charge +1.
  • \ce{Cl^-} — Counter ion (chloride) (none): Ionic species outside the coordination sphere that balances the overall charge of the complex cation.
  • \ce{[Co(NH3)4Cl2]Cl2} — Coordination salt (none): The neutral ionic compound tetraamminedichloridocobalt(III) chloride; the coordination complex written inside square brackets, counter ions outside.

Worked Example

Problem. Name the compound K₂[PtCl₄] using IUPAC rules, and determine the oxidation state of platinum.

\ceK2[PtCl4] = potassium tetrachloridoplatinate(II)\ce{K2[PtCl4]} \text{ = potassium tetrachloridoplatinate(II)}

Answer. potassium tetrachloridoplatinate(II)

Figure
Diagram
IUPAC Naming of [Co(NH₃)₄Cl₂]Cl₂Written Formula: [Co(NH₃)₄Cl₂]Cl₂Square brackets enclose the coordination sphereCATION (named first)Counter ion outside brackets:chloride (Cl⁻ × 2)COMPLEX ION [inside brackets]Co³⁺ + 4 NH₃ + 2 Cl⁻overall charge: +1Ligands (alphabetical order)ammine (×4) then chlorido (×2)Central metal + ox. statecobalt(III) [Co is +3]tetraamminedichlorido(prefix + ligand name)cobalt(III)(metal + Roman numeral)tetraamminedichloridocobalt(III) chloride
Origins · 1 minAlfred Werner and how this was discovered
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Alfred Werner
1866–1919 · Swiss (born French)

Alfred Werner proposed the theory of coordination compounds in 1893, correctly describing the octahedral geometry of six-coordinate metal complexes and the existence of coordination isomers. His theory formed the foundation for the modern understanding and naming of coordination compounds. He was awarded the Nobel Prize in Chemistry in 1913 for this work. Werner proposed the correct octahedral structure of cobalt(III) complexes at age 26 — reportedly after waking from a dream at 2 a.m. and writing out the full theory before dawn.

  • Ligands and Coordination Number — Understanding what ligands are, their donor atoms, and denticity (monodentate vs. polydentate) is essential before applying nomenclature rules correctly.
  • Isomerism in Coordination Compounds — Geometric (cis/trans) and optical isomerism in coordination compounds require specific naming prefixes that are an extension of IUPAC nomenclature.
  • Oxidation States of Transition Metals — Correctly assigning the oxidation state of the central metal — a core part of naming — requires a solid understanding of variable oxidation states in transition elements.
  • Structure and Geometry of Coordination Compounds — The 3D geometry (octahedral, tetrahedral, square-planar) determines the possibility of geometric isomers, which affects the IUPAC name.