IUPAC nomenclature

The essentials

IUPAC rules allow every coordination compound to be named unambiguously. Formulae are written with the central atom first, followed by ligands in alphabetical order, all inside square brackets. Names follow the order below.

Key points

• Name the cation first, then the anion, whether the complex is positive or negative.
• Inside the complex, ligands are named in alphabetical order before the metal. Multiplying prefixes (di-, tri-) are ignored for alphabetising.
• Anionic ligands end in "-o": chlorido (Cl⁻), cyanido (CN⁻), oxalato (C₂O₄²⁻), hydroxido (OH⁻), nitrito-N (NO₂⁻ bound through N).
• Neutral ligands keep their names except aqua (H₂O), ammine (NH₃), carbonyl (CO) and nitrosyl (NO).
• Prefixes di, tri, tetra show number; for ligands whose names already contain a number, use bis, tris, tetrakis with brackets, e.g. bis(ethane-1,2-diamine).
• The oxidation state of the metal is shown in Roman numerals in brackets.
• If the complex is an anion, the metal name ends in "-ate": ferrate (Fe), cuprate (Cu), argentate (Ag), aurate (Au), plumbate (Pb), stannate (Sn), cobaltate (Co).
• Examples: [Co(NH₃)₆]Cl₃ is hexaamminecobalt(III) chloride; K₃[Fe(CN)₆] is potassium hexacyanidoferrate(III); [Ni(CO)₄] is tetracarbonylnickel(0); [CoCl₂(en)₂]Cl is dichloridobis(ethane-1,2-diamine)cobalt(III) chloride.

Exam tips

• Always calculate the oxidation state before writing the Roman numeral.

Common mistakes

• Writing "ammine" as "amine" or forgetting "-ate" in anionic complexes.