IUPAC Nomenclature of Mononuclear Coordination Compounds

Coordination compounds are named according to a systematic set of IUPAC rules that let the complete composition and structure of any mononuclear complex be read directly from its name.

Cation named before anion. As with any ionic compound, the name of the cation is stated first, followed by the name of the anion, regardless of whether the complex itself is the cation or the anion of the compound. In $\text{K}_4[\text{Fe}(\text{CN})_6]$, "potassium" (the simple cation) comes before the name of the complex anion.

Ligands named in alphabetical order, before the metal. Inside the name of the coordination entity itself, every ligand is named first, in strict alphabetical order (ignoring any multiplying prefix), and the central metal's name comes last. This is the mirror image of how the formula is conventionally written, where the metal symbol comes first and the ligands follow.

Naming individual ligands. Anionic ligands take the ending "-o": $\text{Cl}^-$ is chloro, $\text{CN}^-$ is cyano, $\text{OH}^-$ is hydroxo, $\text{C}_2\text{O}_4^{2-}$ is oxalato. (A later, 2005 IUPAC recommendation instead uses the ending "-ido" for these, e.g. chlorido, cyanido — both conventions are still encountered, but the "-o" forms used throughout this chapter remain the most widely taught form.) Most neutral ligands keep their ordinary molecular name unchanged and are enclosed in parentheses in the formula, but a handful of very common neutral ligands take special traditional names instead: $\text{H}_2\text{O}$ is aqua, $\text{NH}_3$ is ammine (note the double "m", to distinguish it from an organic amine), $\text{CO}$ is carbonyl, and $\text{NO}$ is nitrosyl.

Multiplying prefixes. For simple ligand names, the ordinary Greek prefixes di-, tri-, tetra-, penta-, hexa- indicate how many of that ligand are present, e.g. hexaammine for six $\text{NH}_3$, pentaamminechloro for five $\text{NH}_3$ plus one $\text{Cl}^-$. When the ligand's own name already contains one of these syllables, or when the ligand name is itself a composite word (as with ethylenediamine), the alternative prefixes bis-, tris-, tetrakis- are used instead, with the ligand name enclosed in parentheses, to avoid ambiguity — three ethylenediamine ligands are named tris(ethylenediamine), never "triethylenediamine" (which would incorrectly suggest a single, different ligand made of three linked ethylenediamine units).

Oxidation state of the metal. The oxidation state of the central metal is given as a Roman numeral in parentheses immediately following the metal's name, with no space, e.g. cobalt(III), iron(II), nickel(0). …