IUPAC nomenclature of coordination compounds

A complex can have a metal, several kinds of ligand, an oxidation state, and counter-ions all in one formula. To talk about it unambiguously, chemists follow an agreed recipe so that a name maps to exactly one structure and back again. That recipe is the IUPAC nomenclature of coordination compounds.

The core rules, applied within the coordination sphere (the part in square brackets): name the ligands first, in alphabetical order, then the metal. Anionic ligands take an -o ending (chloride to chlorido, cyanide to cyanido, hydroxide to hydroxido); common neutral ligands keep special names (water becomes aqua, ammonia becomes ammine with two m's, carbon monoxide becomes carbonyl). The number of each ligand is shown by a prefix: di-, tri-, tetra- for simple ligands, but bis-, tris-, tetrakis- for ligands whose own names already contain a number prefix or are complicated (as in tris(ethylenediamine)). The prefixes do not count for alphabetizing — you alphabetize by the ligand name itself. After the ligands comes the metal, with its oxidation state in Roman numerals in parentheses. One twist: if the whole complex is an anion, the metal name takes an -ate ending (and sometimes a Latin stem, so iron becomes ferrate, copper cuprate). For a salt, the cation is named before the anion, exactly as for any ionic compound.

This may feel fussy, but it pays off: the name [Co(NH3)5Cl]Cl2 reads as pentaamminechloridocobalt(III) chloride, and from that name alone you can rebuild the formula, knowing five ammonias and one chloride sit on a cobalt(III), with two more chlorides outside as counter-ions. Systematic naming is essential because isomers share a formula — only a careful name (with cis-, trans-, fac-, mer-, or mu- where needed) can tell two isomers apart on paper.