Naming of Mononuclear Coordination Compounds

Naming a coordination compound follows the principles of additive nomenclature: every group attached to the central atom is identified individually and then listed, as a prefix, in front of the name of the central atom itself, along with a multiplier that says how many of that group are present. A fixed set of conventions governs how this is done.

Cation before anion, ligands before metal

• The cation is always named first, regardless of whether the coordination entity itself is the positively charged part or the negatively charged part of the compound.
• Within the coordination entity, the ligands are named in alphabetical order, ahead of the central atom's name. This is the mirror image of formula-writing, where the central atom comes first and the ligands follow — in naming, that order is reversed.

Naming the ligands themselves

• Names of anionic ligands (negatively charged) take the ending "-o".
• Names of neutral and cationic ligands are generally used unchanged, with a handful of ligands that keep special traditional names instead of a systematic "-o"/unchanged form: water is named aqua, ammonia is named ammine, carbon monoxide is named carbonyl, and nitric oxide is named nitrosyl. When such a ligand's formula is written out, it is enclosed in parentheses.

A later (2004) IUPAC draft recommends that anionic ligand names instead end in "-ido" — so, for instance, chloro would become chlorido. Both conventions describe the same ligands; the "-o" form is the one used through the rest of this chapter.

Counting the ligands

• The simple multiplying prefixes mono, di, tri, and so on are used to state how many of a given ligand are present.
• If a ligand's own name already contains a numerical term of this kind (for example, a substituted or polydentate ligand name), then to avoid confusion the multipliers bis, tris, tetrakis are used instead, with the ligand's name enclosed in parentheses. As an example of this convention, $\ce{[NiCl2(PPh3)2]}$ is named dichloridobis(triphenylphosphine)nickel(II).

Oxidation state and the metal's own name

• The oxidation state of the central metal, whether the coordination entity is a cation, an anion, or electrically neutral, is written as a Roman numeral enclosed in parentheses immediately after the name of the metal.
• If the coordination entity is a cation, the metal keeps its ordinary elemental name (chromium remains chromium, platinum remains platinum).
• If the coordination entity is an anion, the metal's name instead takes the suffix "-ate" — for example, the complex anion $[Co(SCN)_4]^{2-}$ is named cobaltate. For a few metals this suffix is added to the metal's Latin name rather than its English name — iron, for example, becomes ferrate in an anionic complex.
• A neutral coordination molecule (one carrying no overall charge) is named following exactly the same pattern used for a complex cation.

The book illustrates these rules with three worked namings: …