Nomenclature of Coordination Compounds (IUPAC Rules)

Why a Systematic Name Is Needed

A coordination entity is a central metal atom or ion bonded to a fixed number of ions or molecules — the ligands — enclosed within square brackets, with any ionisable counter ions written outside. The chemistry of how those names are assigned is governed by the recommendations of the International Union of Pure and Applied Chemistry (IUPAC). The names of coordination compounds are derived by the principle of additive nomenclature: the groups surrounding the central atom are identified and listed as prefixes to the metal name, each with its appropriate multiplier.

The stakes for NEET are concrete. The two compounds $\ce{[CoCl2(NH3)4]+}$ that Werner studied have identical empirical formula $\ce{CoCl3.4NH3}$ yet distinct properties — they are isomers. Only an unambiguous naming and formula convention lets us write one without ambiguity over the other. Once the convention is fixed, naming becomes a mechanical sequence; the marks are lost not to difficulty but to a handful of recurring slips, all of which this article isolates.

The IUPAC Naming Rules (Name from Formula)

NCERT §5.3.2 lays down the rules for naming mononuclear coordination compounds. They are best memorised as an ordered checklist, because every step has exactly one position in the final name.

Naming the Ligands: Anionic, Neutral, Cationic

Before any name can be assembled, each ligand's IUPAC name must be known. Anionic ligands replace their ending with –o; the 2004 IUPAC draft adopted by NCERT and NEET appends –ido, so that chloro becomes chlorido, cyano becomes cyanido, and so on. Neutral ligands generally keep their molecular name, but four high-frequency ones have special names that must be memorised verbatim.

For an ambidentate ligand — one with two different donor atoms, either of which can ligate — the donor atom is specified with an italic letter. Thus $\ce{NO2-}$ bound through nitrogen is nitrito-N and through oxygen is nitrito-O; $\ce{SCN-}$ through sulphur is thiocyanato-S. NCERT's worked example names $\ce{[Pt(NH3)2Cl(NO2)]}$ as diamminechloridonitrito-Nplatinum(II), and $\ce{Hg[Co(SCN)4]}$ as mercury(I) tetrathiocyanato-Scobaltate(III).

Multiplying Prefixes: di/tri vs bis/tris

The count of each ligand is indicated by a numerical prefix. The choice between the two prefix families is one of the most frequently tested points in the chapter, and it is purely a matter of avoiding ambiguity.

NCERT's signature illustration is $\ce{[NiCl2(PPh3)2]}$, named dichloridobis(triphenylphosphine)nickel(II). The chloride takes the simple prefix di, but triphenylphosphine — a substituted name — takes bis and is enclosed in parentheses. Likewise $\ce{[Co(en)3]^3+}$ is named with tris(ethane-1,2-diamine) because "diamine" already contains a numeral; writing "triethanediamine" would be unreadable.

One subtlety often missed: the multiplying prefix is ignored when alphabetising. You alphabetise by the ligand's own name, not by its prefix. In diamminechlorido… the order is decided by ammine versus chlorido, not by "di" versus "chlorido".

The Metal, the -ate Suffix and Oxidation State

Once the ligands are named and ordered, the metal is appended. Whether the metal's name changes depends on the net charge of the coordination entity, not the compound as a whole.

For several metals the anionic form uses the Latin stem rather than the English name. NCERT highlights how the same metal is named differently in a cation and an anion — for example silver remains "silver" in $\ce{[Ag(NH3)2]+}$ (diamminesilver(I)) but becomes "argentate" in $\ce{[Ag(CN)2]-}$ (dicyanidoargentate(I)).

The oxidation state follows the metal name as a Roman numeral in parentheses, with no intervening space. It is the charge the central atom would carry if every ligand were removed with its shared electron pair. Operationally:

Oxidation state of metal = (charge on the coordination entity) − (sum of the charges of all ligands).

For $\ce{[Cu(CN)4]^3-}$, the four $\ce{CN-}$ ligands carry $4\times(-1)=-4$; with an overall $-3$ charge the copper is $-3-(-4)=+1$, written Cu(I). Neutral ligands (aqua, ammine, en, CO) contribute zero, which is why for $\ce{[Cr(NH3)3(H2O)3]Cl3}$ the chromium oxidation state simply equals the $+3$ charge of the cation.

Worked Examples — Building Names

Writing Formulae from Names

Going from a name to a formula reverses the priorities. NCERT §5.3.1 sets out the rules for writing formulae of mononuclear coordination entities, and the single most important change is that the central atom is written first, with ligands following in alphabetical order regardless of charge.

Worked Examples — Building Formulae

High-Yield NEET Traps

Across years, examiners recycle the same four errors. Internalising them is worth more than memorising more examples.

A closely related conceptual point that NEET frames through naming is the homoleptic versus heteroleptic distinction: a complex with only one kind of ligand (e.g. potassium trioxalatoaluminate(III), $\ce{K3[Al(ox)3]}$) is homoleptic, while one with mixed ligands (e.g. triamminetriaquachromium(III) chloride) is heteroleptic. The 2023 paper tested exactly this by giving four IUPAC names and asking which describes a homoleptic complex.