IUPAC naming of coordination compounds — rules with 5 examples

Question

State the IUPAC rules for naming coordination compounds. Name the following: (i) $[\text{Co(NH}_3)_6]\text{Cl}_3$ , (ii) $\text{K}_3[\text{Fe(CN)}_6]$ , (iii) $[\text{CoCl}_2\text{(en)}_2]\text{Cl}$ , (iv) $[\text{Pt(NH}_3)_2\text{Cl}_2]$ , (v) $\text{K}_2[\text{PdCl}_4]$ .

(CBSE 2024, 5-mark question)

Solution — Step by Step

Cation is named first, then anion (regardless of which is complex).
Within the coordination sphere, name ligands first (in alphabetical order), then the metal.
Anionic ligands end in -o (chloro, cyano, hydroxo). Neutral ligands keep their names except: NH $_3$ = ammine, H $_2$ O = aqua, CO = carbonyl, NO = nitrosyl.
Prefixes: di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for complex ligands (like en = ethylenediamine).
Metal in anionic complex gets the suffix -ate (ferrate, cobaltate, platinate).
Oxidation state of metal is written in Roman numerals in parentheses.

(i) $[\text{Co(NH}_3)_6]\text{Cl}_3$

Cation: $[\text{Co(NH}_3)_6]^{3+}$
Ligand: 6 ammine → hexaammine
Metal: cobalt(III)
Hexaamminecobalt(III) chloride

(ii) $\text{K}_3[\text{Fe(CN)}_6]$

Cation: K $^+$ (potassium)
Anion: $[\text{Fe(CN)}_6]^{3-}$
Ligand: 6 cyano → hexacyano
Metal in anionic complex: ferrate(III)
Potassium hexacyanoferrate(III)

(iii) $[\text{CoCl}_2\text{(en)}_2]\text{Cl}$

Ligands in alphabetical order: chloro (2 Cl) + en (2 ethylenediamine)
bis- for en (complex ligand): bis(ethylenediamine)
dichloro for 2 Cl
Dichlorobis(ethylenediamine)cobalt(III) chloride

(iv) $[\text{Pt(NH}_3)_2\text{Cl}_2]$

Neutral complex (no counter-ion)
Ligands: ammine (2) + chloro (2) → alphabetical: diamminedichloro
Diamminedichloroplatinum(II)

(v) $\text{K}_2[\text{PdCl}_4]$

Cation: potassium
Anion: $[\text{PdCl}_4]^{2-}$
Potassium tetrachloropalladate(II)

Why This Works

IUPAC nomenclature is systematic — once you know the rules, any coordination compound can be named unambiguously. The alphabetical ordering of ligands, the -ate suffix for anionic complexes, and the Roman numeral oxidation state eliminate all confusion.

The naming convention is essential because coordination compounds can have isomers (geometric, optical) that have the same formula but different arrangements. Proper naming distinguishes them.

Alternative Method — Working Backwards (Formula from Name)

Given the name “tetraamminecarbonatocobalt(III) chloride”:

Cation: $[\text{Co(NH}_3)_4\text{CO}_3]^+$ (4 ammine + 1 carbonato + Co(III))
Anion: Cl $^-$
Formula: $[\text{Co(NH}_3)_4\text{CO}_3]\text{Cl}$

For CBSE boards, always write the oxidation state calculation explicitly: in $[\text{Co(NH}_3)_6]\text{Cl}_3$ , let oxidation state of Co = $x$ . NH $_3$ is neutral, Cl $^-$ contributes -3. So $x + 0 = +3$ , hence Co is +3. This step-by-step approach fetches full marks.

Common Mistake

The most frequent error: using “di-” for ethylenediamine instead of “bis-”. For simple ligands (Cl, NH $_3$ ), use di-, tri-, tetra-. For complex/polysyllabic ligands (ethylenediamine, oxalate), use bis-, tris-, tetrakis- and enclose the ligand name in parentheses. Writing “diethylenediamine” instead of “bis(ethylenediamine)” is marked wrong.

Question

Solution — Step by Step

Why This Works

Alternative Method — Working Backwards (Formula from Name)

Common Mistake

Try These Next