Naming Complex Ions

Spec Mapping — OCR H432 Module 5.3.1 — Transition elements (complex-ion nomenclature), covering the systematic IUPAC naming of transition-metal complex ions: alphabetical ligand order using ligand-root letters (ignoring multiplying prefixes); standard ligand names (aqua, ammine, chloro/chlorido, hydroxo, cyano, carbonyl) with anionic ligands ending in -o; the Greek multiplying prefixes di-, tri-, tetra-, penta-, hexa- for simple ligands and bis-, tris-, tetrakis- for complicated or bidentate ligand names; the Latin-stem + -ate suffix (cuprate, ferrate, plumbate, aurate, stannate, manganate) for the metal in anionic complexes; the Roman-numeral oxidation state of the central metal enclosed in parentheses; and the two-way translation between formula (e.g. [Cu(NH₃)₄(H₂O)₂]²⁺) and name (tetraamminediaquacopper(II)) (refer to the official OCR H432 specification document for exact wording).

Naming complex ions is one of the few corners of A-Level chemistry where there is no flexibility — the IUPAC rules are precise, the spelling of ammine (two m's) must be exact, and a missing Roman numeral costs a mark every time it appears in a mark scheme. The good news is that the rules are simple, finite, and entirely learnable: five steps, six ligand names, six Latin metal stems, and a small set of Greek multiplying prefixes. The bad news is that examiners use this topic to make subtle decisions between band-discriminator candidates — hexaaquacopper(II) with single m loses the mark; hexaaquacopper(2) with an Arabic number loses the mark; tetrachlorocopper(II) without the -ate suffix on an anionic complex loses the mark. This lesson presents the rules systematically, gives ten worked examples covering every variation OCR has asked, and rehearses the formula↔name translations in both directions so that the candidate can read a complex name aloud, write its formula, and predict its structure without hesitation.

Key Definition — Nomenclature of complex ions is the systematic IUPAC method of naming a transition-metal complex by (1) listing ligands alphabetically by root with Greek multiplying prefixes; (2) appending the metal name (Latin stem + -ate if the complex is anionic; English name otherwise); and (3) adding the metal oxidation state in Roman numerals inside parentheses. The reverse process — converting a name into a formula — uses the same rules: identify each ligand and its count, identify the metal and its oxidation state, assemble inside square brackets with a superscript overall charge.

By the end of this lesson you should be able to:

• Apply the five-step IUPAC nomenclature algorithm to any first-row transition-metal complex
• Recall the six standard ligand names OCR examines: aqua (H₂O), ammine (NH₃), chloro/chlorido (Cl⁻), hydroxo (OH⁻), cyano (CN⁻), carbonyl (CO)
• Use Greek multiplying prefixes (di-, tri-, tetra-, penta-, hexa-) for simple ligands and bis-/tris-/tetrakis- for complicated/bidentate names
• Distinguish naming of cationic, neutral, and anionic complexes (Latin stem + -ate for anions: cuprate, ferrate, plumbate, aurate, stannate, manganate)
• Calculate the metal oxidation state from overall charge and ligand charges, and quote it as a Roman numeral in parentheses
• Convert fluently between formula and name in both directions for OCR's canonical complexes

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The Five-Step IUPAC Naming Algorithm

Every complex name you write follows the same five-step recipe. Memorise the recipe; the rest of the lesson is just practice applying it.

1. Identify each ligand in the complex and look up its IUPAC name from the table below.
2. Arrange ligands alphabetically by the first letter of the ligand root (ignore the Greek prefix when alphabetising).
3. Add Greek multiplying prefixes (di-, tri-, tetra-, penta-, hexa-) to show how many of each ligand are present.
4. Add the metal name: use the normal English name for cationic or neutral complexes; use the Latin stem + -ate for anionic complexes.
5. Append the oxidation state of the metal as a Roman numeral in parentheses, calculated from the overall charge and the ligand charges.

graph TD
  A["Complex ion formula<br/>e.g. [Cu(NH3)4(H2O)2]2+"] --> B["Step 1: identify ligands<br/>4 NH3 + 2 H2O"]
  B --> C["Step 2: alphabetise by root<br/>ammine before aqua"]
  C --> D["Step 3: Greek prefixes<br/>tetraammine + diaqua"]
  D --> E["Step 4: metal name<br/>cationic, so 'copper'"]
  E --> F["Step 5: Roman numeral<br/>+2 oxidation state = (II)"]
  F --> G["Final name:<br/>tetraamminediaquacopper(II)"]

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Standard Ligand Names

The ligand names used in nomenclature are not always the same as the molecule or ion names. Anionic ligands end in -o (older IUPAC) or -ido (newer 2005 IUPAC); OCR accepts both, but the -o form is what appears on past papers.

Ligand formula	Ligand name in complex	Donor atom	Charge
H₂O	aqua	O	0
NH₃	ammine (two m's!)	N	0
Cl⁻	chloro (or chlorido)	Cl	−1
OH⁻	hydroxo (or hydroxido)	O	−1
CN⁻	cyano (or cyanido)	C	−1
CO	carbonyl	C	0
F⁻	fluoro	F	−1
Br⁻	bromo	Br	−1
I⁻	iodo	I	−1
NO₂⁻	nitro (N-bound) or nitrito (O-bound)	N or O	−1
O²⁻	oxo	O	−2
H⁻	hydrido	H	−1
en	en or 1,2-diaminoethane (bis/tris)	2 × N	0
C₂O₄²⁻	ethanedioato (oxalato) (bis/tris)	2 × O	−2
EDTA⁴⁻	edta or ethylenediaminetetraacetato	2 × N + 4 × O	−4

Spelling alert: ammine has two m's (the NH₃ ligand); amine has one m (the organic R–NH₂ functional group). The double-m spelling distinguishes the two senses of the word — OCR mark schemes are unforgiving about this.

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Greek Multiplying Prefixes

Use the Greek prefixes for simple ligand names:

Number	Prefix	Example
1	(none)	[Ag(CN)₂]⁻ — dicyanoargentate(I), one Ag
2	di-	diaqua, dichloro, diammine
3	tri-	trichloro, triammine
4	tetra-	tetraaqua, tetrachloro, tetraammine
5	penta-	pentaaqua, pentaammine
6	hexa-	hexaaqua, hexaammine, hexacyano

Use the bis-/tris-/tetrakis- prefixes only when:

• The ligand name itself already contains a Greek prefix (e.g. en = 1,2-diaminoethane; using tri- would give "tri-1,2-diaminoethane", confusable with "trideaminoethane")
• The ligand name is multi-syllabic and the brackets aid clarity ([Ni(en)₃]²⁺ = tris(1,2-diaminoethane)nickel(II))
• The ligand name is a complex polydentate molecule

Number	bis-/tris- prefix	Example
2	bis-	bis(ethanedioato), bis(1,2-diaminoethane)
3	tris-	tris(ethanedioato), tris(1,2-diaminoethane)
4	tetrakis-	tetrakis(pyridine) (not common in OCR)

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Metal Names in Anionic Complexes — Latin Stems

When the overall charge on the complex is negative, the metal takes its Latin stem with the -ate suffix:

Metal	English name	Latin root	Anionic name	Example complex
Fe	iron	ferr-	ferrate	[Fe(CN)₆]³⁻ = hexacyanoferrate(III)
Cu	copper	cupr-	cuprate	[CuCl₄]²⁻ = tetrachlorocuprate(II)
Ag	silver	argent-	argentate	[Ag(CN)₂]⁻ = dicyanoargentate(I)
Au	gold	aur-	aurate	[AuCl₄]⁻ = tetrachloroaurate(III)
Pb	lead	plumb-	plumbate	[Pb(OH)₄]²⁻ = tetrahydroxoplumbate(II)
Sn	tin	stann-	stannate	[Sn(OH)₆]²⁻ = hexahydroxostannate(IV)
Mn	manganese	mangan-	manganate	MnO₄⁻ = tetraoxomanganate(VII) (permanganate)

Other transition metals (Co, Ni, Cr, Zn) keep their normal English name + -ate: cobaltate, nickelate, chromate, zincate. The -ate suffix is the cue that tells the reader the overall charge is negative — without it, a name like "hexacyanoiron(III)" is ambiguous (is the complex positive or negative?).

Cationic and neutral complexes keep the normal metal name with no suffix: copper, iron, cobalt, chromium, nickel, manganese.

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Calculating the Metal Oxidation State

The metal oxidation state is not explicitly written in the formula — you must calculate it from the overall complex charge and the sum of the ligand charges. The rule:

$x + \\sum \\text{(ligand charges)} = \\text{(overall complex charge)}$x+sumtext(ligandcharges)=text(overallcomplexcharge)

Where x is the metal oxidation state. Solve for x.

Complex	Ligand charges	Equation	Metal oxidation state
[Cu(H₂O)₆]²⁺	6 × 0 = 0	x + 0 = +2	+2 (II)
[CuCl₄]²⁻	4 × (−1) = −4	x − 4 = −2	+2 (II)
[Fe(CN)₆]³⁻	6 × (−1) = −6	x − 6 = −3	+3 (III)
[Co(NH₃)₆]³⁺	6 × 0 = 0	x + 0 = +3	+3 (III)
[Cr(OH)₆]³⁻	6 × (−1) = −6	x − 6 = −3	+3 (III)
MnO₄⁻	4 × (−2) = −8	x − 8 = −1	+7 (VII)
[Pt(NH₃)₂Cl₂]	0 + 2(−1) = −2	x − 2 = 0	+2 (II)
[Ni(en)(C₂O₄)₂]²⁻	0 + 2(−2) = −4	x − 4 = −2	+2 (II)
[Fe(EDTA)]⁻	−4	x − 4 = −1	+3 (III)

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Ten Worked Examples — Formula → Name

Example 1: [Cu(H₂O)₆]²⁺

• Ligands: 6 × H₂O = hexaaqua
• Metal: Cu, cationic, "copper"
• Oxidation state: x + 0 = +2 → x = +2 → (II)
• Name: hexaaquacopper(II)

Example 2: [Cu(NH₃)₄(H₂O)₂]²⁺

• Ligands: 4 × NH₃ + 2 × H₂O. Alphabetical: ammine before aqua (compare letters: amm < aqu)
• Greek prefixes: tetraammine + diaqua → "tetraamminediaqua"
• Metal: Cu, cationic, "copper"
• Oxidation state: x + 0 = +2 → (II)
• Name: tetraamminediaquacopper(II)

Example 3: [CuCl₄]²⁻

• Ligands: 4 × Cl⁻ = tetrachloro
• Metal: Cu, anionic (overall −2), so use Latin stem + -ate: "cuprate"
• Oxidation state: x + 4(−1) = −2 → x = +2 → (II)
• Name: tetrachlorocuprate(II)

Example 4: [Fe(CN)₆]⁴⁻

• Ligands: 6 × CN⁻ = hexacyano
• Metal: Fe, anionic (overall −4), so Latin stem + -ate: "ferrate"
• Oxidation state: x + 6(−1) = −4 → x = +2 → (II)
• Name: hexacyanoferrate(II) (older name: ferrocyanide — note that Fe(II)/Fe(III) complexes of CN⁻ have distinct historical names ferro-/ferri-)

For comparison, [Fe(CN)₆]³⁻ has Fe(III): x + 6(−1) = −3 → x = +3 → hexacyanoferrate(III) (ferricyanide).

Example 5: [Co(NH₃)₆]³⁺

• Ligands: 6 × NH₃ = hexaammine
• Metal: Co, cationic, "cobalt"
• Oxidation state: x + 0 = +3 → (III)
• Name: hexaamminecobalt(III)

Example 6: [Ag(NH₃)₂]⁺

• Ligands: 2 × NH₃ = diammine
• Metal: Ag, cationic, "silver"
• Oxidation state: 2(0) + x = +1 → x = +1 → (I)
• Name: diamminesilver(I) (this is Tollens' reagent, used to test for aldehydes)

Example 7: [Cr(OH)₆]³⁻

• Ligands: 6 × OH⁻ = hexahydroxo
• Metal: Cr, anionic (overall −3); Cr doesn't have a distinct Latin stem, so the form is "chromate" with -ate
• Oxidation state: x + 6(−1) = −3 → x = +3 → (III)
• Name: hexahydroxochromate(III)

This is the species formed when Cr(OH)₃ dissolves in excess NaOH (amphoteric behaviour, lesson 5).

Example 8: [Fe(H₂O)₅(OH)]²⁺

• Ligands: 5 × H₂O + 1 × OH⁻. Alphabetical: aqua before hydroxo
• Greek prefixes: pentaaqua + (mono)hydroxo → "pentaaquahydroxo"
• Metal: Fe, cationic, "iron"
• Oxidation state: 5(0) + (−1) + x = +2 → x = +3 → (III)
• Name: pentaaquahydroxoiron(III)

This species is formed when [Fe(H₂O)₆]³⁺ hydrolyses slightly in acidic aqueous solution (the OCR Module 5.3.1 qualitative-analysis link to acidic Fe³⁺ solutions).

Example 9: [Ni(en)₃]²⁺

• Ligands: 3 × en (1,2-diaminoethane). Because the ligand name itself starts with "di-", we use tris (not "tri") to multiply: tris(1,2-diaminoethane)
• Metal: Ni, cationic, "nickel"
• Oxidation state: 3(0) + x = +2 → x = +2 → (II)
• Name: tris(1,2-diaminoethane)nickel(II)

Example 10: [Cr(C₂O₄)₃]³⁻

• Ligands: 3 × C₂O₄²⁻ (ethanedioato). Bidentate, so use tris: tris(ethanedioato)
• Metal: Cr, anionic (overall −3), so chromate
• Oxidation state: 3(−2) + x = −3 → x = +3 → (III)
• Name: tris(ethanedioato)chromate(III) (an OCR favourite — chiral, optically active, Δ/Λ enantiomers; lesson 7)

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Alphabetical Order of Ligands — Detailed Rules