Naming Complex Ions

Learning Objectives (OCR Spec 5.3.1)

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

• Apply IUPAC nomenclature rules to first-row transition metal complex ions
• Use the correct ligand prefixes: aqua, ammine, chloro, hydroxo, cyano, carbonyl
• Use Greek multiplying prefixes: di, tri, tetra, penta, hexa (and bis, tris, tetrakis for bidentate)
• State the oxidation state in Roman numerals in the name
• Distinguish naming of cationic, neutral and anionic complexes
• Convert a given name to a formula and vice versa

The Naming Rules

The full IUPAC nomenclature has many subtleties but at A-Level OCR you need five straightforward rules:

1. Name the ligands first, alphabetically (by root, not by prefix).
2. Use the correct ligand names (see table below).
3. Use Greek prefixes (di, tri, tetra, penta, hexa) to show how many of each ligand.
4. Name the metal next; if the complex is anionic (overall negative charge), use the Latin root for the metal and add -ate (e.g. cuprate for copper, ferrate for iron).
5. Follow the metal name with the oxidation state in Roman numerals in brackets, e.g. (II), (III), (VII).

Ligand Names

The ligand names used in nomenclature differ from the molecule or ion names:

Ligand formula	Ligand name in complex
H2O	aqua
NH3	ammine (two m's)
Cl-	chloro
OH-	hydroxo
CN-	cyano
CO	carbonyl
F-	fluoro
Br-	bromo
I-	iodo
NO2-	nitro
O2-	oxo
H-	hydrido

Spelling matters: ammine (two m's) is the NH3 ligand; amine (one m) is an organic functional group R-NH2. The same word would confuse both meanings, hence the double m.

Metal Names in Anionic Complexes

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

Metal	Latin root	Anionic complex name
Fe	ferr-	ferrate
Cu	cupr-	cuprate
Ag	argent-	argentate
Au	aur-	aurate
Pb	plumb-	plumbate
Sn	stann-	stannate

Other metals keep their normal English name + -ate: cobaltate, nickelate, manganate, chromate (the -ate suffix is what tells you the overall charge is negative).

Cationic and neutral complexes keep the normal metal name: iron, copper, cobalt, chromium, nickel, etc.

Worked Examples

Example 1: [Cu(H2O)6]2+

Ligands: 6 x H2O = hexaaqua Metal: Cu, cationic, so "copper" Oxidation state: +2 (6 x 0 + x = +2, so x = +2) = (II)

Name: hexaaquacopper(II)

Example 2: [Cu(NH3)4(H2O)2]2+

Ligands: 4 x NH3 + 2 x H2O. Alphabetical order by root: ammine before aqua. So "tetraamminediaqua". Metal: Cu, cationic, so "copper" Oxidation state: +2

Name: tetraamminediaquacopper(II)

Example 3: [CuCl4]2-

Ligands: 4 x Cl- = tetrachloro Metal: Cu, anionic (overall -2), so use Latin root + -ate: "cuprate" Oxidation state: 4(-1) + x = -2, so x = +2 -> (II)

Name: tetrachlorocuprate(II)

Example 4: [Fe(CN)6]3-

Ligands: 6 x CN- = hexacyano Metal: Fe, anionic (overall -3), so use Latin root + -ate: "ferrate" Oxidation state: 6(-1) + x = -3, so x = +3 -> (III)

Name: hexacyanoferrate(III) (old name: ferricyanide)

Example 5: [Co(NH3)6]3+

Ligands: 6 x NH3 = hexaammine Metal: Co, cationic, "cobalt" Oxidation state: +3

Name: hexaamminecobalt(III)

Example 6: [Ag(NH3)2]+

Ligands: 2 x NH3 = diammine Metal: Ag, cationic, "silver" Oxidation state: 2(0) + x = +1, x = +1 -> (I)