🟣 This is Higher Level (HL) content.
📘 Key Definitions
| Term | Definition |
|---|---|
| Complex ion | A central transition metal atom/ion bonded to surrounding molecules or ions called ligands |
| Ligand | A neutral molecule or negative ion with at least one lone pair of electrons that it donates to the metal. Acts as a Lewis base. |
| Coordinate bond | A dative covalent bond where both electrons come from the ligand to an empty orbital on the metal |
| Coordination number | The total number of coordinate bonds formed between the metal ion and its ligands |
Common Ligands
| Ligand | Formula | Type | Name in Complex |
|---|---|---|---|
| Water | H₂O | Neutral | aqua |
| Ammonia | NH₃ | Neutral | ammine |
| Carbon monoxide | CO | Neutral | carbonyl |
| Chloride | Cl⁻ | Anionic | chlorido |
| Cyanide | CN⁻ | Anionic | cyanido |
| Hydroxide | OH⁻ | Anionic | hydroxido |
Geometric Shapes
| Coord. No. | Shape | Bond Angle | Example |
|---|---|---|---|
| 6 | Octahedral | 90° | [Fe(H₂O)₆]³⁺ |
| 4 | Tetrahedral | 109.5° | [CuCl₄]²⁻, [CoCl₄]²⁻ |
| 4 | Square planar | 90° | [Ni(CN)₄]²⁻ |
| 2 | Linear | 180° | [Ag(NH₃)₂]⁺ |
Note: Larger ligands (e.g. Cl⁻) tend to form tetrahedral complexes (coord. no. 4) because fewer can fit around the metal. Smaller ligands (e.g. H₂O, NH₃) form octahedral complexes (coord. no. 6).
Octahedral Complex [Fe(H₂O)₆]³⁺
IUPAC Naming Rules
- Ligands first (in alphabetical order), then the metal
- Use Greek prefixes: di-, tri-, tetra-, penta-, hexa-
- Anionic ligands end in -ido or -o (e.g. chlorido, cyanido)
- Neutral ligands keep their name (except: H₂O = aqua, NH₃ = ammine, CO = carbonyl)
- Metal oxidation state in Roman numerals in parentheses
- If the complex is an anion, the metal name ends in -ate (e.g. cuprate, ferrate, cobaltate)
Worked Examples
[CuCl₄]²⁻
4 chlorido ligands + copper in +2 oxidation state + overall anionic complex
= tetrachloridocuprate(II)
[CoCl(NH₃)₅]²⁺
5 ammine + 1 chlorido (alphabetical: ammine before chlorido) + cobalt in +3 + cationic
= pentaamminechloridocobalt(III)
⚠️ Exam Tip
The coordination number is NOT always the same as the number of ligands. Some ligands (bidentate, polydentate) can form more than one coordinate bond. At HL, you mainly deal with monodentate ligands where coord. no. = no. of ligands.