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. |
| coordination bond | A coordination covalent bond where both electrons come from the ligand to an empty orbital on the metal |
| Coordination number | The total number of coordination 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
Tetrachloridocuprate(II): [CuCl4]2−
4 chlorido ligands (\(\text{Cl}^-\)) + copper in +2 oxidation state + overall negative complex charge.
= tetrachloridocuprate(II) ion (anionic suffix "-ate")
Pentaamminechloridocobalt(III): [CoCl(NH3)5]2+
5 neutral ammine (\(\text{NH}_3\)) + 1 chlorido (\(\text{Cl}^-\)) + cobalt in +3 state + cationic charge.
= pentaamminechloridocobalt(III) ion (alphabetical ligand ordering)
Coordination Number vs Number of Ligands
The coordination number is the number of coordinate bonds attached to the central transition metal ion, NOT simply the number of ligand molecules. For bidentate ligands (e.g. 1,2-ethanediamine, \(\text{en}\)) or hexadentate ligands (e.g. \(\text{EDTA}^{4-}\)), the coordination number exceeds the ligand count (e.g. \([\text{Fe(en)}_3]^{3+}\) has 3 ligands but coordination number 6).
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