HL Extension
Complex Ion Terminology
- Complex Ion: Central metal cation bonded to surrounding ligands via coordination bonds.
- Ligand: Neutral molecule or anion containing at least one lone pair that donates to the central metal ion (Lewis base).
- Coordination Number: Total number of coordination bonds formed to the central metal cation (typically 6 octahedral, 4 tetrahedral/square planar, 2 linear).
How Complex Ions Form
Transition metal cations have empty d orbitals that can accept lone pairs from ligands. Each ligand acts as a Lewis base and donates an electron pair to the metal ion (Lewis acid), forming a coordination bond.
Formula Convention
Writing Complex Ion Formulas
Enclosed in square brackets \([\;]\) with overall charge outside:
\[[\text{Fe}(\text{H}_2\text{O})_6]^{3+}, \quad [\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}, \quad [\text{Fe}(\text{CN})_6]^{4-}\]Common Ligands
| Ligand | Formula | Charge | Lone pairs donated |
|---|---|---|---|
| Water | H₂O | 0 | 1 (from O) |
| Ammonia | NH₃ | 0 | 1 (from N) |
| Chloride | Cl⁻ | -1 | 1 (from Cl) |
| Cyanide | CN⁻ | -1 | 1 (from C) |
Deducing the Charge on a Complex Ion
This is a key exam skill. The overall charge is calculated as:
HL Extension
Deducing Metal Oxidation State and Complex Charge
\[\mathbf{\text{Overall Charge} = \text{Oxidation State of Metal} + \sum(\text{Charges on all Ligands})}\]
Worked Examples
| Complex ion | Metal charge | Ligand charges | Overall charge | Coord. no. |
|---|---|---|---|---|
| [Cu(H₂O)₆]²⁺ | Cu²⁺ = +2 | 6 x 0 = 0 | +2 | 6 |
| [Fe(CN)₆]³⁻ | Fe³⁺ = +3 | 6 x (-1) = -6 | -3 | 6 |
| [CuCl₄]²⁻ | Cu²⁺ = +2 | 4 x (-1) = -4 | -2 | 4 |
| [Ag(NH₃)₂]⁺ | Ag⁺ = +1 | 2 x 0 = 0 | +1 | 2 |
Coordination Number and Geometry
| Coordination number | Common geometry | Example |
|---|---|---|
| 6 | Octahedral | [Cu(H₂O)₆]²⁺ |
| 4 | Tetrahedral or square planar | [CuCl₄]²⁻ (tetrahedral) |
| 2 | Linear | [Ag(NH₃)₂]⁺ |
Key Insight
Deducing Cobalt Oxidation State in Hexamminecobalt
In \([\text{Co}(\text{NH}_3)_6]\text{Cl}_3\), the complex ion is \([\text{Co}(\text{NH}_3)_6]^{3+}\). Since \(\text{NH}_3\) is a neutral ligand (charge = 0), the oxidation state of cobalt is +3.
Examiner Trap
Coordination Number vs Oxidation State
- Neutral ligands: Remember \(\text{H}_2\text{O}\) and \(\text{NH}_3\) have charge 0.
- Coordination number ≠ Oxidation state: In \([\text{Fe}(\text{CN})_6]^{3-}\), coordination number = 6, but Fe oxidation state = +3.
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