Key Definitions
- Complex ion: A charged species consisting of a central metal cation bonded to surrounding molecules or anions (ligands) via coordination bonds.
- Ligand: A molecule or anion that donates a lone pair to the central metal ion, acting as a Lewis base.
- Coordination number: The total number of coordinate bonds formed between the ligands and the central metal ion.
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.
Writing Complex Ion Formulas
Complex ion formulas are written in square brackets [ ] with the overall charge as a superscript outside. The metal ion is written first, followed by the ligands.
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:
Charge Formula
Overall charge = (charge on metal ion) + (sum of charges on all ligands)
Neutral ligands (H₂O, NH₃) contribute 0. Charged ligands (Cl⁻, CN⁻) contribute their charge.
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]⁺ |
Think About It
Given the formula [Co(NH₃)₆]Cl₃, what is the charge on the complex ion and the oxidation state of cobalt?
There are 3 Cl⁻ ions outside the brackets, so the complex ion [Co(NH₃)₆] must have a charge of +3 to balance. Since NH₃ is neutral, cobalt must be Co³⁺ (oxidation state +3).
Common Exam Mistakes
- Forgetting that neutral ligands (H₂O, NH₃) do not contribute to the overall charge.
- Confusing coordination number with oxidation state. Coordination number = number of bonds to the metal.
- Not writing the complex ion in square brackets with the charge outside.
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