The 18-Electron Rule
Concept Overview
The 18-Electron Rule is the inorganic equivalent of the octet rule. Transition metals have 9 valence orbitals (1s + 3p + 5d). Filling all with paired electrons requires exactly 18 electrons, giving a highly stable closed-shell configuration.
Two counting methods exist:
- Neutral Atom (Covalent) Method: Metal is treated as neutral (oxidation state 0). Ligands as neutral radicals.
- Donor-Pair (Ionic) Method: Metal is assigned its formal oxidation state. Ligands as closed-shell ions.
Important exception: 16-electron complexes are very stable for metals in square planar geometry ().
Key Equations
| Ligand | Covalent (e⁻) | Ionic (e⁻) |
|---|---|---|
| CO | 2 | 2 |
| PR₃ | 2 | 2 |
| Cl⁻ | 1 | 2 |
| η⁵-Cp | 5 | 6 |
| η²-C₂H₄ | 2 | 2 |
| H⁻ | 1 | 2 |
| M–M bond | 1 | 1 |
Worked Examples
Electron Counting for Ferrocene
Count valence electrons for using the ionic method.
Electron Counting for Mn₂(CO)₁₀
Count valence electrons per Mn center using the covalent method.
Common Misconceptions
❌ Misconception
Any complex without 18 electrons is unstable and cannot exist.
✅ Correction
The 18e rule is a guideline. Many stable complexes exist with 16e (square planar catalysts) or even 19-20e (metallocenes with anti-bonding occupancy).
Interactive Visual
Select a metal and add ligands to count valence electrons. Toggle between ionic and covalent methods — both yield the same total!
Add Ligands:
Current Ligands: