What it does
Most stable organometallic and coordination complexes of the d-block hold 18 valence electrons around the metal, the same number as the next noble gas. Square-planar d⁸ complexes such as Wilkinson’s catalyst and cisplatin are the main exception at 16. Counting the electrons tells you whether a complex is saturated, whether it can bind another ligand, and what oxidation state the metal is in.
This tool uses the covalent (neutral ligand) method with Green’s covalent bond classification:
- An L ligand gives two electrons and carries no charge: CO, phosphines, amines, water, alkenes, H₂.
- An X ligand gives one electron and takes one away from the metal’s oxidation state: halides, hydride, alkyl, cyanide.
- Bigger ligands are combinations: η⁵-Cp is L₂X, η³-allyl and acac are LX, η⁶-benzene is L₃, oxo is X₂ and nitrido X₃.
Valence electrons = group number of the metal + 2 × L + X + metal–metal bonds − charge. Oxidation state = charge + X. The d-electron count is the group number minus the oxidation state.
The page also shows the ionic count (each X ligand as an anion giving two electrons), so you can see that both methods give the same answer, the [MLₗXₓ] classification and the ligand bond number.
How to use it
- Pick a worked example, or choose the metal and type the overall charge of the complex ion (0 for a neutral molecule).
- Add the ligands and how many of each. Count a chelating ligand such as en or acac once per ligand, and a ring once per ring with its hapticity.
- For a metal–metal bonded dimer, count one metal with its own ligands and add its metal–metal bonds.
Worked examples
- Ferrocene, Cp₂Fe: 8 + 2 × (4 + 1) = 18 electrons, Fe(II), d⁶.
- Wilkinson’s catalyst, RhCl(PPh₃)₃: 9 + 6 + 1 = 16, Rh(I), d⁸, square planar.
- [Co(NH₃)₆]³⁺: 9 + 12 − 3 = 18, Co(III), d⁶.
- Mn₂(CO)₁₀: each Mn has 7 + 10 + 1 (Mn–Mn) = 18, Mn(0).
- Titanocene dichloride: 4 + 10 + 2 = 16, Ti(IV), d⁰.
- Cobaltocene: 19 electrons, which is why it is so easily oxidised to the 18-electron cobaltocenium ion.
Questions
Why do some books give different numbers? Older texts count NO, carbenes and some rings differently, and the ionic method can give a different oxidation state for ligands such as NO. The total count does not change. This tool leaves NO out because its linear and bent forms count differently.
Does every complex obey the rule? No. Early metals (d⁰ to d²), bulky ligands, square-planar d⁸ complexes and many simple Werner complexes such as [Ni(H₂O)₆]²⁺ (20 electrons) do not.
Is anything sent anywhere? No. Everything is calculated in your browser, and the page works offline once loaded.
Where the numbers come from
- Electron counting: Green's covalent bond classification (L and X ligands), as in standard textbooks.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-08.