Effective Atomic Number Rule
The observation that stable metal carbonyls tend to have a total electron count equal to that of the next noble gas.
Formula / equation
EAN = Z − oxidation state + 2n(ligands)
Unit
Dimensionless
In depth
Equivalent to the 18-electron rule, it reflects the filling of all nine valence orbitals — one s, three p and five d. Complexes obeying it are generally stable and diamagnetic, and deviations often signal high reactivity or a coordinatively unsaturated centre.
Examples in the real world
Ni(CO)₄ reaches 36, matching krypton; Fe(CO)₅ and Cr(CO)₆ likewise achieve 36.