number that represents the total number of electrons surrounding the nucleus of a metal atom in a metal complex. It is composed of the metal atom's electrons and the bonding electrons from the surrounding electron-donating atoms and molecules. Thus the effective atomic number of the cobalt atom in the complex 【Co(NH₃)₆】³⁺ is 36, the sum of the number of electrons in the trivalent cobalt ion (24) and the number of bonding electrons from six surrounding ammonia molecules, each of which contributes an electron pair (2 × 6 =12).

The English chemist Nevil V. Sidgwick (Sidgwick, Nevil Vincent) made the observation, since known as the EAN rule, that in a number of metal complexes the metal atom tends to surround itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal is situated. This rule seems to hold for most of the metal complexes with carbon monoxide, the metal carbonyls as well as many organometallic compounds. By using this rule it is possible to predict the number of ligands in these types of compounds and also the products of their reactions. The EAN rule is often referred to as the “18-electron rule” since, if one counts only valence electrons (6 for Co³⁺ and 2 × 6 = 12 for 6 NH₃), the total number is 18.