The sum of the number of electrons, donated by all ligands and those present on the central metal ion or atom in complex is called as effective atomic number (EAN).
• Generally EAN of central metal ion will be equal to the number of electrons in the nearest noble gas.
• If the EAN of the central metal is equal to the number of electrons in the nearest noble gas then the complex possess greater stability.
EAN = [(atomic number of central metal) – (the oxidation state of the metal) + (the number of electrons gained by the metal from the ligands through co-ordination)] = [Z metal – (ox.state of the metal) + 2(coordination number of the metal)]. Except in case of NO 3 is multiplied to coordination number of the metal.
Here:
Z metal (Ni) = 28, coordination number of the metal { n(CN) = 4},and ox.state of the metal = 2
EAN = 28 – (2) + 2(4) = 34