Question

What is EAN? Write its formula.

Answer 1

EAN is a number which gives the net value of electrons present in a complex. It is generally used to determine if the number of ligand atoms the metal ion tends to surround itself with is equal to the atomic number of nearest noble gas or not.
Complete step by step answer:
EAN is the abbreviation for Effective Atomic Number. It is the total number of electrons present around the central metal ion in a complex. We can also write that-
It is the sum of the number of electrons, donated by all the ligands present in the complex.
- Effective atomic number of the central metal ion in any complex is generally equal to the number of electrons in the nearest noble gas which is in the same period as that of the metal ion.
We can calculate the effective atomic number of any given atom using the simple formula which is-
EAN = [(atomic number of the central metal ion) – (oxidation state of the metal ion in the complex) + (the number of electrons gained by the metal ion from the ligand through bonding)]
We can also write the above formula as-
EAN = [(atomic number of the central metal ion) – (oxidation state of the metal ion in the complex) + (2$\times$ co-ordination number of the central metal ion)]
Coordination number = Number of ligands present inside the coordination sphere.
Let’s take an example, ${{K}_{4}}Fe{{(CN)}_{6}}$
Atomic number of iron = 26
Oxidation state of iron = +2
Number of electrons shared by the ligand = $2\times 6$= 12 (since there are 6 carbonyl groups so the coordination number will be 6)
$\therefore$EAN= [26-2+12] = 36
EAN number of the central metal ion, iron is 36.
As we know, iron lies in the fourth period and the noble gas corresponding to the fourth period is krypton. Atomic number of krypton is 36, which is equal to the EAN number of the central metal ion, Fe which lies in the fourth period too.
If the metal ion has the EAN number equal to the atomic number of nearest noble gas, the complex is stable.

Note: It is important to know here that EAN number may not be equal to the nearest noble gas configuration too. There are a few examples like potassium ferricyanide , where the EAN number comes out to be 35, which is not equal to the nearest noble gas configuration.
It is also important to remember here that coordination number is the number of atoms or ions bonded to the central metal ion, inside the coordination sphere.