Chemistry - Coordination Chemistry Question with Solution | TestHub

The Effective Atomic Number (EAN) rule states that the sum of the atomic number of the central metal atom and the electrons donated by the ligands should equal the atomic number of the nearest noble gas.

For $[Fe(CO{)}_5]$:

Fe (Z=26) contributes 26 electrons.

5 CO ligands contribute $5\times 2=10$ electrons.

EAN = $26+10=36$.

For $[Fe(CO{)}_2(NO{)}_2]$:

Fe (Z=26) contributes 26 electrons.

2 CO ligands contribute $2\times 2=4$ electrons.

2 NO ligands contribute $2\times 3=6$ electrons (NO is a 3-electron donor).

EAN = $26+4+6=36$.

Difference in EAN = $36-36=0$.