What is EAN of metal ion in hexacyanoferrate (II) ion?

The Effective Atomic Number (EAN) for the metal ion in $[\text{Fe}(\text{CN})_6]^{4-}$ is determined through systematic analysis of its electronic configuration.

The central metal ion is iron with atomic number $Z = 26$. The oxidation state calculation yields $x + 6 \times (-1) = -4$, resulting in $x = +2$ for $\text{Fe}^{2+}$.

The number of electrons in $\text{Fe}^{2+}$ is $26 - 2 = 24$. Each cyanide ligand donates 2 electrons, with 6 ligands contributing $6 \times 2 = 12$ electrons.

EAN $= 24 + 12 = 36$

Therefore, the Effective Atomic Number is $\boxed{36}$.