Q.Calculate the Effective Atomic Number (EAN) of iron in $[\text{Fe}(\text{CN})_6]^{4-}$ and state which noble gas it corresponds to.

[!TLDR] $\text{EAN} = Z - x + 2(\text{CN})$; Fe is $+2$ here (from the $-4$ complex charge and six $-1$ CN$^-$ ligands). [!ANS …

To apply the EAN formula, iron's oxidation state in $[\text{Fe(CN)}_6]^{4-}$ must first be found: six $\text{CN}^-$ ligands contribute $6\times(-1) = -6$ to the overall $-4$ charge of the complex, so $x + (-6) = -4 \Rightarrow x = +2$. Iron's atomic number is $Z = 26$, and the coordination number is 6 (six monodentate cyanide ligands). Substituting into $\text{EAN} = Z - x + 2(\text{CN})$: $\text{EAN} = 26 - 2 + 2(6) = 24 + 12 = 36$. The noble gas with atomic nu …

Determine the metal's oxidation state from overall charge balance; substitute $Z$, $x$, and the coordination …

Using the wrong oxidation state for iron by forgetting to first solve the charge-balance equation, or forgetting the factor of 2 (each donor atom contri …