high vs low spin

Question

[Fe(H2O)6]2+ vs [Fe(CN)6]4−: Which statement is correct?

RankGuru · Accepted Answer

1. **Metal Center**: Both complexes contain $Fe^{2+}$ with a $d^6$ electronic configuration.
2. **Spectrochemical Series**: Ligands vary in their ability to split the $d$-orbitals (Crystal Field Splitting Energy, $\Delta_o$).
3. **Analysis of $[Fe(H_2O)_6]^{2+}$**:
   - Water ($H_2O$) is a **weak field ligand**. $\Delta_o < Pairing Energy (P)$.
   - Electrons do not pair up; they occupy both $t_{2g}$ and $e_g$ levels ($t_{2g}^4 e_g^2$). 
   - Presence of 4 unpaired electrons $\implies$ **High Spin**.
4. **Analysis of $[Fe(CN)_6]^{4-}$**:
   - Cyanide ($CN^-$) is a **strong field ligand**. $\Delta_o > P$.
   - Electrons prefer to pair up in the lower $t_{2g}$ orbitals ($t_{2g}^6$).
   - Zero unpaired electrons $\implies$ **Low Spin**.

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high vs low spin

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