Question

\( [\text{Cr(NH}_3) _6]^{3+} \) is paramagnetic while \( [\text{Ni(CN)}_4]^{2-} \) is diamagnetic. Explain why.

Hint:

- Strong field ligands like \( CN^- \) cause pairing of electrons, leading to diamagnetic behaviour. - Weak field ligands like \( NH_3 \) do not cause pairing, leading to paramagnetism.

Answer 1

\( [\text{Cr(NH}_3)_6]^{3+} \) is paramagnetic due to unpaired electrons, while \( [\text{Ni(CN)}_4]^{2-} \) is diamagnetic due to electron pairing.
Step 1: Electronic Configuration of Cr\(^{3+}\) and Ni\(^{2+}\) \[ \text{Cr}^{3+} = 3d^3, \quad \text{Ni}^{2+} = 3d^8 \] Step 2: Effect of Ligands on Magnetic Properties - \( NH_3 \) is a weak field ligand, so it does not cause electron pairing in \( [\text{Cr(NH}_3)_6]^{3+} \), leaving unpaired electrons in the \( t_{2g} \) orbitals. - \( CN^- \) is a strong field ligand, so it pairs electrons in \( [\text{Ni(CN)}_4]^{2-} \), making it diamagnetic.