Question

Which complex is expected to be diamagnetic?
• \( [\mathrm{Fe(H_2O)_6}]^{2+} \)
• \( [\mathrm{CoF_6}]^{3-} \)
• \( [\mathrm{Ni(CN)_4}]^{2-} \)
• \( [\mathrm{Mn(H_2O)_6}]^{2+} \)

• A. \( [\mathrm{Fe(H_2O)_6}]^{2+} \)
• B. \( [\mathrm{CoF_6}]^{3-} \)
• C. \( [\mathrm{Ni(CN)_4}]^{2-} \)
• D. \( [\mathrm{Mn(H_2O)_6}]^{2+} \)

Hint:

Strong field ligands like: \[ \mathrm{CN^-}, \mathrm{CO} \] cause electron pairing and often produce diamagnetic complexes. Weak field ligands like: \[ \mathrm{H_2O}, \mathrm{F^-} \] usually give paramagnetic complexes.

Answer 1

The Correct Option is C

Concept: A substance is:
• Paramagnetic if it contains unpaired electrons.
• Diamagnetic if all electrons are paired. In coordination compounds, magnetic behavior depends upon:
• Oxidation state of the metal
• Electronic configuration
• Nature of ligand (strong field or weak field) Strong field ligands cause pairing of electrons, whereas weak field ligands usually do not. In the spectrochemical series: \[ \mathrm{CN^-} \] is a strong field ligand, while: \[ \mathrm{H_2O}, \mathrm{F^-} \] are weak field ligands.

Step 1: Analyze Option (1): \( [\mathrm{Fe(H_2O)_6}]^{2+} \) Oxidation state of iron: \[ x + 6(0) = +2 \] \[ x = +2 \] Electronic configuration of Fe: \[ \mathrm{Fe}: [Ar]3d^64s^2 \] Thus: \[ \mathrm{Fe^{2+}}: 3d^6 \] Water is a weak field ligand. Therefore, electrons remain unpaired in high-spin configuration. Hence, the complex is paramagnetic. Thus, Option (1) is incorrect.

Step 2: Analyze Option (2): \( [\mathrm{CoF_6}]^{3-} \) Oxidation state: \[ x + 6(-1) = -3 \] \[ x = +3 \] Thus: \[ \mathrm{Co^{3+}}: 3d^6 \] Fluoride is a weak field ligand. Therefore, the complex becomes high spin with unpaired electrons. Hence, it is paramagnetic. Thus, Option (2) is incorrect.

Step 3: Analyze Option (3): \( [\mathrm{Ni(CN)_4}]^{2-} \) Oxidation state: \[ x + 4(-1) = -2 \] \[ x = +2 \] Thus: \[ \mathrm{Ni^{2+}}: 3d^8 \] Cyanide (\(\mathrm{CN^-}\)) is a very strong field ligand. It causes pairing of electrons. The complex adopts square planar geometry with complete pairing of electrons. Hence, no unpaired electrons are present. Therefore, the complex is diamagnetic. Thus, Option (3) is correct.

Step 4: Analyze Option (4): \( [\mathrm{Mn(H_2O)_6}]^{2+} \) Oxidation state: \[ x + 6(0) = +2 \] \[ x = +2 \] Thus: \[ \mathrm{Mn^{2+}}: 3d^5 \] Water is a weak field ligand. Hence, all five electrons remain unpaired. Therefore, the complex is strongly paramagnetic. Thus, Option (4) is incorrect. Final Conclusion: The diamagnetic complex among the given options is: \[ \boxed{ [\mathrm{Ni(CN)_4}]^{2-} } \] Hence, the correct answer is: \[ \boxed{(3)} \]