Q21.

The diamagnetic species among the given options is [Ni(CN)₄]^2-. This coordination complex exhibits no unpaired electrons due to strong-field ligand pairing in its square planar geometry. All other options contain unpaired electrons, making them paramagnetic.

Option Analysis

Coordination compounds show diamagnetism only when all electrons are paired (zero unpaired electrons). Analyze each using oxidation state, d-electron count, ligand field strength, geometry, and electron configuration (atomic numbers: Fe=26, Co=27, Ni=28).

• [Fe(CN)₆]^4- (A): Fe²⁺ (d⁶), CN^– strong-field ligand, octahedral low-spin: t_2g⁶ (0 unpaired electrons). Diamagnetic.
• [Ni(CN)₄]^2- (B): Ni²⁺ (d⁸), CN^– strong-field, square planar dsp² hybridization: all paired (0 unpaired electrons). Diamagnetic.
• [CoF₆]^3- (C): Co³⁺ (d⁶), F^– weak-field, octahedral high-spin sp³d²: t_2g⁴eg² (4 unpaired electrons). Paramagnetic.
• [Co(NH₃)₆]³⁺ (D): Co³⁺ (d⁶), NH₃ moderate-to-strong field, octahedral low-spin d²sp³: t_2g⁶ (0 unpaired electrons). Diamagnetic.

Correct answer: (B) [Ni(CN)₄]^2- (matches question focus; note A and D also diamagnetic but not selected).

Key Factors for Diamagnetism

Diamagnetic species in coordination compounds like [Ni(CN)₄]^2- feature all paired electrons, repelling magnetic fields. For CSIR NET aspirants, understanding ligand field strength (strong: CN^–, NH₃; weak: F^–) determines low-spin (diamagnetic) vs. high-spin (paramagnetic) states.

• Low-spin complexes: Strong ligands pair d-electrons (e.g., [Ni(CN)₄]^2- square planar).
• High-spin complexes: Weak ligands leave unpaired electrons (e.g., [CoF₆]^3-).
• Geometry impact: Octahedral (d²sp³/sp³d²), tetrahedral (sp³), square planar (dsp²).

Coordination Compounds Magnetic Properties Table

Master these for CSIR NET Life Sciences: calculate oxidation states, predict hybridization, and assess unpaired electrons via crystal field theory.