Question

Identify the complex having the highest number of unpaired electrons from following.

• A. \([\text{Co(NH}_3)_6]^{3+}\)
• B. \([\text{CoF}_6]^{3-}\)
• C. \([\text{NiCl}_4]^{2-}\)
• D. \([\text{Ni(CN)}_4]^{2-}\)

Hint:

Compare Co3+ high spin d6 in fluoride with Co3+ low spin and Ni2+ complexes.

Answer 1

The Correct Option is B

Step 1: Counts
Co3+ and Ni2+ have d6 and d8 respectively.

Step 2: Spin state
NH3 and CN- are strong field, so they pair electrons: 0 unpaired. F- is weak so d6 stays high spin: 4 unpaired. Cl- tetrahedral Ni2+: 2 unpaired.

Step 3: Compare
0, 4, 2, 0, so the largest is for the fluoro complex: option (B).

Final Answer:
Option (B). \[ \boxed{[\text{CoF}_6]^{3-}} \]