The total number of unpaired electrons present in [Co(NH₃)₆]Cl₂ and [Co(NH₃)₆]Cl₃ is :

Solution

Chemistry ma’am · 3 steps

Find cobalt's oxidation state first. Then ask whether NH₃ can force pairing at that charge.

1. Oxidation states. NH₃ is neutral, so the number of chloride ions gives the charge on cobalt. [Co(NH₃)₆]Cl₂ has Co²⁺ (3d⁷) and [Co(NH₃)₆]Cl₃ has Co³⁺ (3d⁶).
2. Field strength. NH₃ sits in the middle of the spectrochemical series. With the higher charge on Co³⁺, the splitting Δₒ is larger than the pairing energy, so the electrons pair up. With Co²⁺, Δₒ is smaller than the pairing energy, so the electrons stay spread out.

   Complex	Metal ion	Spin and configuration	Unpaired electrons
   [Co(NH₃)₆]Cl₂	Co²⁺, d⁷	high spin, t_(2g)⁵ e_g² (sp³d²)	3
   [Co(NH₃)₆]Cl₃	Co³⁺, d⁶	low spin, t_(2g)⁶ e_g⁰ (d²sp³)	0

3. So. The total number of unpaired electrons is 3 + 0 = 3.

The common slip: 1. Treating NH₃ as strong field for both complexes makes Co²⁺ low spin (t_(2g)⁶ e_g¹, 1 unpaired), so the total comes out as 1, not 3.

The idea to keep. Whether a ligand pairs electrons depends on the metal too. A higher charge on the metal gives a larger Δₒ. That is why NH₃ pairs electrons on Co³⁺ but not on Co²⁺.

Answer from the NTA final answer key. Solution by PrepWiser.