The hybridization and magnetic behaviour of cobalt ion in [Co(NH₃)₆]³⁺ complex, respectively is :

Solution

Chemistry ma’am · 4 steps

Find the d count of Co, then ask whether ammonia is strong enough to pair those electrons up.

1. Oxidation state. NH₃ is neutral, so cobalt carries the full +3 charge. Co is [Ar] 3d⁷ 4s², so Co³⁺ is 3d⁶.
2. Ammonia pairs electrons. With a 3+ metal ion, NH₃ acts as a strong-field ligand. The six 3d electrons pair up into three orbitals, leaving two 3d orbitals empty.

   Co³⁺ (3d⁶): ↑↓ ↑↓ ↑↓_(paired) □ □_(used by NH₃)

3. Hybridization. The six NH₃ lone pairs go into two inner 3d orbitals, one 4s and three 4p orbitals. That is d²sp³, an inner-orbital octahedral complex.
4. So. No electron is left unpaired, so the complex is diamagnetic. The answer is (c): d²sp³ and diamagnetic.

The idea to keep. For Co³⁺ (d⁶) with ligands like NH₃ or CN⁻, the electrons pair up, so you get inner-orbital d²sp³ and zero unpaired electrons. Weak F⁻ gives outer sp³d² instead.

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