Using valence bond theory, explain the hybridisation, shape and magnetic behaviour of [Co(NH₃)₆]³⁺ and [CoF₆]³⁻. Calculate the spin-only magnetic moment of each.

1. 1 · State the principle. In VBT, the metal mixes empty (n−1)d, ns, np or nd orbitals into hybrid orbitals, which accept lone pairs from the ligands.
2. 2 · Write the configuration. Co (Z = 27): [Ar] 3d⁷ 4s². Co³⁺: 3d⁶, with 4 unpaired electrons as a free ion.
3. 3 · [Co(NH₃)₆]³⁺. NH₃ pairs the electrons: 3d ↑↓ ↑↓ ↑↓ _ _. d²sp³, octahedral, inner orbital, 0 unpaired, diamagnetic, μ = 0.
4. 4 · [CoF₆]³⁻. F⁻ is weak, no pairing: 3d ↑↓ ↑ ↑ ↑ ↑. sp³d² using 4d, octahedral, outer orbital, 4 unpaired, μ = √24 = 4.90 BM.
5. 5 · Draw the orbital boxes. For each complex, draw boxes for 3d, 4s, 4p (and 4d); show metal electrons as arrows and ligand pairs as crosses.
6. 6 · Link to real life. Strongly paramagnetic complexes, such as gadolinium chelates, are used as MRI contrast agents.
7. 7 · Close in one line. So the same Co³⁺ ion is diamagnetic or paramagnetic depending on whether the ligand forces pairing.

Scoring tip: Show the orbital boxes before and after bonding. The electron count earns most of the marks.