Question

Using valence bond approach, predict the shape and magnetism (i.e., paramagnetic or diamagnetic) of [Co(CN)₄]⁻.

Answer 1

1. The cyanide (CN⁻) ligand is a monodentate ligand with a charge of −1.
2. The overall charge of the complex is −1, so for [Co(CN)₄]⁻, the charge on cobalt must be +3.
3. Cobalt has an atomic number of 27, so the electron configuration of neutral Co is:
   Co: [Ar] 3d⁷ 4s²
4. For Co³⁺, cobalt loses three electrons (two from the 4s orbital and one from the 3d orbital), giving the electron configuration:
   Co³⁺: [Ar] 3d⁶
5. In [Co(CN)₄]⁻, the coordination number of cobalt is 4, as it is surrounded by four cyanide (CN⁻) ligands.
6. A coordination number of 4 suggests that the complex could adopt either a tetrahedral or square planar geometry.
7. Cyanide (CN⁻) is a strong field ligand, it causes significant pairing of electrons in the metal ion’s d-orbitals.
8. The Co³⁺ ion has 6 electrons in the 3d-orbitals. To minimize electron repulsion, these electrons will pair up in the lower-energy orbitals, leading to a low-spin configuration.
9. Given the strong-field ligands, cobalt will undergo d²sp³ hybridisation, resulting in square planar geometry (common for d⁶ systems with strong field ligands).
10. The electron configuration of Co³⁺ is 3d⁶.
11. Cyanide is a strong field ligand; it will cause the electrons to pair up in the t_2g​ orbitals (low-spin configuration).
12. As a result, all six electrons in the Co³⁺ ion will be paired.

Since there are no unpaired electrons in the complex, it is diamagnetic.