Question

Using the valence bond approach, predict the shape and magnetic character of [Ni(CO)₄].

Answer 1

1. The carbon monoxide (CO) ligand is a neutral ligand. 
2. The complex [Ni(CO)₄] has no overall charge, the oxidation state of nickel is 0.
3. Nickel has an atomic number of 28, so the electron configuration of neutral Ni is:
   Ni : [Ar] 3d⁸ 4s²
4. The electron configuration of Ni⁰ remains the same as that of neutral nickel:
   Ni⁰ : [Ar] 3d⁸ 4s²
5. Ni⁰ has 10 valence electrons (8 in the 3d orbitals and 2 in the 4s orbitals).
6. In the complex [Ni(CO)₄, the coordination number of Ni is 4, as there are four CO ligands coordinated to the nickel ion.
7. A coordination number of 4 suggests that the complex can adopt a tetrahedral or square planar geometry.
8. CO is a neutral ligand and a strong field ligand, it will cause significant electron pairing.
9. In Ni⁰, due to the strong-field nature of CO, nickel will undergo sp³ hybridisation, using one s-orbital and three p-orbitals to form four hybrid orbitals.
10. These four hybrid orbitals will overlap with the lone pairs of electrons from the four CO ligands, leading to a tetrahedral geometry.
11. The Ni⁰ ion has 10 valence electrons in its 3d⁸ 4s² electron configuration. CO is a strong field ligand, it induces pairing of electrons in the d-orbitals of nickel.
12. In this case, all the electrons in the Ni⁰ ion will be paired, meaning there are no unpaired electrons.

Thus, [Ni(CO)₄] is diamagnetic because there are no unpaired electrons.