Question

[NiCl₄]²⁻ is paramagnetic, while [Ni(CO)₄] is diamagnetic, though both are tetrahedral. Why? (Atomic number of Ni = 28)

Why is [NiCl₄]^2– paramagnetic but [Ni(CO)₄] is diamagnetic? (At. nos.: Cr = 24, Co = 27, Ni = 28)

[NiCl₄]²⁻ is paramagnetic, while [Ni(CO)₄] is diamagnetic, though both are tetrahedral. Why?

Answer 1

In [Ni(CO)₄], nickel is in zero oxidation state, whereas in [NiCl₄]²⁻ it is in +2 oxidation state. In the presence of CO ligand, the unpaired d-electrons of nickel get paired, but Cl^– being a weak ligand is not able to pair the unpaired electrons. Hence, there is no unpaired electron present in [Ni(CO)₄], so it is diamagnetic and due to the presence of unpaired electrons in [NiCl₄]²⁻, it is paramagnetic.

Answer 2

In [NiCl₄]²⁻, the oxidation state of Ni is +2. Chloride is a weak field ligand and does not cause pairing up of electrons against the Hund’s rule of maximum multiplicity. As a result, two unpaired electrons are present in the valence d-orbitals of Ni, which impart paramagnetic character to the complex. On the other hand, carbonyl is a strong field ligand and causes pairing up of electrons against the Hund’s rule of maximum multiplicity. As a result, no unpaired electrons are present, and hence, the complex is diamagnetic.

Answer 3

[NiCl₄]²⁻ and [Ni(CO)₄] both are tetrahedral. But their magnetic characters are different. This is due to differences in the nature of the ligands.

Ni⁺² = [Ar] 4s⁰3d⁸

Ni⁺² has 2 unpaired electrons; hence, this complex is paramagnetic.

In Ni(CO)₄, Ni is in the zero oxidation state, i.e., it has a configuration of 3d⁸4s².

Ni = [Ar] 4s² 3d⁸

But CO is a strong field ligand. Therefore, it causes the pairing of unpaired 3d electrons. Also, it causes the 4s electrons to shift to the 3d orbital, thereby giving rise to sp³ hybridisation. Since no unpaired electrons are present in this case, [Ni(CO)₄] is diamagnetic.