(a) What type of isomerism is shown by the complex [Co(NH₃)₅(SCN)]²⁺?

(b) Why is [NiCl₄]^2– paramagnetic while [Ni(CN)₄]^2– is diamagnetic?

(Atomic number of Ni = 28)

(c) Why are low spin tetrahedral complexes rarely observed?

(a) [Co(NH₃)₅(SCN)]²⁺ shows linkage isomerism.

Linkage isomerism: Isomerism of this type occurs in compounds containing ambidentate ligands, i.e., when more than one atom in a unidentate ligand may function as a donor. For example, in the case of ligand SCN^-, either nitrogen or an sulphur atom may act as a donor giving two different isomers.

Thus, two different have been prepared each containing the SCN group in the complex ion.

Note: Unidentate ligands are the ligands which have only one donor site. Example: F^-, Cl^- etc.

Ambidentate ligands are the ligands which can attach them with the central metal atom by two different atoms.

Example:

(b) In , Nickel is in +2 oxidation state, i.e., nickel is present as Ni²⁺ ion.

Theatomic number of Nickel (Ni) is Z = 28

The electronic configuration of ₂₈Ni= [Ar] 3d⁸ 4s²

And, the electronic configuration of Ni²⁺=[Ar] 3d⁸

The formation of may be explained through hybridization as follows:

Here, we can see that in , Cl- is a weak ligand. It is therefore, unable to pair up the unpaired electrons of the 3d orbital. Hence, the hybridization involved is sp³ (involving one 4s and three 4p orbitals) and the shape is tetrahedral. Since, there are two unpaired electrons, hence it is paramagnetic.

In , Nickel is in +2 oxidation state, i.e., nickel is present as Ni²⁺ ion.

Theatomic number of Nickel (Ni) is Z = 28

The electronic configuration of ₂₈Ni= [Ar] 3d⁸ 4s²

And, the electronic configuration of Ni²⁺=[Ar] 3d⁸

The formation of may be explained through hybridization as follows:

Here, we can see that in , CN- is a strong ligand. It is therefore approaches the metal ion, the electrons must pair up. Hence, the hybridization involved is dsp² (involving one 3d, one 4s and two 4p orbitals) and the shape is square planar. There are no unpaired electrons, hence it is diamagnetic.

(c) low spin tetrahedral complexes are rarely observed.

Crystal field theory for tetrahedral complexes:

The splitting pattern for tetrahedral complexes is just the reverse of the splitting pattern of the octahedral complexes. Moreover the spliiting is much smaller than that in case of octahedral complexes. The difference of energy, represented by Δ_t ≈ 4/9 Δ_° . This energy is so small that it is unable to force the electrons to pair up. Hence, low spin tetrahedral complexes are rarely observed. have high spin configuration.