Discuss the nature of bonding in the following coordination entities on the basis of valence bond theory:

(i) [Fe(CN)6]4−

In the above coordination complex, iron exists in the +II oxidation state.

Fe2+ : Electronic configuration is 3d6

Orbitals of Fe2+ ion:

As CN− is a strong field ligand, it causes the pairing of the unpaired 3d electrons.

Since there are six ligands around the central metal ion, the most feasible hybridization is d2sp3.

d2sp3 hybridized orbitals of Fe2+ are:

6 electron pairs from CN− ions occupy the six hybrid d2sp3orbitals.

Then,

Hence, the geometry of the complex is octahedral and the complex is diamagnetic (as there are no unpaired electrons).

(ii) [FeF6]3−

In this complex, the oxidation state of Fe is +3.

Orbitals of Fe+3 ion:

There are 6 F− ions. Thus, it will undergo d2sp3 or sp3d2 hybridization. As F− is a weak field ligand, it does not cause the pairing of the electrons in the 3d orbital. Hence, the most feasible hybridization is sp3d2.

sp3d2 hybridized orbitals of Fe are:

Hence, the geometry of the complex is found to be octahedral.

(iii) [Co(C2O4)3]3−

Cobalt exists in the +3 oxidation state in the given complex.

Orbitals of Co3+ ion:

Oxalate is a weak field ligand. Therefore, it cannot cause the pairing of the 3d orbital electrons. As there are 6 ligands, hybridization has to be either sp3d2 or d2sp3 hybridization.

sp3d2 hybridization of Co3+:

The 6 electron pairs from the 3 oxalate ions (oxalate anion is a bidentate ligand) occupy these sp3d2 orbitals.

Hence, the geometry of the complex is found to be octahedral.

(iv) [CoF6]3−

Cobalt exists in the +3 oxidation state.

Orbitals of Co3+ ion:

Again, fluoride ion is a weak field ligand. It cannot cause the pairing of the 3d electrons. As a result, the Co3+ ion will undergo sp3d2 hybridization.

sp3d2 hybridized orbitals of Co3+ ion are:

Hence, the geometry of the complex is octahedral and paramagnetic.