Explain on the basis of valence bond theory that [Ni(CN)4]2– ion with square planar structure is diamagnetic and the [NiCl4]2– ion with tetrahedral geometry is paramagnetic.

To understand why the [Ni(CN)4]²⁻ ion exhibits a square planar structure and is diamagnetic, while the [NiCl4]²⁻ ion has a tetrahedral geometry and is paramagnetic, we can delve into the concepts of valence bond theory and crystal field theory. Both of these ions involve nickel in the +2 oxidation state, but their different ligands and geometries lead to distinct electronic configurations.

The Basics of Valence Bond Theory

Valence bond theory helps us explain the bonding in coordination complexes by considering how atomic orbitals combine to form bonds. In this context, the arrangement of ligands around the central metal ion and the nature of these ligands significantly affect the electronic structure of the complex.

Analyzing [Ni(CN)4]²⁻

• Nickel’s Electron Configuration: Nickel (Ni) has the electron configuration of [Ar] 3d⁸ 4s². In the +2 oxidation state, it loses two electrons, resulting in 3d⁸.
• Effect of CN^-: The cyanide ion (CN^-) is a strong field ligand, which means it causes a significant splitting of the 3d orbitals. This strong field splitting leads to the pairing of electrons in the lower energy 3d orbitals.
• Geometry and Hybridization: In the case of [Ni(CN)4]²⁻, the complex adopts a square planar geometry. The hybridization involves the 3d²sp² orbitals, which accommodates the paired electrons. As a result, all electrons are paired, leading to a diamagnetic property.

Paramagnetism in [NiCl4]²⁻

• Chloride Ligand Influence: In contrast, the chloride ion (Cl^-) is a weak field ligand. This weak field does not promote significant pairing of the 3d electrons, allowing unpaired electrons to remain in the higher energy orbitals.
• Tetrahedral Geometry: The [NiCl4]²⁻ complex adopts a tetrahedral structure, which involves sp³ hybridization. Here, the 3d orbitals do not participate as much in bonding, allowing for more unpaired electrons in the 3d orbitals.
• Electronic Configuration: In this tetrahedral arrangement, nickel retains two unpaired electrons in its d orbitals, resulting in a paramagnetic behavior due to the presence of these unpaired electrons.

Summary of Key Differences

In summary, the diamagnetism of [Ni(CN)4]²⁻ arises from strong field ligands causing electron pairing, while the weak field ligands in [NiCl4]²⁻ lead to unpaired electrons and paramagnetism. The differing geometries—square planar for [Ni(CN)4]²⁻ and tetrahedral for [NiCl4]²⁻—are fundamentally tied to the nature of the ligands involved and their effect on the electron configuration of the nickel ion.

This illustrates how the choice of ligands and the resulting molecular geometry significantly influence the electronic properties of coordination complexes.