Problem 164 - A Complete Resource Book in Chemistry for JEE Main

Question

Nickel \((\mathrm{Z}=28)\) combines with a uninegative monodentate ligand \(\mathrm{X}^{-}\)to form a paramagnetic complex \(\left[\mathrm{Ni} \mathrm{X}_{4}\right]^{2-}\), the number of unpaired electrons in nickel and the geometry of this complex ion is [2006] (a) one, tetrahedral (b) two, tetrahedral (c) one, square planar (d) two, square planar

Step-by-Step Solution

Answer

(b) two, tetrahedral

Step 1: Understand the Nickel Electron Configuration

Begin by identifying the electron configuration for Nickel with atomic number 28. The ground state configuration is \[ ext{Ni}: [ ext{Ar}] ext{3d}^8 ext{4s}^2. \] When forming the complex, Nickel loses 2 electrons from the \(4s\) to become \([Ni]^{2+}\) with the configuration \[ [ ext{Ar}] ext{3d}^8. \]

Step 2: Determine Electronic Configuration in Complex

The complex \([ ext{NiX}_4]^{2-}\) shows paramagnetism, indicating unpaired electrons remain. In this complex, Nickel maintains its \(3d^8\) configuration, as ligands fill \(4s\) and \(4p\) orbitals, \(3d\) remains unaffected.

Step 3: Assess Electron Pairing and Geometry Influence

Since \(Ni^{2+}\) retains paramagnetism and \([ ext{NiX}_4]^{2-}\) is tetrahedral (following VSEPR and ligand field strength of uninegative ligands), the geometry favors a high spin state resulting in 2 unpaired electrons. \(d^8\) usually leads to square planar with strong field ligands, but here's confirmed tetrahedral.

Key Concepts

Paramagnetic Complex

A paramagnetic complex is characterized by the presence of unpaired electrons in its structure, which makes it attracted by a magnetic field. Paramagnetism arises because unpaired electrons create magnetic dipoles, which align themselves in a magnetic field, contributing to the magnetic properties of the complex.
For the Nickel complex \([\text{NiX}_4]^{2-}\), the paramagnetic nature suggests that there are unpaired electrons in the metal's d-orbital.
This serves as an important clue when predicting the geometry and the electronic distribution of the complex.
- **Key takeaway:** Always look for unpaired electrons when determining if a complex is paramagnetic or not.
- **Magnetic Properties:** The more unpaired electrons, the stronger the paramagnetism.

Electron Configuration

Electron configuration plays a crucial role in understanding the behavior of atoms and molecules when they interact with other substances to form complexes. For Nickel, with atomic number 28, its ground state electron configuration is \[ \text{Ni}: [\text{Ar}]\ 3d^8\ 4s^2 \].
When Nickel forms a complex and oxidizes to the \( \text{Ni}^{2+} \) state, it loses two electrons, typically from the 4s orbital, leading to an electron configuration of \[ \text{Ni}^{2+}: [\text{Ar}]\ 3d^8 \].
This information is pivotal, as it determines the positioning of the d-orbital electrons, impacting the magnetic and chemical properties of the complex. In the case of \([\text{NiX}_4]^{2-}\), the presence of unpaired electrons in the 3d orbital aligns with its paramagnetic character.
- **Electrons and Magnetism:** The configuration tells us how many electrons are unpaired, which affects magnetism.
- **Use in Complexes:** Knowing the electron configuration helps predict the behavior and type of bonding in complexes.

Tetrahedral Geometry

Tetrahedral geometry in complexes like \([\text{NiX}_4]^{2-}\) generally results from the arrangement of ligands around a central metal ion in four regions of space, all pointing towards the corners of a tetrahedron. This shape is often influenced by the type and strength of the ligands involved.
- **Factors Influencing Geometry:** The spatial arrangement in a tetrahedral complex is largely determined by ligands that typically are weaker field and not strong enough to cause electron pairing within the 3d orbitals. For our nickel complex, the uninegative and monodentate nature of the ligand \( \text{X}^- \) contributes to this geometric arrangement.
- **High Spin and Unpaired Electrons:** Because of the specific arrangement and the type of ligands, the tetrahedral geometry in \([\text{NiX}_4]^{2-}\) supports a high-spin configuration, allowing the retention of two unpaired electrons and hence confirming its paramagnetic nature.
- **Common Attributes:** Tetrahedral arrangements usually do not lead to electron pairing in the same extent as square planar. Thus, for complexes like ours, expect unpaired electrons and resultant paramagnetism.