Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination compound is a complex substance which contains a central metal atom or ion surrounded by oppositely charged ions or neutral molecules. These compounds exhibit structural as well as stereoisomerism. Hybridisation theory explains the geometry of the complex. Crystal field theory explains the colour of complexes and magnetic properties. Identify the correct statement (a) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) is tetrahedral and paramagnetic (b) \(\left[\mathrm{NiCl}_{4}\right]^{2-}\) is square planar and paramagnetic (c) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) is square planar and paramagnetic (d) \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\) is tetrahedral and diamagnetic

Step-by-Step Solution

Answer

The correct statement is (d): \([\mathrm{Cu}(\mathrm{CN})_{4}]^{3-}\) is tetrahedral and diamagnetic.

Step 1: Analyze each complex

We need to examine each given complex to determine its geometry and magnetic properties. We'll use hybridization theory for geometry and crystal field theory for magnetism.

Step 2: Examine \([\mathrm{Ni}(\mathrm{CN})_{4}]^{2-}\)

Nickel (Ni) in this complex is in a +2 oxidation state. CN⁻ is a strong field ligand, usually leading to square planar structures and low spin configurations. Hybridization would be \(dsp^2\). Low spin results in diamagnetic properties.

Step 3: Examine \([\mathrm{NiCl}_{4}]^{2-}\)

Nickel (Ni) is in the +2 oxidation state, and Cl⁻ is a weak field ligand, which leads to a tetrahedral structure with \(sp^3\) hybridization. Weak field causes high spin, resulting in paramagnetic properties.

Step 4: Examine \([\mathrm{Ni}(\mathrm{CO})_{4}]\)

Nickel (Ni) here is in the 0 oxidation state. CO is a strong field ligand, indicating low spin and usually leads to a tetrahedral geometry with \(sp^3\) hybridization. Low spin means diamagnetic properties.

Step 5: Examine \([\mathrm{Cu}(\mathrm{CN})_{4}]^{3-}\)

Copper (Cu) here is in the +1 oxidation state. CN⁻ is a strong field ligand. The square planar shape and low spin configuration (\(dsp^2\) hybridization) suggest the complex is diamagnetic.

Step 6: Identify the correct statement

Based on our analysis: (a) is incorrect as \([\mathrm{Ni}(\mathrm{CN})_{4}]^{2-}\) is square planar and diamagnetic; (b) is incorrect as \([\mathrm{NiCl}_{4}]^{2-}\) is tetrahedral and paramagnetic; (c) is incorrect as \([\mathrm{Ni}(\mathrm{CO})_{4}]\) is tetrahedral and diamagnetic; (d) is correct because \([\mathrm{Cu}(\mathrm{CN})_{4}]^{3-}\) fits as tetrahedral and diamagnetic.

Key Concepts

Hybridization Theory

Hybridization theory is a fundamental concept that helps us understand the geometry of coordination complexes. By merging atomic orbitals, we form hybrid orbitals that orient themselves in specific shapes.
This theory gives insight into how the atoms arrange in space around a central metal ion or atom. When focusing on coordination compounds, hybridization is crucial because the shape determines many of the compound's chemical properties.
For instance, in the case of \([Ni(CN)_4]^{2-}\), the CN⁻ ligands are strong field ligands. Due to this, hybridization will involve the \(dsp^2\) configuration, leading to a square planar shape. Similarly, weak field ligands like Cl⁻ in \([NiCl_4]^{2-}\) result in \(sp^3\) hybridization, resulting in a tetrahedral shape.

• Strong field ligands usually lead to low spin and the \(dsp^2\) hybridization, forming square planar structures.
• Weak field ligands typically lead to high spin configurations, favoring a tetrahedral shape with \(sp^3\) hybridization.

Crystal Field Theory

Crystal Field Theory provides us with a glimpse into the electronic structures of complexes by focusing on the ligand's influence on the metal ion's d-orbitals.
When ligands form bonds with a metal ion, they split the degeneracy of the d-orbitals into different energy levels.
This splitting of d-orbitals is expressed in terms of crystal field splitting energy, and it directly influences the color, stability, and magnetism of the complex.
High spin or low spin configurations are determined based on the size of this energy split and the strength of the ligands involved. Using our examples:

• For \([Ni(CN)_4]^{2-}\), the strong field ligand CN⁻ causes a large splitting, usually resulting in a low spin, diamagnetic complex.
• Conversely, the weaker field Cl⁻ in \([NiCl_4]^{2-}\) causes a smaller split, resulting in a high spin, paramagnetic complex.

This is why you might observe colors in some complexes when they absorb certain wavelengths according to the energy difference in their d-orbitals.

Structural Isomerism

Coordination compounds often display a fascinating aspect called structural isomerism. This is when the compounds have the same chemical formula but different structures, leading to different properties.
It can manifest in various ways in coordination chemistry.

• Linkage isomerism: Occurs when an ambidentate ligand can attach to the central metal ion in more than one way.
• Coordination isomerism: Happens when you have a mis-match in the distribution of ligands between the central ions of two different coordination complexes.

These isomers help explain why compounds with the same chemical constituents can behave differently in chemical reactions.
Recognizing the type of isomerism helps predict the behavior and property changes that can occur within a coordination compound.

Magnetic Properties of Complexes

The magnetic properties of coordination complexes are intricately linked to electronic arrangements within their structure. This property, whether a complex is magnetic or not, stems from the presence of unpaired electrons in the compound.
Coordination complexes can be classified as either diamagnetic or paramagnetic.

• Diamagnetic: All electrons are paired, and the complex is not attracted to a magnetic field. This is typical for low spin complexes like \([Ni(CN)_4]^{2-}\) due to strong field ligands causing electron pairing.
• Paramagnetic: Contains unpaired electrons making the complex attracted to magnetic fields. It often occurs with high spin complexes, where weak field ligands, such as Cl⁻ in \([NiCl_4]^{2-}\), do not cause significant electron pairing.

Understanding the magnetic property of a complex is crucial not only for its theoretical implications but also for practical applications, such as in materials science and medicine.