Problem 46 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The species having tetrahedral shape is (a) \(\left[\mathrm{PdCl}_{4}\right]^{2-}\) (b) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) (c) \(\left[\mathrm{Pd}(\mathrm{CN})_{4}\right]^{2-}\) (d) \(\left[\mathrm{NiCl}_{4}\right]^{2-}\)

Step-by-Step Solution

Answer

The species with a tetrahedral shape is \( \left[\mathrm{NiCl}_{4}\right]^{2-} \).

Step 1: Identify Coordination Number

Determine the coordination number for each complex, which is the number of ligand donor atoms bonded to the central metal atom. For all the complexes given, the coordination number is 4 because each metal is surrounded by 4 ligands.

Step 2: Understand the Coordinate Geometry

For coordination number 4, there are two possible geometries: tetrahedral and square planar. Tetrahedral complexes are more common for lighter metals or those with a d10 electronic configuration.

Step 3: Analyze Electronic Configurations

Examine the electron configuration of the central metal atoms in each complex. - Pd is in d8 state in both \( \left[\mathrm{PdCl}_{4}\right]^{2-} \) and \( \left[\mathrm{Pd(CN)}_{4}\right]^{2-} \) , typically forming square planar complexes. - Ni is in d8 state in \( \left[\mathrm{Ni(CN)}_{4}\right]^{2-} \), forming square planar due to the strong field of \(CN^-\).- Ni in \( \left[\mathrm{NiCl}_{4}\right]^{2-} \) is a d10 configuration making it tetrahedral.

Step 4: Identify Ligand Field Strength

Consider the field strength of the ligands: \(CN^-\) is a strong field ligand often leading to square planar configurations. \(Cl^-\) is a weak field ligand, which does not typically cause pairing of electrons, favoring tetrahedral geometry.

Step 5: Determine the Shape

Based on coordination number, metal electron configuration, and ligand field strength, deduce the geometry: - \( \left[\mathrm{NiCl}_{4}\right]^{2-} \) is tetrahedral.- The others are square planar due to either high d-block electron count or strong field ligands.

Key Concepts

Coordination Number

In coordination chemistry, the coordination number is a fundamental concept that helps us understand how many atoms or ions are bonded to a central metal ion in a complex. It is determined by counting the number of ligand donor atoms that are directly attached to the metal center. In the context of the exercise, each complex has a coordination number of 4. This is because each metal center is bound to four ligands:

• Coordination number 4 allows complexes to exhibit distinct geometries. These geometries can significantly affect a complex's chemical properties.
• The common geometries for these numbers include tetrahedral and square planar arrangements, depending on various factors such as the metal's electronic configuration and the type of ligands.

Understanding coordination numbers can help predict the structural arrangement and properties of a complex ion, which is crucial for understanding their reactivity and function.

Tetrahedral Geometry

Tetrahedral geometry is one of the possible structures for complexes with a coordination number of 4. The name tetrahedral comes from the geometry where four atoms are symmetrically arranged around a central atom, forming a shape that is reminiscent of a pyramid with a triangular base. This shape is very common in coordination complexes, especially those with certain electronic configurations:

• A tetrahedral shape is favored in cases where the metal center avoids electron-electron repulsion as opposed to planar arrangements.
• In a tetrahedral complex, the bond angles are approximately 109.5°, which helps minimize these repulsions.

From the exercise, we see that \(\left[\mathrm{NiCl}_{4}\right]^{2-}\) forms a tetrahedral shape due to its d10 electron configuration. This configuration lacks significant electron pairing influences, allowing for a higher flexibility in its spatial arrangement. Understanding this geometry is crucial since it directly affects the compound's reactivity, optical activity, and thermal stability.

Ligand Field Strength

Ligand field strength is a key concept for determining the geometry of a metal complex. It refers to the ability of a ligand to split the d-orbitals of the central metal ion. The field strength of a ligand can influence whether a complex will form a tetrahedral or square planar shape:

• Strong field ligands, like \(CN^{-}\), typically lead to large splitting of d-orbitals. This generally results in a preference for square planar complexes as seen in\(\left[\mathrm{Ni(CN)}_{4}\right]^{2-}\).
• Weak field ligands, such as \(Cl^{-}\), result in smaller splitting and thus often favor the formation of tetrahedral complexes, as they do not lead to efficient pairing of d-electrons.

The field strength impacts not just the geometry but also the stability and spectra of the complex. Recognizing the impact of ligand field strength helps chemists to predict how a complex will behave in various chemical environments, making it an essential concept in coordination chemistry.