Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of isomers possible for complex \(\mathrm{K}_{2}[\mathrm{Pd}\) \(\left.\mathrm{Cl} \mathrm{Br}(\mathrm{SCN})_{2}\right]\) is (a) 1 (b) 2 (c) 3 (d) 4

Step-by-Step Solution

Answer

Option (b) 2 is the answer.

Step 1: Understand the Complex

The complex given is \( ext{K}_{2}[ ext{Pd} ext{Cl} ext{Br}( ext{SCN})_2]\). This complex involves palladium (Pd) as the central metal atom coordinated to different ligands, specifically two thiocyanates (SCN), one chlorine (Cl), and one bromine (Br).

Step 2: Determine the Geometry

The central metal in the coordination complex is palladium, which is often involved in square planar coordination with four ligands.

Step 3: Apply the Isomerism Concept

In square planar complexes like this, isomers can exist due to the different possible arrangements of ligands around the central metal. Specifically, geometric isomerisms are possible, resulting in different spatial arrangements.

Step 4: Analyze Possible Arrangements

For the complex \([ ext{Pd} ext{Cl} ext{Br}( ext{SCN})_2]\), you can have the following possible arrangements of ligands: (1) Cl and Br are adjacent (cis to each other), and (2) Cl and Br are opposite (trans to each other). Each arrangement results in a different isomer.

Step 5: Count the Isomers

Given the above arrangements, check for distinct geometries: (1) cis (with SCNs opposite) and (2) trans (with SCNs adjacent). These are two distinct arrangements, thus leading to two isomers.

Key Concepts

Square Planar Geometry

Square planar geometry is a common structural arrangement in coordination chemistry, especially for complexes involving transition metals. In this configuration, the central metal atom is surrounded by four ligands, forming a square-like shape on a single plane. Think of it as a flat space, with your metal atom in the center and each ligand positioned at right angles to its neighbors.
This geometry is particularly stable for certain metal ions, such as palladium, platinum, and nickel, which allows them to easily engage with four ligands. In a square planar complex, the 90-degree angles between ligands are crucial for arranging different types of ligands in various spatial patterns, paving the way for geometric isomerism.

Geometric Isomerism

Geometric isomerism arises when the spatial arrangement of ligands around a central atom in a coordination complex can differ, while the actual connections remain the same. This form of isomerism is quite prevalent in square planar complexes.
For instance, in the complex \(\[\text{PdClBr(SCN)}_2\]\), the different positions of the Cl and Br ligands can create unique isomers. These arrangements are termed *cis* and *trans* isomers.

• *Cis* isomers occur when identical ligands are adjacent.
• *Trans* isomers occur when ligands are opposite one another.

In our example, two distinct geometric arrangements are possible – cis and trans – due to how Cl and Br can be positioned. This flexibility results in multiple ways the ligands can be organized, leading to the appearance of two isomers.

Ligands

Ligands are molecules or ions that attach to the central metal atom in a coordination complex through coordinate bonding. These entities donate one or more pairs of electrons to the metal, creating a coordinate covalent bond.
Ligands can vary greatly in type, size, charge, and electron-withdrawing or -donating abilities. Common examples include water, ammonia, halides (like Cl and Br), and thiocyanate (SCN), as seen in our complex example.

• The nature and arrangement of ligands significantly affect the properties, reactivity, and stability of the complex.
• In a square planar arrangement, as with palladium, the positioning of these ligands is critical for determining the isomers formed.

This intricate dance of ligands determines the final structure and spatial design of the entire coordination complex.

Palladium Complexes

Palladium is a transition metal known for forming stable complexes, often exhibiting square planar geometry. This makes it ideal for coordinating with four ligands in various chemical environments.
Palladium complexes are quite prevalent in the field of catalysis and organic synthesis due to their versatile bonding capabilities. When palladium interacts with ligands such as Cl, Br, and SCN, as in the complex \(\[\text{PdClBr(SCN)}_2\]\), it produces a structure that allows for geometric isomerism.

• The ability of palladium to stabilize in square planar arrangements facilitates the exploration of geometric isomers.
• This makes palladium complexes incredibly useful for studying the subtle nuances of coordination chemistry and the effects of different ligand arrangements.

Hence, palladium not only holds the ligands together but also serves as the pivot point around which these isomers are generated, presenting an excellent case study for understanding geometric isomerism in depth.