Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

In \(\left[\mathrm{Cr}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]^{3-}\), the isomerism shown is (a) optical (b) ionization (c) geometrical (d) ligand

Step-by-Step Solution

Answer

The isomerism shown is optical (a).

Step 1: Understanding the Compound

The compound given is \([ ext{Cr}( ext{C}_2 ext{O}_4)_3]^{3-}\). This is a coordination complex where "Cr" stands for chromium and "C2O4" represents the oxalate ion as a ligand. The charge of the complex is 3-.

Step 2: Identifying Ligand Type

The ligand \( ext{C}_2 ext{O}_4\) is a bidentate ligand, meaning it can coordinate to the metal center at two points, forming a chelate ring with the metal. This is important when considering the types of isomerism possible.

Step 3: Considering Types of Isomerism

Coordination complexes can exhibit several types of isomerism: optical, geometrical, ligand, and ionization. Optical isomerism occurs when a compound can exist in such a form that it is non-superimposable on its mirror image, often due to chiral arrangements in the compound.

Step 4: Determining if Optical Isomerism is Possible

Check if the compound can form non-superimposable mirror images, which happens if there's an asymmetrical arrangement of ligands around the metal center. For \([ ext{Cr}( ext{C}_2 ext{O}_4)_3]^{3-}\), the bidentate nature of oxalate allows for such arrangements, making optical isomerism possible.

Key Concepts

Optical Isomerism

In coordination chemistry, optical isomerism is a fascinating phenomenon where certain compounds exist as non-superimposable mirror images, much like our hands. This property is often the result of chiral geometries formed around a central atom, usually a metal, in coordination complexes. When a compound exhibits optical isomerism, it can interact differently with plane-polarized light, causing it to rotate the light either to the left or right depending on the direction of its chiral form. Thus, these isomers are called enantiomers. In the complex \([\text{Cr}(\text{C}_2\text{O}_4)_3]^{3-}\), optical isomerism is possible due to its geometric configuration around the chromium center. In this case, each oxalate ligand acts as a bridge, connecting to the central atom at two different points, leading to an arrangement that lacks a plane of symmetry. Without intrinsic symmetry, the molecule can create two distinct enantiomers that are mirror images of each other.

Coordination Complexes

Coordination complexes are molecules consisting of a central metal atom or ion connected to surrounding molecules or ions called ligands. These formations are essential in many fields of chemistry, including bioinorganic and organometallic chemistry. The metal center provides empty orbitals that can form coordinate covalent bonds with electron-rich ligands.

• This results in diverse structures, ranging from simple arrangements to complex networks.
• Common central metals include transition metals like chromium (in \([\text{Cr}(\text{C}_2\text{O}_4)_3]^{3-}\)).

The nature of the ligands and the number of coordination sites affect the properties and reactivity of the complex. For instance, coordination complexes can display unique forms of isomerism (such as optical isomerism), highlighting their fascinating symmetry or lack thereof. Understanding the formation and structure of these complexes is crucial for applications, including catalysis and material science.

Bidentate Ligands

Bidentate ligands are a type of ligand that can form two bonds with a metal center in a coordination complex. The term "bidentate" means "two-toothed," indicating that these ligands have two sites available for bonding to the same metal, which greatly stabilizes the coordination compound.

• An example of a bidentate ligand is the oxalate ion \(\text{C}_2\text{O}_4^{2-}\) found in the compound \([\text{Cr}(\text{C}_2\text{O}_4)_3]^{3-}\).
• Because it binds at two sites, it can form a ring structure with the metal, termed a chelate ring.

This chelation makes bidentate ligands particularly effective at forming stable complexes, as the multi-point attachment reduces the chance of the ligand releasing easily compared to monodentate ligands.Additionally, the presence of bidentate ligands can lead to interesting spatial arrangements around the metal center, allowing for the possibility of optical isomerism, as seen in the given exercise.Understanding how different ligands interact with metal centers is key to predicting and explaining the behavior of coordination complexes.