Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pairs represents linkage isomers? (a) \(\left[\mathrm{Pd}\left(\mathrm{P} \mathrm{Ph}_{3}\right)_{2}(\mathrm{NCS})_{2}\right]\) and \(\left[\mathrm{Pd}\left(\mathrm{P} \mathrm{Ph}_{3}\right)_{2}(\mathrm{SCN})_{2}\right]\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{NO}_{3}\right] \mathrm{SO}_{4}\) and \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{SO}_{4}\right] \mathrm{NO}_{3}\) (c) \(\left[\mathrm{Pt} \mathrm{Cl}_{2}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Br}_{2}\) and (d) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{Pt} \mathrm{Cl}_{4}\right]\) and \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{CuCl}_{4}\right]\)

Step-by-Step Solution

Answer

Option (a) represents linkage isomers.

Step 1: Understand Linkage Isomerism

Linkage isomerism occurs when a ligand can coordinate to a central metal ion in multiple ways, due to the presence of more than one possible donor atom in the ligand. Examples include ligands like NO2, which can bind through nitrogen (nitro) or oxygen (nitrito).

Step 2: Analyze Option (a)

In option (a), \([\mathrm{Pd}(\mathrm{PPh}_{3})_{2}(\mathrm{NCS})_{2}]\) and \([\mathrm{Pd}(\mathrm{PPh}_{3})_{2}(\mathrm{SCN})_{2}]\), the NCS ligand can bind through nitrogen or sulfur. This is a classic example of linkage isomers as the ligand (NCS) changes the binding atom (N vs. S) with the metal.

Step 3: Analyze Option (b)

In option (b), \([\mathrm{Co}(\mathrm{NH}_{3})_{5} \mathrm{NO}_{3}] \mathrm{SO}_{4}\) and \([\mathrm{Co}(\mathrm{NH}_{3})_{5} \mathrm{SO}_{4}] \mathrm{NO}_{3}\), this is an example of an ionization isomer, not a linkage isomer, since it's the counter ion outside the coordination sphere that changes.

Step 4: Analyze Option (c)

In option (c), \([\mathrm{Pt} \mathrm{Cl}_{2}(\mathrm{NH}_{3})_{4}] \mathrm{Br}_{2} \), there is no indication of different ligands binding through different atoms. Thus, this pair does not exhibit linkage isomerism.

Step 5: Analyze Option (d)

In option (d), \([\mathrm{Cu}(\mathrm{NH}_{3})_{4}] [\mathrm{Pt} \mathrm{Cl}_{4}]\) and \([\mathrm{Pt}(\mathrm{NH}_{3})_{4}] [\mathrm{CuCl}_{4}]\), this pair represents a change in metal centers and not the binding site of a ligand, so it represents neither linkage isomerism nor any kind of isomerism related to ligand donor atoms.

Key Concepts

Coordination Chemistry

Coordination Chemistry is a branch of chemistry that focuses on the study of compounds formed between metal ions and ligands. These compounds, known as coordination compounds or complexes, have a central metal ion bonded to surrounding molecules or ions called ligands. The nature and behavior of these ligands with metal ions govern much of the structure and properties of the coordination compound. Here's a brief look at the aspects of coordination chemistry:

• **Central Metal Ion:** The metal atom or ion acts as the center of the complex, usually transitioning metals like cobalt or nickel. These metals have empty d-orbitals that allow them to form coordinate bonds.
• **Ligands:** These are atoms, ions, or molecules that donate a pair of electrons to the metal ion. They can vary from simple ions like chloride (Cl-) to complex organic molecules.
• **Coordinate Bonds:** The bond formed between the metal ion and the ligands isn't a typical ionic or covalent bond, but a special type known as a coordinate covalent bond, involving the donation of a pair of electrons from the ligand to the empty d-orbitals of the metal.

This fundamental building of coordination chemistry is essential in understanding more complex phenomena like isomerism in coordination compounds and ligand coordination modes.

Isomerism in Coordination Compounds

Isomerism is a fascinating concept in coordination chemistry where compounds with the same formula display different properties because of a different arrangement of atoms. In coordination compounds, isomerism presents itself in various forms:

• **Linkage Isomerism:** This occurs when a ligand can bind to the metal ion in more than one way. For example, the ligand \(NO_2\) can attach through either nitrogen or oxygen, forming nitro or nitrito isomers.
• **Geometric Isomerism:** Arises when the spatial arrangement of ligands around the central metal differs. It's common in square planar and octahedral complexes, such as cis (adjacent) and trans (opposite) configurations.
• **Optical Isomerism:** Occurs when a coordination compound can exist in chiral forms, which are mirror images of each other, like left and right hands.

Understanding these types of isomerism helps explain why two compounds with identical formulas might exhibit drastically different chemical and physical properties, an important consideration when synthesizing new materials or drugs.

Ligand Coordination Modes

Ligands are diverse in how they can attach to metal ions, a feature that impacts the structure and reactivity of the resulting coordination complexes. Below are some of the common coordination modes:

• **Monodentate Ligands:** Have a single donor atom that forms one bond to the central metal ion. Examples include water \((H_2O)\) and ammonia \((NH_3)\).
• **Bidentate Ligands:** These ligands contain two donor atoms that can attach to the metal ion. A classic example is ethylenediamine \((en)\), which grips the metal like a "bite."
• **Polydentate Ligands:** Can attach to metal ions through multiple donor atoms. EDTA is an example that can bind through six donor sites, often used in chelation therapy.
• **Ambidentate Ligands:** These ligands have more than one type of donor atom but can only use one at a time. A prominent example is thiocyanate \((NCS^-),\) which can bind through nitrogen or sulfur, demonstrating linkage isomerism as explored in the original exercise.

Each coordination mode impacts the stability and color of the coordination compound, a key feature addressed in applications like dye manufacturing and catalysis.