Problem 36 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among the complex ions, \(\left[\mathrm{Co}\left(\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\right)_{2} \mathrm{Cl}_{2}\right]^{+}, \quad\left[\mathrm{CrCl}_{2}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\right]^{3-}, \quad\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right]^{+}\), \(\left[\mathrm{Fe}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{CN})_{4}\right]^{-}\) \(\left[\mathrm{Co}\left(\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\right)_{2}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}\right]^{2+}\) and \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}\right]^{2+}\), the number of complex ion(s) that show(s) cis- trans isomerism is

Step-by-Step Solution

Answer

3 complex ions show cis-trans isomerism: omenclature{[Co(en)2Cl2]+}, omenclature{[Fe(H2O)4(OH)2]+}, and omenclature{[Co(NH3)4(H2O)Cl]^{2+}}.

Step 1: Understanding Cis-Trans Isomerism

Cis-trans isomerism occurs in certain coordination complexes where different ligands are exchanged for each other that are adjacent ( omenclature{cis}) or opposite ( omenclature{trans}) relative to each other in the coordination sphere.

Step 2: Evaluate Geometry of Complex Ions

Identify the geometry of each complex ion: - omenclature{[Co(en)2Cl2]+} is octahedral. - omenclature{[CrCl2(C2O4)2]^{3-}} is octahedral. - omenclature{[Fe(H2O)4(OH)2]+} is octahedral. - omenclature{[Fe(NH3)2(CN)4]^{-}} is square planar or octahedral but can have cis-trans when square planar. - omenclature{[Co(en)2(NH3)Cl]^{2+}} is octahedral. - omenclature{[Co(NH3)4(H2O)Cl]^{2+}} is octahedral.

Step 3: Check for Cis-Trans Possibility in Octahedral Complexes

In an octahedral complex, cis-trans isomerism can occur if there are two identical ligands which can be positioned either adjacent ( omenclature{cis}) or opposite ( omenclature{trans}) to each other.

Step 4: Analyze Each Complex for Isomerism

- For omenclature{[Co(en)2Cl2]+}, two Cl ions can be cis or trans, forms isomers. - omenclature{[CrCl2(C2O4)2]^{3-}} does not undergo cis-trans due to omenclature{C2O4} bidentate ligand preventing such arrangement. - omenclature{[Fe(H2O)4(OH)2]+} can have omenclature{OH} groups in cis or trans. - omenclature{[Co(en)2(NH3)Cl]^{2+}} no two identical ligands subject to cis-trans towards each other. - omenclature{[Co(NH3)4(H2O)Cl]^{2+}} has NH3 groups which can be cis or trans across Cl and H2O. - omenclature{[Fe(NH3)2(CN)4]^{-}} in square planar can form cis or trans isomers.

Key Concepts

Octahedral Geometry

In coordination chemistry, octahedral geometry refers to a molecular shape where a central metal atom is surrounded by six ligands positioned at the corners of an octahedron. This highly symmetrical arrangement allows several possible ways for ligands to be arranged. Depending on the type and number of ligands, cis-trans isomerism can be observed.

• **Cis-trans isomerism**: Occurs when ligands can occupy adjacent positions (cis) or opposite positions (trans) relative to each other.
• **Identical ligands**: The presence of two identical ligands is crucial for such isomerism in octahedral complexes.

Many metal ions, like cobalt and iron, form octahedral complexes because of their ability to bond with multiple ligands. When examining octahedral complexes like the examples provided in the exercise, it's essential to recognize this geometry's role in potential isomer formations.

Square Planar Complexes

Square planar complexes are a common geometry especially for certain transition metals, like those in the d8 configuration. Here, the central metal atom is surrounded by four ligands arranged in a square plane, which is particularly conducive to cis-trans isomerism.

• **Cis Isomer**: Where identical ligands are adjacent, found on a single side of the square, forming a 90-degree angle.
• **Trans Isomer**: Where identical ligands are opposite each other across the metal center, forming a 180-degree angle.

When analyzing square planar complexes for cis-trans isomerism, it is essential to note ligand arrangements, as this unique planar geometry allows distinctly different isomeric forms. For example, the complex \([Fe(NH_3)_2(CN)_4]^{-}\) given in the exercise can potentially exhibit such isomerism.

Coordination Complexes

Coordination complexes feature a central metal atom or ion bonded to surrounding molecules or ions known as ligands. These complexes can vary in shape, size, and the nature of bonds, influencing their chemical and physical properties.

• The **central metal** is typically a transition metal, which can accept electrons from ligands to form coordinate covalent bonds.
• **Ligands** can be neutral or anionic, and they are essential in determining the geometry of the complex.
• **Isomerism**: In coordination complexes, different spatial arrangements of ligands lead to isomers, such as cis-trans isomers in octahedral or square planar geometries.

Understanding these aspects is vital when exploring complex ions' behavior, stability, and reactivity, as seen through the examples in the given exercise. The diversity in ligand types and the arrangement highlights the complexity and fascinating nature of these coordination complexes.