Question

Which of the following octahedral complexes show geometric isomerism? If geometric isomers are possible, draw their structures. (a) \(\left[\mathrm{Co}(e n) \mathrm{Cl}_{4}^{-}\right]\) (b) \(\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{ClBr}^{4-}\right]\) (c) \(\left[\mathrm{Cd}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{4}^{2-}\right]\)

Short answer

Answer: The octahedral complexes in cases (b) and (c) exhibit geometric isomerism due to the presence of various types of ligands in an octahedral arrangement. In case (a), the complex is homoleptic and cannot exhibit geometric isomers. The geometric isomers for cases (b) and (c) can be represented by cis and trans configurations based on the relative positions of ligands around the central metal ion.

Step-by-step solution

Case (a) - \(\left[\mathrm{Co}(e n) \mathrm{Cl}_{4}^{-}\right]\)

For this complex, there is only one type of ligand (Cl) surrounding the central metal atom (Co). This means that the complex is homoleptic and cannot exhibit geometric isomers. The structure of the complex would be octahedral, with Cl atoms surrounding Co at each vertex.

Case (b) - \(\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{ClBr}^{4-}\right]\)

In this complex, there are three types of ligands: Cl, Br, and \(\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\). This complex can exhibit geometric isomers because it is not homoleptic. We can represent the Cl and Br ligands in two distinct configurations, resulting in two distinct geometric isomers, with ethylenediaminetetraacetate ligands staying trans to each other: 1. Cis configuration: Cl and Br ligands are adjacent to each other (either in the plane or diagonally adjacent), and trans configuration: Cl and Br ligands are opposite each other on the octahedral structure. 2. Trans configuration: Cl and Br ligands are opposite each other (either horizontally or vertically), and the cis configuration: Cl and Br ligands are in the same plane with ethylenediaminetetraacetate ligands.

Case (c) - \(\left[\mathrm{Cd}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{4}^{2-}\right]\)

This complex has two types of ligands: Cl and \(\left(\mathrm{NH}_{3}\right)\). Since the complex is not homoleptic, geometric isomers are possible. We can draw two geometric isomers for this complex: cis and trans configurations based on the positions of ammonia (NH3) ligands: 1. Cis configuration: \(\mathrm{NH}_{3}\) ligands are adjacent to each other (either in the plane or diagonally adjacent). 2. Trans configuration: \(\mathrm{NH}_{3}\) ligands are opposite each other on the octahedral structure. In conclusion, case (b) and case (c) complexes have geometric isomers due to the presence of various types of ligands in an octahedral arrangement. The geometric isomers are possible based on the relative positions of ligands around the central metal ion.

Key concepts

Octahedral Complexes

In the world of coordination chemistry, an octahedral complex is a molecule where a central atom is surrounded by six ligands at the corners of an imaginary octahedron. The metal atom or ion sits at the center, and the ligands which can be the same or different molecules, are typically arranged at 90-degree angles to each other.

Imagine having a ball (the metal ion) and sticking six rods (the ligands) at equal angles from the center; this model represents the shape of an octahedral complex. Most transition metal complexes with a d^6 electron configuration tend to form octahedral geometries because this arrangement minimizes the repulsion between the electron pairs according to VSEPR theory (Valence Shell Electron Pair Repulsion theory).

This geometry becomes the playground for various isomerism types, including geometric isomerism, which arises from the different possible arrangements of ligands around the central atom. Octahedral complexes can demonstrate a rich variety of isomers further lending to the complexity and beauty of coordination chemistry.

Ligands

The term 'ligand' in chemistry refers to an ion or molecule that binds to a central metal atom to form a coordination complex. Ligands are like the hands that hold onto the metal ion, creating a more stable structure. They can be neutral molecules like water (H2O) or ammonia (NH3), or negatively charged ions like chloride (Cl-) or hydroxide (OH-).

Ligands are classified based on the number of points at which they attach to the central atom. Monodentate ligands, such as Cl-, attach at a single point, while multidentate ligands, often referred to as chelating agents, like ethylenediamine (en), can grab onto the metal at multiple points, forming a more secure hold. This firm grip can significantly influence the stability and properties of the final complex.

An essential feature of ligands is their ability to donate a pair of electrons to the metal ion, forming a coordinate covalent bond. This electron pair donation is a defining characteristic of a ligand and is central to the formation of coordination complexes.

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs when molecules have the same formula but different spatial arrangements of atoms. Think about placing two identical stickers on opposite sides of a ball versus placing them next to each other—this is akin to trans and cis arrangements, respectively.

In coordination complexes, cis-trans isomerism manifests when you have pairs of identical or different ligands that can either be next to each other (cis) or across from each other (trans) in the octahedral framework. The isomers have distinct physical and chemical properties, despite having the same molecular formula. For example, cisplatin, a chemotherapy drug, and its trans isomer have very different biological activities; the cis isomer is an effective cancer treatment, while the trans isomer lacks this therapeutic effect.

Recognizing and drawing the isomers might seem challenging, but with practice, students learn to visualize the three-dimensional arrangements that can exist, and thus better understand the fascinating interplay between structure and function in chemical compounds.