Question

Draw geometrical isomers of each of the following complex ions. a. \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{-}\) b. \(\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{I}_{2}^{2+}\) c. \(\operatorname{Ir}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\) d. \(\operatorname{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}^{+}\)

Short answer

The geometrical isomers for each complex ion are as follows: a. \(\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{-}\): cis and trans isomers with oxalates adjacent or opposite. b. \(\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{I}_{2}^{2+}\): cis and trans isomers with iodides adjacent or opposite. c. \(\operatorname{Ir}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\): fac and mer isomers with the same type of ligands on the same face or meridian of the octahedral. d. \(\operatorname{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\mathrm{I}_{2}^{+}\): cis and trans isomers with iodides adjacent or opposite.

Step-by-step solution

Identify the coordination sphere

The central metal ion is Co, and the ligands are two oxalates and two water molecules. The coordination sphere is octahedral, with 6 ligands.

Draw the isomers

The possible geometrical isomers for this complex ion are cis and trans. In the cis isomer, both oxalates will be adjacent to each other, and in the trans isomer, they will be opposite to each other. Draw the cis and trans isomers with their respective arrangements of ligands. b. \(\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{I}_{2}^{2+}\)

Identify the coordination sphere

The central metal ion is Pt, and the ligands are four ammine and two iodide ions. The coordination sphere is octahedral, with 6 ligands.

Draw the isomers

The possible geometrical isomers for this complex ion are cis and trans. In the cis isomer, both iodides will be adjacent to each other, and in the trans isomer, they will be opposite to each other. Draw the cis and trans isomers with their respective arrangements of ligands. c. \(\operatorname{Ir}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\)

Identify the coordination sphere

The central metal ion is Ir, and the ligands are three ammines and three chloride ions. The coordination sphere is octahedral, with 6 ligands.

Draw the isomers

The possible geometrical isomers for this complex ion are facial (fac) and meridional (mer). In the fac isomer, the same type of ligands will be on the same face of the octahedral, and in the mer isomer, the same type of ligands will be on the meridian of the octahedral. Draw the fac and mer isomers with their respective arrangements of ligands. d. \(\operatorname{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\mathrm{I}_{2}^{+}\)

Identify the coordination sphere

The central metal ion is Cr, and the ligands are one ethylenediamine (en), two ammines, and two iodide ions. The coordination sphere is octahedral, with 6 ligands.

Draw the isomers

The possible geometrical isomers for this complex ion are cis and trans. In the cis isomer, both iodides will be adjacent to each other, and in the trans isomer, they will be opposite to each other. Draw the cis and trans isomers with their respective arrangements of ligands.

Key concepts

Geometrical Isomerism

In coordination chemistry, geometrical isomerism occurs when ligands attach differently around a central metal atom, resulting in different spatial arrangements. Even though they have the same formula, the structure’s shape affects its properties and reactivity.

This type of isomerism is possible in coordination complexes, specifically in cases where ligands can occupy different positions, while bonded to the central atom. Geometrical isomerism does not involve breaking of bonds; instead, it merely changes the ligand position.

Consider the complex \({\mathrm{Pt}({\mathrm{NH}_{3}})_{4}{\mathrm{X}_{2}}}\) where X could be halides like Iodide (I^{-}). Depending on whether the two iodides are adjacent or on the opposite sides, this complex can form different geometrical isomers. These spatial arrangements lead to the existence of cis and trans forms, which exhibit different chemical and physical properties, despite having the same molecular formula.

Octahedral Complexes

Octahedral complexes are a prevalent structure in coordination chemistry where a central metal ion is surrounded by six ligands at the corners of an octahedron.

These six coordinates ensure a symmetrical arrangement, which is common for transition metals like Co, Pt, and Ir. The geometry offers a variety of isomeric forms, including cis and trans, as well as fac and mer isomers.

• Cis-Trans: Ligands in adjacent positions (cis) or opposite positions (trans).
• Fac-Mer: Ligands grouped on a single face (fac) or linearly across the structure (mer).

For instance, in the case of \({\mathrm{Co}({\mathrm{C}_{2} \mathrm{O}_{4}})_{2}({\mathrm{H}_{2} \mathrm{O}})_{2}^{-}}\), the isomerism arises by positioning identical ligands next to or away from each other. The octahedral arrangement makes it possible for the overall spatial configuration to influence how these complexes interact with their surroundings.

Coordination Sphere

The coordination sphere in coordination chemistry refers to the central metal atom and its directly attached ligands. This crucial component determines the binding geometry and overall charge, heavily influencing the complex’s properties.

The ligands in the coordination sphere participate in the formation of covalent bonds with the metal. In an octahedral complex, this means coordinating exactly six ligands. The arrangement of these ligands is pivotal:

• Symmetry and arrangement impact physical properties like color and magnetism.
• Defined geometry determines the type and number of isomers possible.

For example, in \(\operatorname{Ir}({\mathrm{NH}_{3}})_{3} {\mathrm{Cl}_{3}}\), understanding the coordination sphere allows for visualizing the distinct fac and mer isomers, each with unique ligand arrangements influencing the chemical behavior of the complex.

Cis-Trans Isomerism

Cis-trans isomerism is a form of geometrical isomerism found in octahedral and square planar coordination complexes. It arises when similar ligands can be positioned either next to each other or across from each other.

This forms different isomers known as "cis" (adjacent) and "trans" (opposite). These two configurations not only influence the compound’s geometry but also its chemical reactivity and interaction with external agents.

• Cis Isomer: Ligands are positioned near each other, often resulting in a more compact structure.
• Trans Isomer: Ligands are directly opposite, giving a more extended shape.

Taking \(\operatorname{Cr}({\mathrm{en}})({\mathrm{NH}_{3}})_{2}{\mathrm{I}_{2}^{+}}\), where two iodide ions can form either a cis or trans arrangement, each isomer exhibits distinct characteristics, offering varied potential uses in chemical synthesis and applications.