Question

Which one of the following has largest number of isomers? (a) \(\left[\mathrm{Ru}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right]^{2+}\) (c) \(\left[\mathrm{Ir}\left(\mathrm{PR}_{3}\right)_{2} \mathrm{H}(\mathrm{CO})\right]^{2+}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right]^{+}\) \([\mathrm{R}=\) alkyl group, en \(=\) ethylenediamine \(]\)

Short answer

Option (c) has the largest number of isomers with 4 possible forms.

Step-by-step solution

Identify Isomer Possibilities for Option (a)

The complex \([\mathrm{Ru}(\mathrm{NH}_{3})_{4}\mathrm{Cl}_{2}]^{+}\) is of the form \([\text{Ma}_{4}\text{b}_{2}]\). For this geometry, there can be two structural isomers: cis and trans, resulting in 2 possible isomers.

Evaluate Isomer Types for Option (b)

The complex \([\mathrm{Co}(\mathrm{NH}_{3})_{5}\mathrm{Cl}]^{2+}\) has the form \([\text{Ma}_{5}\text{b}]\), which allows for only one type of isomeric form, leading to 1 isomer.

Consider Isomers for Option (c)

The complex \([\mathrm{Ir}(\mathrm{PR}_{3})_{2}\mathrm{H}(\mathrm{CO})]^{2+}\) is of square-planar geometry. It allows for possible geometric isomers based on the positions of the PR extsubscript{3}, H, and CO groups, resulting in 4 isomers (cis, trans, facial, and meridional).

Calculate Isomers for Option (d)

The complex \([\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}]^{+}\) is an octahedral complex. Since it contains bidentate ethylenediamine ligands and two Cl- ligands, it can form cis and trans isomers as well, resulting in 2 isomers.

Key concepts

Isomerism in Coordination Chemistry

In coordination chemistry, isomerism is an exciting concept that refers to the occurrence of compounds with the same chemical formula but different arrangements of atoms. This can lead to distinctive physical and chemical properties. Coordination compounds are particularly rich in isomerism, primarily due to their complex structures that often incorporate metal centers and a variety of ligands.

Isomers in coordination compounds can be broadly classified into two categories:

• Structural Isomers: These isomers differ in the connectivity of atoms, meaning the bonds between atoms differ. For example, coordination position isomerism happens when ligands are attached through different donor atoms.
• Stereoisomers: These isomers have the same connectivity but differ in the spatial arrangement of atoms. Geometrical and optical isomerism fall under this category. Geometrical isomers, such as cis and trans, arise from different arrangements of ligands around the central metal atom.

Understanding these types of isomerism helps in predicting and explaining the diverse properties of coordination compounds.

Geometrical Isomerism

Geometrical isomerism is a specific form of stereoisomerism where the spatial arrangement of ligands differs around the central metal atom. In coordination chemistry, this often occurs in square-planar and octahedral complexes. It's pivotal in differentiating between physical and chemical properties of compounds because of the positioning of ligands.

For instance, in an octahedral complex like \([\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}]^{+}\), the ligands can form either a cis arrangement, where similar ligands are adjacent, or a trans arrangement, where they are opposite each other. These differences affect solubility, reactivity, and even color.

Similarly, in square-planar complexes, the distinctions between cis and trans isomers are equally crucial. Although these complexes have fewer coordination sites than octahedral ones, their potential for isomerism remains vital to understanding their behavior in chemical reactions.

Square-Planar Geometry

Square-planar geometry is characteristic of coordination complexes where four ligands are arranged around a central metal ion in a plane, forming 90° angles with each other. This geometry is common in particular types of ligands and transition metals, especially those with a d8 electron configuration, such as platinum and iridium.

A fascinating aspect of square-planar geometry is its potential for various isomers due to the flexibility of ligand positions. In a complex like \([\mathrm{Ir}(\mathrm{PR}_{3})_{2}\mathrm{H}(\mathrm{CO})]^{2+}\), different combinations of cis, trans, facial, and meridional arrangements are possible. These isomers result from distinct ligand positions across the central metal, profoundly influencing the chemical behavior and crystal field splitting of the compound.

The relative positions can lead to notable changes in properties, including magnetic behavior, absorption spectra, and kinetic reactivity, making the study of square-planar complexes essential in coordination chemistry.

Octahedral Complexes

Octahedral complexes are one of the most common and versatile geometries observed in coordination chemistry. They involve six ligands symmetrically distributed around a central metal ion, forming the shape of an octahedron. This geometry is prevalent among transition metals due to the stability it offers.

The symmetry of octahedral complexes allows for interesting types of isomerism, notably cis-trans and optical isomerism. In the complex \([\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}]^{+}\), there are two bidentate ethylenediamine ligands, which can form cis and trans isomers based on their positions relative to the chloride ions.

These isomers can have drastically different properties. For example, one isomer may be more stable, less reactive, or display a different color than another, attributes that are essential in applications ranging from catalysis to materials science. Understanding the octahedral geometry's inherent isomerism is crucial for professionals working with coordination compounds.