Question

Which of these octahedral coordination complexes can exhibit geometric isomerism? (a) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3} \mathrm{Cl}_{3}\right]\) (b) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\)

Short answer

Both complexes (a) and (b) can exhibit geometric isomerism.

Step-by-step solution

Understanding Coordination Complexes

A coordination complex consists of a central metal atom or ion bonded to surrounding molecules or anions called ligands. The coordination number is the total number of points at which ligands are attached to the central metal.

Interpreting Geometric Isomerism

Geometric isomerism occurs in coordination complexes when different arrangements of the ligands around the central metal atom are possible, resulting in different spatial configurations (i.e., cis and trans forms). Generally, this occurs when there is a possibility of having at least two different ligands around the central atom.

Analyzing Complex (a)

The complex \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3} \mathrm{Cl}_{3}\right] \) has a central chromium ion with three water ligands and three chloride ligands. Since the ratio of the two types of ligands (\(\mathrm{H}_2\mathrm{O}\) and \(\mathrm{Cl}^–\)) is equal, it can lead to multiple possible spatial arrangements (cis and trans), allowing for geometric isomerism.

Analyzing Complex (b)

The complex \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^+ \) comprises four water ligands and two chloride ligands. Because there are two kinds of ligands and the number of each type is not equal, different spatial arrangements (cis and trans) with respect to the two chloride ions are possible, thereby allowing geometric isomerism.

Conclusion on Geometric Isomerism

After analyzing both complexes, we find that both complexes can exhibit geometric isomerism due to the distinct arrangements possible with different types of ligands surrounding the central metal ion.

Key concepts

Coordination Complexes

Coordination complexes are fascinating structures composed of a central metal atom or ion surrounded by molecules or ions known as ligands. These ligands are attached to the metal through coordinate covalent bonds, where both electrons in the bond come from the ligand. The arrangement of these bonds gives the complex various unique properties. In a coordination complex, the coordination number is crucial. It refers to the number of ligand attachment points to the central metal, determining the complex's shape. Coordination complexes are quite prevalent in chemistry and serve essential roles in biological, industrial, and environmental processes. Understanding these complexes enables us to predict and explain their reactivity and function.

Octahedral Coordination

Octahedral coordination is a common spatial arrangement in coordination chemistry, where six ligands symmetrically surround a central metal atom or ion. The octahedral shape forms due to the six bond sites resulting in 90-degree angles between adjacent ligands. This geometric configuration is significant because it leads to various isomeric possibilities, especially when different ligands occupy the coordination sites. When different types of ligands are present, this arrangement can result in geometric isomers, such as cis (same types of ligands adjacent) and trans (same types of ligands opposite) forms. Geometric isomerism in octahedral complexes can drastically impact the properties and function of the compound.

Ligands

Ligands play a crucial role in the formation and properties of coordination complexes. They are ions or molecules that have lone pairs of electrons available to form coordinate covalent bonds with the central metal. Ligands can vary widely, from simple ions like chloride (\( ext{Cl}^- \)) to larger, more complex structures like ethylenediamine.The nature and strength of the ligand-metal bond can affect the stability and reactivity of a complex. For example:

• Monodentate ligands: These have a single donor atom that binds to the metal, such as ammonia (\( ext{NH}_3 \)).
• Polydentate ligands (Chelates): These bind through multiple atoms, increasing the stability of the complex. An example is ethylenediamine which binds through two nitrogen atoms.

Ligands also influence the color, magnetic properties, and solubility of coordination complexes, making them central to the chemical and physical attributes of these species.

Chromium Complexes

Chromium complexes offer a rich area of study due to their varied chemistry and application. Chromium, in its various oxidation states, can form numerous coordination compounds. It acts as the central metal atom in complexes, often surrounded by different ligands that influence its properties.For example, the complexes mentioned in the exercise, \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3} \mathrm{Cl}_{3}\right]\) and \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^+\), exhibit octahedral coordination involving water and chloride ligands. These complexes can exhibit geometric isomerism due to the presence of different ligand types, affecting how these complexes interact with light and other chemicals. Chromium complexes have applications in industrial processes, pigmentation, and even in biological systems where their redox properties are vital for certain biological functions.