Question

Which of the following complexes can exist as diastereoisomers? (a) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{4}\right]^{-}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{S} \mathrm{Br}\right]^{2+}\) (c) \(\left[\mathrm{FeCl}_{2}(\mathrm{NCS})_{2}\right]^{2-}\) (tetrahedral) (d) \(\left[\mathrm{PtCl}_{2} \mathrm{Br}_{2}\right]^{2-}\) (square planar)

Short answer

Complex (a) and (d) can exist as diastereoisomers.

Step-by-step solution

Analyze Complex (a)

The complex \( \left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Cl}_{4}\right]^{-} \) is an octahedral coordination compound. For a complex to exhibit diastereoisomerism, it often requires the presence of chiral centers or different spatial arrangements that aren't mirror images (i.e., geometric isomerism). This complex can have different arrangements of \( \mathrm{NH}_3 \) and \( \mathrm{Cl} \) ligands as cis and trans-isomers, which are not mirror images. Thus, this complex can exist as diastereoisomers.

Analyze Complex (b)

The complex \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\mathrm{Br}\right]^{2+} \) is incorrect as presented. Assuming it should be \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\mathrm{Br}\right]^{2+} \), it is an octahedral complex. This complex cannot form diastereoisomers because there are no different spatial arrangements for a single \( \mathrm{Br} \) that can create non-superimposable, non-mirror image stereochemistry.

Analyze Complex (c)

The complex \( \left[\mathrm{FeCl}_{2}(\mathrm{NCS})_{2}\right]^{2-} \) is tetrahedral. Tetrahedral complexes with two different types of ligands do not show geometric isomerism because all angles are 109.5°, leading to equivalent positions for all ligands, thus no diastereoisomers are possible.

Analyze Complex (d)

The complex \( \left[\mathrm{PtCl}_{2}\mathrm{Br}_{2}\right]^{2-} \) is square planar. In square planar complexes, when there are two pairs of different monodentate ligands, it can exhibit geometric isomerism (cis and trans forms). Therefore, this complex can exist as diastereoisomers.

Key concepts

Diastereoisomers

Diastereoisomers are a fascinating aspect of chemistry, especially in coordination complexes. These are stereoisomers that are not mirror images of each other, which allows them to have distinct spatial arrangements. They can arise in coordination complexes featuring metal centers with ligands arranged differently.
One key point about diastereoisomers is that they usually involve different geometries such as cis and trans forms, as seen in some octahedral and square planar complexes.

• In an octahedral complex, diastereoisomerism often occurs when different ligands can be swapped between adjacent (cis) and opposite (trans) positions around a metal center.
• For square planar complexes, the arrangement on a flat plane allows certain complexes to exhibit cis and trans forms depending on the arrangement of differing ligands.

This difference in spatial arrangement leads them to have different physical and chemical properties, making them significant in various chemical reactions and industrial applications.

Geometric Isomerism

Geometric isomerism is a type of stereoisomerism that is crucial in understanding the structural arrangement of coordination complexes. It occurs due to different positions of ligands around the central metal atom without breaking any bonds.

• For example, in octahedral complexes, which have six ligands around a metal, such isomerism can lead to cis and trans configurations, particularly when the ligands are not identical.
• In square planar complexes, geometric isomerism happens when there are two pairs of identical ligands, allowing for different spatial configurations.

The distinct configuration affects the physical and chemical properties of the complexes, such as color, magnetic properties, and reactivity. Understanding geometric isomerism is therefore essential for both academic studies and practical applications in chemistry.

Octahedral Complexes

Octahedral complexes are a common and intriguing category within coordination chemistry. These complexes consist of a central metal atom surrounded symmetrically by six ligands, forming a structure similar to an octahedron.
This geometry allows for a variety of isomeric forms, which impacts their chemical behavior. Geometric isomerism is prevalent among such complexes due to the multiple possible arrangements of ligands.

• Cis and trans isomers can form when there are two or more different types of ligands, which do not equally share all positions around the metal.
• These complexes are particularly common with transition metals, and their properties can be precisely tailored by careful selection of ligands.

The versatility and diversity of octahedral complexes make them a prime area of study in both developing new catalytic compounds and understanding fundamental chemical principles.

Square Planar Complexes

Square planar complexes are structured with four ligands positioned on a single plane around the central metal atom, forming a square shape. This configuration is particularly prevalent with d8 metal ions, such as platinum or palladium.
Unlike tetrahedral complexes, square planar complexes can readily display geometric isomerism.

• The classic example involves the presence of two pairs of different ligands, which can be arranged to form cis or trans isomers.
• These geometric forms lead to significantly different properties such as solubility, stability, and even biological activity, making these isomers critical in pharmaceuticals.

Understanding square planar complexes is essential because they form the basis for many important catalytic and therapeutic compounds, highlighting their importance in industrial and medical chemistry.