Problem 38 - Chemistry and Chemical Reactivity

Question

Which of the following complexes containing the oxalate ion is (are) chiral? (a) \(\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right) \mathrm{C}_{4}\right]^{2-}\) (b) \(\operatorname{cis}-\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]^{2-}\) (c) \(\operatorname{trans}-\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]^{2-}\)

Step-by-Step Solution

Answer

Only complex (b) is chiral.

Step 1: Understand Chiral Compounds

A chiral compound is one that cannot be superimposed on its mirror image. In coordination complexes, chirality can often be determined by looking for a lack of symmetry. Specifically, geometric isomers of complexes can exhibit chirality, often in the case of cis and trans configurations.

Step 2: Analyze Complex (a)

The complex (a) \( \left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right) \mathrm{Cl}_{4}\right]^{2-} \) contains one oxalate (\( \mathrm{C}_2 \mathrm{O}_4^{2-} \)) ligand and four chloride ions (\( \mathrm{Cl}^- \)). This complex typically forms in an octahedral geometry. Since the chlorides are all the same ligands, this complex is symmetrical and does not exhibit chirality.

Step 3: Analyze Complex (b) - cis configuration

The complex (b) \( \operatorname{cis}-\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]^{2-} \) has a cis configuration. The two oxalate ligands and two chloride ions are arranged such that they are next to each other. In this arrangement, the complex lacks a plane of symmetry and is chiral. There are no internal planes that can divide and mirror the entire complex in half, so it is non-superimposable on its mirror image.

Step 4: Analyze Complex (c) - trans configuration

The complex (c) \( \operatorname{trans}-\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]^{2-} \) involves trans configuration. Here, the two oxalate ligands and two chloride ions are directly opposite each other. The geometry forms a plane of symmetry that makes the complex superimposable with its mirror image, hence it is achiral.

Key Concepts

Chirality

Chirality is a concept often encountered in chemistry, describing a molecule that cannot be superimposed on its mirror image. Think of it like your hands: the left hand is a mirror image of the right, but they cannot be perfectly aligned in the same space. In coordination chemistry, chiral complexes lack symmetry, meaning there are no planes or points within the complex that can divide the structure into two identical halves. Non-superimposable mirror images are called enantiomers.

In coordination compounds, chirality is also influenced by the arrangement of ligands around a central metal atom. Geometric isomerism, such as in cis and trans forms, can impact chirality. For example, certain configurations might lead to non-superimposable mirror images, making them chiral, while other configurations may create symmetrical structures that are achiral.

Geometric Isomerism

Geometric isomerism refers to the different spatial arrangements of ligands around a central metal atom in a coordination complex. It is based on the spatial arrangement rather than the actual connectivity of the atoms. The most common forms in coordination chemistry are **cis** and **trans** isomers:

• **Cis Isomers**: Ligands are next to each other. This arrangement often leads to less symmetric structures and can make the complex chiral if there is no internal plane splitting the complex into identical halves.
• **Trans Isomers**: Ligands are opposite each other. This usually results in symmetric structures that have a plane of symmetry, often rendering the complex achiral.

Understanding whether a complex is **cis** or **trans** is critical when determining potential chirality. For example, a **cis**-configured complex might be chiral, while a **trans**-configured complex often is not.

Oxalate Ligand

The oxalate ligand \(( ext{C}_2 ext{O}_4^{2-})\) is a common bidentate ligand in coordination chemistry. This means it can attach to the central metal atom through two points of connection. Its unique shape allows it to form stable five-membered rings with the metal center, enhancing the stability of the complex.

Oxalate ligands are often involved in forming complexes that can exhibit geometric isomerism. They are capable of creating both **cis** and **trans** configurations, depending on the other ligands and the overall symmetry of the complex. Their role in determining the chirality of a complex is essential, as the arrangement of oxalate groups in relation to other attached atoms can significantly influence whether the overall complex is chiral or achiral.

The flexibility and versatile bonding of oxalate make it an important ligand in studying both geometric isomerism and chirality.