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Chapter 22: Problem 38

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) \(\operatorname{trans}-\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]^{2-}\)

Short Answer

Expert verified
Only complex (b) is chiral.

Step by step solution

01

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.
02

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.
03

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.
04

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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.

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Most popular questions from this chapter

Excess silver nitrate is added to a solution containing \(1.0 \mathrm{mol}\) of \(\left[\mathrm{Co}\left(\mathrm{NH}_{9}\right)_{4} \mathrm{Cl}_{2}\right] \mathrm{Cl} .\) What amount of \(\mathrm{AgCl}\) (in moles) will precipitate?

An this question, we explore the differences between metal coordination by monodentate and bidentate ligands. Formation constants, \(K_{f},\) for \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}(\mathrm{aq})\) and \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}(\mathrm{aq})\) are as follows: $$\begin{aligned} \mathrm{Ni}^{2+}(\mathrm{aq})+6 \mathrm{NH}_{3}(\mathrm{aq}) & \longrightarrow\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}(\mathrm{aq}) & & K_{f}=10^{8} \\ \mathrm{Ni}^{2+}(\mathrm{aq})+3 \mathrm{en}(\mathrm{aq}) & \longrightarrow\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}(\mathrm{aq}) & & K_{f}=10^{18} \end{aligned}$$ The difference in \(K_{f}\) between these complexes indicates a higher thermodynamic stability for the chelated complex, caused by the chelate effect. Recall that \(K\) is related to the standard free energy of the reaction by \(\Delta G^{\circ}=-R T \ln K\) and \(\Delta G^{\circ}=\Delta H^{\circ}-T \Delta S^{\circ} .\) We know from experiment that \(\Delta H^{\circ}\) for the \(\mathrm{NH}_{3}\) reaction is \(-109 \mathrm{kJ} / \mathrm{mol},\) and \(\Delta H^{\circ}\) for the ethylenediamine reaction is \(-117 \mathrm{kJ} / \mathrm{mol}\). Is the difference in \(\Delta H^{\circ}\) sufficient to account for the \(10^{10}\) difference in \(K_{f}\) ? Comment on the role of entropy in the second reaction.

The complex \(\left[\mathrm{Mn}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) has five unpaired electrons, whereas \(\left[\mathrm{Mn}(\mathrm{CN})_{6}\right]^{4-}\) has only one. Using the ligand field model, depict the electron configuration for each ion. What can you conclude about the effects of the different ligands on the magnitude of \(\Delta_{0} ?\)

In which of the following complexes are geometric isomers possible? If isomers are possible, draw their structures and label them as cis or trans, or as fac or mer. (a) \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (c) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right) \mathrm{Br}_{3}\right]^{-}\) (b) \(\operatorname{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{F}_{3}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}\right]^{2+}\)

Give the formula of a complex constructed from one \(\mathrm{Ni}^{2+}\) ion, one ethylenediamine ligand, three ammonia molecules, and one water molecule. Is the complex neutral or is it charged? If charged, give the charge.
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