Question

Four geometric isomers are possible for \([\mathrm{Co}(\mathrm{en})\) \(\left.\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}\right]^{+} .\) Draw the structures of all four. (Two of the isomers are chiral, meaning that each has a nonsuperimposable mirror image.)

Short answer

The four isomers include two cis and two trans forms. Two cis forms are chiral.

Step-by-step solution

Identifying Ligands and Complex Geometry

The complex ion \([\mathrm{Co}(\mathrm{en})(\mathrm{NH}_{3})_{2}(\mathrm{H}_{2} \mathrm{O}) \mathrm{Cl}]^{+}\) contains cobalt as the central metal. It forms an octahedral geometry because \(\mathrm{Co}^{3+}\) typically forms octahedral complexes. The ligands present are ethylenediamine (en) as a bidentate ligand, two ammonia molecules, one water molecule, and one chloride ion as monodentate ligands.

Understanding Isomer Types

In this complex, the possible isomers include geometrical (cis/trans) isomers and optical isomers due to the presence of bidentate ligand (en) and monodentate ligands arranged around the octahedral Co center.

Drawing Cis Isomers

For the cis isomers, two configurations need consideration: all bidentate and monodentate ligands are adjacent. Draw one where \(\mathrm{NH}_{3}\), \(\mathrm{H}_{2} \mathrm{O}\), and \(\mathrm{Cl}\) are grouped on the same face (cis) and another where only two of the monodentates are on the same face, leading to different optical activity.

Drawing Trans Isomers

In the trans configuration, the two \(\mathrm{NH}_{3}\) ligands are opposite each other. Two distinct isomers can arise where the positioning of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{Cl}\) vary. One will be trans with respect to the bidentate ligand and exhibits distinct symmetry properties.

Determining Chirality

Examine each drawn isomer for chirality: a chiral isomer will lack symmetry elements such as a plane of symmetry or a center of symmetry. Identifying the structures that do not have superimposable mirror images will show which of these isomers are chiral.

Key concepts

Chirality in coordination compounds

Chirality is a fundamental property in coordination chemistry. It occurs when a molecule has non-superimposable mirror images, known as enantiomers. In coordination compounds, chirality can arise due to the spatial arrangement of ligands around a central metal ion. When dealing with chiral coordination compounds, one often needs to determine the arrangement of ligands that leads to chirality.

In the context of the cobalt complex \([\mathrm{Co}(\mathrm{en})(\mathrm{NH}_{3})_{2}(\mathrm{H}_{2} \mathrm{O}) \mathrm{Cl}]^{+}\), chirality comes into play as one of the potential geometric isomers lacks internal planes of symmetry. This lack of symmetry leads to two non-superimposable configurations.

Chiral isomers have significant implications in real-world applications, especially in pharmaceuticals, where different enantiomers of a molecule can have drastically different effects in biological systems. Understanding how to identify and predict the chirality in coordination compounds is essential for chemists, particularly when designing complexes for specific applications.

Octahedral complexes

Octahedral complexes are a common and important structure in coordination chemistry. These complexes have six ligands symmetrically arranged around a central metal ion, creating an octahedral shape. This geometry is prevalent because it allows for varied ligand interactions and enhances the stability of the metal-ligand bonds.

For \([\mathrm{Co}(\mathrm{en})(\mathrm{NH}_{3})_{2}(\mathrm{H}_{2} \mathrm{O}) \mathrm{Cl}]^{+}\), cobalt typically forms an octahedral complex due to its electronic configuration and the ability to surround itself with six ligands. The arrangement of these ligands can lead to different isomers, based on their positions relative to each other.

• **Cis isomers:** Where similar ligands occupy adjacent positions (90° apart).
• **Trans isomers:** Where similar ligands occupy opposite positions (180° apart).

A robust understanding of octahedral complexes with different ligands is vital to predicting and controlling the properties of these compounds. This knowledge is crucial when examining geometric isomerism, as it dictates the possible spatial arrangements.

Bidentate and monodentate ligands

Ligands can be classified based on the number of donor atoms that bind to the central metal ion. Monodentate ligands have a single donor atom that connects to the metal, whereas bidentate ligands have two donor atoms, allowing them to "bite" the metal ion at two sites. This distinction is important when discussing the binding and structural characteristics of complexes.

Within the \([\mathrm{Co}(\mathrm{en})(\mathrm{NH}_{3})_{2}(\mathrm{H}_{2} \mathrm{O}) \mathrm{Cl}]^{+}\) complex, the ethylenediamine (\(\mathrm{en}\)) acts as a bidentate ligand. This allows it to create a more stable, two-point attachment to cobalt. Ammonia, water, and chloride act as monodentate ligands, each offering one point of attachment.

Bidentate ligands can significantly impact geometric isomerism because they occupy two coordination sites and influence how other ligands can be arranged. Recognizing the role of different types of ligands in forming complex structures not only helps in understanding isomerism but also in designing compounds with desired chemical and physical properties.

• **Monodentate Ligands:** Bind through one atom.
• **Bidentate Ligands:** Bind through two atoms, often stabilizing the complex more effectively.

Understanding these concepts is key for students learning to predict and rationalize the formation and behavior of various coordination compounds.