Question

Draw a structure for cis-dichlorobis(ethylenediamine)cobalt(III) ion. Is this ion chiral? Is the trans isomer chiral? Explain.

Short answer

The cis-dichlorobis(ethylenediamine)cobalt(III) ion does have a plane of symmetry and therefore is not chiral. Likewise, the trans isomer also has a plane of symmetry and thus is not chiral.

Step-by-step solution

Identify the Components of the Complex Ion

This compound is called 'cis-dichlorobis(ethylenediamine)cobalt(III) ion'. In this, the cobalt(III) ion is the central metal ion, ethylenediamine (denoted as en, a bidentate ligand) is the ligand, and there are two chloride ions. The term 'cis' indicates that the two similar groups lie on the same side of a molecule.

Draw the Structure of the Complex Ion

Here, coordination number of the cobalt ion is 6 (4 from two ethylenediamine molecules and 2 from two chloride ions). As the coordination number is 6, the structure is likely to be octahedral. In the cis-configuration, arrange the two chloride ions at adjacent positions, and the two ethylenediamine ligands at the remaining positions.

Determine the Chirality of the Cis Isomer

For a molecule to be chiral, it should lack a plane of symmetry or a center of symmetry. Analyzing the drawn structure, it can be seen that the cis isomer does have a plane of symmetry and hence it is not chiral.

Draw the Structure of the Trans Isomer

In the trans isomer, the similar groups are arranged directly opposite to each other on the molecule. So position the two chloride ions opposite to each other and then place the ethylenediamine ligands at the remaining positions in the octahedral structure.

Determine the Chirality of the Trans Isomer

Just like the cis isomer, for the trans isomer to be chiral, it should not have a plane of symmetry. Looking closely at the structure, it can be noted that trans isomer does possess a plane of symmetry. Therefore, the trans isomer is also not chiral.

Key concepts

Chirality

Chirality is a fascinating concept in chemistry. It refers to the property of a molecule that makes it non-superimposable on its mirror image. This is similar to how our left and right hands are mirror images but cannot be perfectly aligned on top of one another.
In coordination chemistry, chirality becomes especially interesting when considering complex ions. For such a complex to be chiral, it must lack a plane or center of symmetry. This means you won't be able to flip or rotate it in a way that looks exactly like its mirror image.
In the exercise example, the cis-dichlorobis(ethylenediamine)cobalt(III) complex ion was explored for chirality. The structure drawn revealed the presence of symmetry, indicating that neither the cis nor the trans isomer could be considered chiral in this context. Understanding chirality is crucial because chiral molecules can have vastly different properties and behaviors despite having the same types of atoms and bonds!

Complex Ion Structures

Complex ion structures may initially seem complex, but with a few principles in mind, they become easier to understand. A complex ion typically consists of a central metal atom or ion surrounded by molecules or ions known as ligands. Ligands can donate electron pairs to the metal center, forming a stable structure.
In the case of the cis-dichlorobis(ethylenediamine)cobalt(III) ion, cobalt is the central metal ion. It forms bonds with two ethylenediamine ligands and two chloride ions, making the overall coordination number six.

• Ethylenediamine acts as a bidentate ligand, meaning it can form two bonds with the metal center.
• The resulting structure is octahedral, which is common for coordination numbers of six.

Such structures play an important role in chemistry as they can exhibit different properties based on their specific configuration, such as color, reactivity, and magnetic behavior. These considerations are key in fields like bioinorganic chemistry and the development of coordination compounds for medical applications.

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism important in the study of coordination complexes. It occurs when ligands attached to a central atom can be positioned differently in space.
In the example provided, we have a complex ion with the formula cis-dichlorobis(ethylenediamine)cobalt(III).

• The term 'cis' indicates that two identical chloride ligands are adjacent to each other in the octahedral structure, leading to some unique properties.
• The 'trans' isomer, on the other hand, positions these chloride ligands opposite each other.

Cis and trans isomers can exhibit different physical and chemical properties, even though they have the same molecular formula. In our exercise, these different spatial arrangements of ligands changed how we assessed chirality. Recognizing and visualizing these spatial differences is crucial for understanding how molecules interact and react in chemical processes.