Question

Early in the 20th century, complexes sometimes were given names based on their colors. Two compounds with the formula \(\mathrm{CoCl}_{3} \cdot 4 \mathrm{NH}_{3}\) were named praseo-cobalt chloride (praseo \(=\) green ) and violio-cobalt chloride (violet color). We now know that these compounds are octahedral cobalt complexes and that they are cis and trans isomers. Draw the structures of these two compounds and name them using systematic nomenclature.

Short answer

Cis: cis-tetrammine-dichlorocobalt(III) chloride; Trans: trans-tetrammine-dichlorocobalt(III) chloride.

Step-by-step solution

Understand the Coordination Complex

The compound \( \mathrm{CoCl}_{3} \cdot 4 \mathrm{NH}_{3} \) represents a coordination complex where cobalt (Co) is the central metal atom surrounded by chloride ions (Cl) and ammonia ligands (NH3). The coordination number is 6, forming an octahedral geometry.

Identify The Isomer Types

Cis and trans isomers occur due to different spatial arrangements of ligands in a complex. In octahedral complexes containing two types of ligands, cis isomers have identical ligands adjacent, while trans isomers have identical ligands opposite each other.

Determine the Cis Isomer

In the cis form, the geometry will have two chloride ions adjacent to each other and two ammonia groups adjacent to each other. For the formula \( \mathrm{CoCl}_2(NH_3)_4^+ \), the remaining \( NH_3 \) groups will occupy adjacent positions.

Sketch the Cis Structure

Draw the octahedral complex where two chloride ions and four ammonia ligands surround the Co center. Position two chloride ions adjacent to each other in one plane while the four ammonia ligands fill the remaining positions.

Determine the Trans Isomer

In the trans form, the two chloride ions are positioned opposite each other, allowing the four ammonia ligands to occupy positions between them.

Sketch the Trans Structure

Draw the octahedral complex where the two chloride ions are placed opposite each other and the four ammonia ligands are positioned on adjacent sides.

Systematic Nomenclature

The cis isomer is named cis-tetrammine-dichlorocobalt(III) chloride and the trans isomer is named trans-tetrammine-dichlorocobalt(III) chloride. The enumeration and location of ligands are based on their positions in the octahedral geometry.

Key concepts

Octahedral Geometry

In coordination chemistry, octahedral geometry is a crucial concept. It refers to the spatial arrangement of ligands surrounding a central metal atom in a complex. This shape is similar to two square pyramids base-to-base. It involves six ligands symmetrically coordinated around the central atom, which makes it very stable.
For the compound \( \mathrm{CoCl}_3 \cdot 4 \mathrm{NH}_3 \), the central metal atom is Cobalt (Co), which forms bonds with six ligands. In this case, they are 4 ammonia molecules (NH3) and 2 chloride ions (Cl), creating an octahedral shape. This particular coordination number is essential because it dictates this octahedral geometry, which affects the compound's properties and reactivity.
The understanding of this geometric arrangement helps predict how molecules will interact, influencing their chemical behavior in reactions.

Cis-Trans Isomerism

Cis-trans isomerism is a kind of stereoisomerism that arises in some coordination complexes due to the different spatial arrangement of ligands. It's crucial for understanding how the physical and chemical properties of these complexes vary. In octahedral complexes like \( \mathrm{CoCl}_3 \cdot 4 \mathrm{NH}_3 \), two types of spatial arrangements are possible: cis and trans.

• **Cis Isomer:** In this configuration, identical ligands are adjacent to each other. For the given coordination complex, two chloride ions are placed next to each other, while the ammonia groups occupy the remaining positions.


• **Trans Isomer:** Here, identical ligands are located opposite each other. This means in the complex, the chloride ions are directly across from one another on the central cobalt atom.

The different arrangements result in distinct physical properties. For instance, the cis and trans isomers of a complex can exhibit different colors, solubility, and reactivity, which is why they were historically named based on color.

Systematic Nomenclature

Systematic nomenclature is essential for the consistent and clear naming of chemical compounds, particularly coordination complexes. Unlike older naming conventions which were based on color, systematic nomenclature provides a detailed account of a compound's composition and structure. For compounds like \( \mathrm{CoCl}_3 \cdot 4 \mathrm{NH}_3 \), it's critical in ensuring universal understanding.
The process involves four main steps:

• **Enumerate Ligands:** List the number of each type of ligand. In this case, there are four ammine (NH3) ligands and two chloride (Cl) ligands.


• **Specify Geometry:** Note the geometric arrangement, here being 'tetrammine-dichlorocobalt(III)'.


• **Indicate Oxidation State:** The oxidation state of the central metal atom, Cobalt, is mentioned in parentheses, for example, '(III)' in this case.


• **Identify Isomers:** Use prefixes 'cis' or 'trans' to denote isomer differences based on ligand arrangement.

Hence, the names become cis-tetrammine-dichlorocobalt(III) chloride and trans-tetrammine-dichlorocobalt(III) chloride, clearly demonstrating different structures.

Coordination Complexes

Coordination complexes play a significant role in chemistry due to their unique structures and functions. They consist of a central metal ion or atom surrounded by molecules or ions, known as ligands. These complexes are the backbone of many chemical reactions and applications, from catalysis to materials science.
In the complex \( \mathrm{CoCl}_3 \cdot 4 \mathrm{NH}_3 \), cobalt serves as the central metal, while chloride ions and ammonia act as ligands attaching to cobalt through coordinate bonds. This forms a stable and defined structure.
Key aspects of coordination complexes include:

• **Bond Formation:** The bonds form through the sharing of a pair of electrons from the ligand to the metal center, which is specially coordinated.


• **Stability:** The arrangement of ligands around the metal affects the stability and properties of the complex, often making such structures more stable than their components alone.


• **Applications:** Coordination complexes are found in biological systems (e.g., hemoglobin), industrial processes (e.g., catalysis), and even therapeutic compounds (e.g., cancer treatments).

Understanding coordination complexes helps explain the diversity and specificity of compounds in chemical reactions.