Problem 47 - Chemistry and Chemical Reactivity

Question

Early in the 20 th century, coordination compounds 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.

Step-by-Step Solution

Answer

The green isomer is cis-tetramminedichlorocobalt(III) chloride, and the violet isomer is trans-tetramminedichlorocobalt(III) chloride.

Step 1: Understand the Problem

The task involves distinguishing between two isomers of a cobalt complex with the formula \(\mathrm{CoCl}_{3} \cdot 4 \mathrm{NH}_{3}\). These are coordination compounds exhibiting octahedral geometry, meaning the central cobalt atom is bonded to six ligands. The problem hints at two isomers: cis and trans. We need to draw and name these isomers using systematic nomenclature.

Step 2: Identify the Cis and Trans Configurations

For octahedral complexes, the cis isomer has similar ligands (in this case \(\mathrm{NH}_{3}\)) adjacent to each other, while the trans isomer has these ligands opposite to each other. Given the formula, two of the ammine (i.e., \(\mathrm{NH}_{3}\)) ligands are replaced by chloride ions to form the complex. Therefore, in the cis isomer, two chloride ions will be adjacent, while in the trans, they will be opposite.

Step 3: Draw the Structures

For cis-\([\mathrm{CoCl}_2(\mathrm{NH}_3)_4]\)Cl, two of the \(\mathrm{Cl}^-\) ions are adjacent giving it a praseo (green) color. For trans-\([\mathrm{CoCl}_2(\mathrm{NH}_3)_4]\)Cl, the \(\mathrm{Cl}^-\) ions are opposite, which corresponds to the violio (violet) color. These can be drawn using 3D representations with \(\mathrm{Cl}^-\) ions and \(\mathrm{NH}_3\) ligands in appropriate positions around the cobalt ion.

Step 4: Name the Isomers Using Systematic Nomenclature

According to IUPAC nomenclature, the cis isomer is named as "cis-tetramminedichlorocobalt(III) chloride" and the trans isomer is called "trans-tetramminedichlorocobalt(III) chloride". The names derive from the number of ammine ligands and the two chloride ions in the coordination sphere, while the oxidation state of cobalt is indicated by (III) since it is coordinating with 6 ligands (4 \(\mathrm{NH}_3\) and 2 \(\mathrm{Cl}^-\)) and still binds one extra chloride outside the sphere.

Key Concepts

Cis-trans isomers

In coordination chemistry, cis-trans isomers, also referred to as geometric isomers, are molecules with the same chemical formula but different spatial arrangements of atoms. This can lead to distinct chemical and physical properties. For octahedral complexes, like the cobalt complexes in the given exercise, the arrangement of ligands around the central metal atom creates these isomers. In the cis isomer, ligands of the same type, such as ammonia ( \(\mathrm{NH_3} \)) in the case of our cobalt complexes, are adjacent to each other. Think of them being right next to each other in the 3D space around the metal center. On the other hand, in the trans isomer, the same ligands are positioned across from each other, like in a straight line that passes through the center of the metal atom. This spatial difference can cause these isomers to have different colors and reactivities, making their differentiation important in coordination chemistry.

Octahedral geometry

Octahedral geometry is a common molecular shape in coordination compounds, especially those involving transition metals like cobalt. In octahedral geometry, a central metal atom is surrounded by six ligands placed at the vertices of an octahedron (essentially two back-to-back pyramids). This arrangement allows for maximum spatial separation and stabilization of the ligands through symmetrical distribution. The six coordinate bonds formed in this manner create a compact and robust structure, allowing for diverse ligand arrangements such as cis and trans isomers. The beautiful geometrical symmetry in octahedral complexes leads to various unique properties, including the possibility of different color manifestations (as seen in our cis and trans cobalt complexes). Understanding the spatial arrangement in octahedral geometry can help in visualizing and predicting the behavior of complex compounds.

Systematic nomenclature

Systematic nomenclature in coordination chemistry follows rules set forth by the International Union of Pure and Applied Chemistry (IUPAC). This helps in consistently naming compounds so that anyone familiar with the rules can understand their structure. In our exercise, the cobalt complexes are named based on the ligands attached to the central metal, their position (cis or trans), and other structural features.

• The prefix 'tetra-' indicates four ammonia ligands.
• 'Di-' is used for the two chloride ligands in the coordination sphere.
• The term 'cobalt(III)' specifies the oxidation state of the metal.
• The suffix 'chloride' denotes the presence of an additional chloride ion outside the coordination sphere.

When naming, the order is: position of ligands (cis/trans), number and type of ligands, metal and its oxidation state, and any external ions, such as extra chloride. Following IUPAC's systematic approach ensures accurate communication of the compounds' structures and helps differentiate between isomers like cis and trans forms.