Problem 77 - General Chemistry: Principles and Modern Applications

Question

Name the following coordination compounds. Which one(s) exhibit(s) isomerism? Explain. (a) \(\left[\operatorname{CoBr}\left(\mathrm{NH}_{3}\right)_{5}\right] \mathrm{SO}_{4}\) (b) \(\left[\operatorname{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right]\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{3}\right] \mathrm{Cl}_{3}\)

Step-by-Step Solution

Answer

Name of compounds: A. Pentaamminebromocobalt(III) sulfate, B. Hexaamminechromium(III) hexacyanocobaltate(III), C. Sodium hexanitritocobaltate(III), and D. Tris(ethylenediamine)cobalt(III) chloride. Only D. Tris(ethylenediamine)cobalt(III) chloride is capable of displaying isomerism.

Step 1 Title - Name the Coordination Compounds

To name the compounds, follow the IUPAC rules: Name the ligands first, in alphabetical order, and then the metal atom/ion. Prefixes indicating the number of ligands are not used for ordering. Also, use Roman numerals in parentheses to indicate the oxidation state of the metal. A. The name for \(\left[\operatorname{CoBr}(\mathrm{NH}_{3})_{5}\right]\mathrm{SO}_{4}\) is 'Pentaamminebromocobalt(III) sulfate' B. The name for \(\left[\operatorname{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right] \left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) is 'Hexaamminechromium(III) hexacyanocobaltate(III)' C. The name for \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right]\) is 'Sodium hexanitritocobaltate(III)' D. The name for \(\left[\mathrm{Co}(\mathrm{en})_{3}\right] \mathrm{Cl}_{3}\) is 'Tris(ethylenediamine)cobalt(III) chloride'

Step 2 Title - Identify the Isomers

An understanding of isomers in coordination chemistry is required. Isomers are distinct compounds that share the same chemical formula. However, their atoms, ions or molecules are arranged differently. A. \(\left[\operatorname{CoBr}(\mathrm{NH}_{3})_{5}\right]\mathrm{SO}_{4}\) does not exhibit isomerism. B. \(\left[\operatorname{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) does not exhibit isomerism. C. \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right]\) does not exhibit isomerism. D. \(\left[\mathrm{Co}(\mathrm{en})_{3}\right] \mathrm{Cl}_{3}\) can exhibit geometric isomerism due to the presence of the bidentate ligand ethylenediamine \( \mathrm{en} \). Different arrangements of the \( \mathrm{en} \) around the \(\mathrm{Co}\) creates geometric isomers (fac- and mer-).

Key Concepts

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) provides a systematic way to name chemical compounds. For coordination compounds, naming follows specific rules:

• Ligands first: Name the ligands in alphabetical order before the central metal atom or ion. For example, in the compound \( \left[\operatorname{CoBr}(\mathrm{NH}_{3})_{5}\right]\mathrm{SO}_{4} \), 'bromo' (Br) and 'ammine' (\( \mathrm{NH}_3 \)) are the ligands.


• Prefix usage: Use prefixes like 'penta-', 'hexa-' to indicate the number of identical ligands. However, these prefixes do not affect the ordering of the ligands.


• Metal oxidation state: Indicate the oxidation state of the central metal with Roman numerals in parentheses. For instance, 'cobalt(III)' is written as 'cobalt(III)', indicating cobalt has a +3 oxidation state.


• Space group suffix: For ionic compounds, the anionic part comes after the cationic coordination part. In our example, 'sulfate' follows the name of the coordination sphere 'pentaamminebromocobalt(III)'.

These guidelines help in creating a universal understanding and recognition of chemical compounds.

Isomerism in Chemistry

Isomerism is a fascinating concept in chemistry where compounds have the same molecular formula but differ in arrangement.

• Structural isomers: They differ in how atoms are connected or the order of connectivity, often affecting their chemical behavior.


• Stereoisomers: They have the same sequence of bonded atoms but differ in spatial orientation. This type of isomerism is common in coordination compounds.

Isomerism can significantly influence the physical and chemical properties of a compound. Recognizing isomers is crucial as they can have different biological activities or chemical reactivities. Understanding these variations helps in harnessing the right compound for specific industrial or pharmaceutical purposes.

Geometric Isomerism

Geometric isomerism is a kind of stereoisomerism. It mainly occurs in compounds with a rigid structure.

• Occurrence in coordination compounds: Geometric isomerism often appears in coordination compounds with different spatial arrangements of ligands.


• Facial and meridional forms: In compounds with bidentate ligands like ethylenediamine ('en'), fac-isomers have ligands adjacent to each other, and mer-isomers have them positioned across the center.


• Physical and chemical properties: Geometric isomers have distinct physical and chemical properties, making understanding these differences crucial in fields like material science and pharmacology.

The concept of geometric isomerism helps in visualizing these distinct but related compounds, aiding in targeted applications.

Coordination Chemistry

Coordination chemistry revolves around metal complexes, where central metal ions are surrounded by molecules or anions known as ligands. The field is essential for understanding the behavior of these complexes.

• Coordination number: It indicates the number of ligand bonds to the central metal atom. This can range from 2 to 12, influencing the compound's geometry.


• Ligands: They can be neutral molecules like water or ammonia, or ions such as chloride. Ligands donate electron pairs to the metal, forming coordinate covalent bonds.


• Applications: Coordination compounds have vast applications: from catalysis to material synthesis, and play vital roles in biological systems like hemoglobin.

Grasping the principles of coordination chemistry is vital for chemists as it sheds light on the interactions between metals and ligands, which is fundamental to various chemical and biological processes.