Problem 48 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct name for the coordination compound, \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]\) is (a) tris-(ethylenediamine)chromate(III) trioxalato-cobalt(III) (b) tris-(ethylenediamine) chromium(III) trioxalatoco-baltate (III) (c) tris-(ethylenediamine)chromate(III)trioxalatocobaltate(III) (d) tris-(ethylenediamine)chromaium(III)trioxalatocobalt(III)

Step-by-Step Solution

Answer

(c) tris-(ethylenediamine)chromate(III)trioxalatocobaltate(III)

Step 1: Understand the Components

The compound is a coordination complex consisting of two parts in a double ionic coordination formula: \([\mathrm{Cr}(\mathrm{en})_{3}]^{3+}\) and \([\mathrm{Co}(\mathrm{C}_{2}\mathrm{O}_{4})_{3}]^{3-}\). The chromium is coordinated with ethylenediamine (\(\mathrm{en}\)), and the cobalt is coordinated with oxalate (\(\mathrm{C}_{2}\mathrm{O}_{4}\)).

Step 2: Naming the Cation Complex

The cationic complex \([\mathrm{Cr}(\mathrm{en})_{3}]^{3+}\) includes chromium (\(\mathrm{Cr}\)) bonded with three ethylenediamine ligands. According to naming conventions, we use the term 'tris-' for three identical neutral ligands. The metal in cationic complexes retains its elemental name with its oxidation state, (III) here, indicated in Roman numerals: **tris-(ethylenediamine)chromium(III)**.

Step 3: Naming the Anionic Complex

The anionic complex \([\mathrm{Co}(\mathrm{C}_{2}\mathrm{O}_{4})_{3}]^{3-}\) consists of cobalt (\(\mathrm{Co}\)) and three oxalate ligands. Using 'trioxalato-' for the three oxalate ligands, and since the complex is an anion, cobalt is named "cobaltate." Hence, we suffix "-ate" to its name. Its oxidation state (III) is also indicated: **trioxalatocobaltate(III)**.

Step 4: Combine the Names

Combine the cation and anion names to form the full compound name: tris-(ethylenediamine)chromium(III) trioxalatocobaltate(III). Ensure there is no extra spacing between the parts.

Key Concepts

Complex Compounds Naming

In coordination chemistry, naming complex compounds follows specific rules to ensure consistency and clarity in communication. The compound in question, \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{3}\right]\), demonstrates a systematic approach required in naming coordination complexes.

• Cation Naming: If the complex ion is a cation, the name for the metal remains as it is in the periodic table along with its oxidation state indicated in parentheses using Roman numerals.
• Anion Naming: Conversely, if the complex is an anion, the metal's name typically ends in "-ate" followed by its oxidation state.
• Order and Syntax: The cationic complex's name is placed first, followed by the anionic complex's name without any extra spacing in between.

These conventions create an ordered and unambiguous way to express the structure and composition of coordination compounds.

Ligands in Coordination Compounds

Ligands are molecules or ions that bind to a central metal atom or ion in a coordination complex. They play a crucial role in determining the properties and reactivity of these compounds. Understanding the role of ligands like ethylenediamine and oxalate is vital.

• Types of Ligands: Ligands can be simple ions like chloride (\(\mathrm{Cl}^-\)) or more complex molecules like ethylenediamine (\(\mathrm{en}\)). They can be neutral or carry a charge.
• Neutral Ligands: Ethylenediamine, a neutral ligand, coordinates using two donor nitrogen atoms, forming a stable complex with metals like chromium.
• Anionic Ligands: Oxalate (\(\mathrm{C}_2\mathrm{O}_4^{2-}\)), a bidentate ligand, uses two oxygen atoms to form chelate rings around electrophilic metal centers such as cobalt.

By donating electrons to the metal, ligands influence the color, magnetism, and other chemical properties of the complex.

Oxidation States in Coordination Compounds

Determining the oxidation state in coordination compounds is crucial for naming them correctly and understanding their chemical behavior. The oxidation state is an indicator of the degree of oxidation or reduction of an atom within the complex. Here's how you can figure it out:

• Assign Known Charges: First, assign known charges. For instance, ethylenediamine (\(\mathrm{en}\)) is a neutral ligand, meaning it has no charge, while oxalate (\(\mathrm{C}_2\mathrm{O}_4^{2-}\)) carries a \(-2\) charge.
• Calculate Oxidation States: The charge on the entire complex can be used to find the oxidation states of the central metals. For example, in \([\mathrm{Cr}(\mathrm{en})_{3}]^{3+}\), with ethylenediamine being neutral, the oxidation state of chromium must be \(+3\) to match the complex's charge.
• Apply to Anion: Similarly, in \([\mathrm{Co}(\mathrm{C}_2\mathrm{O}_4)_{3}]^{3-}\), each \(\mathrm{C}_2\mathrm{O}_4\) contributes \(-2\), thus cobalt must be in the \(\mathrm{+3}\) oxidation state to balance the overall \(-3\) charge of the anionic complex.

Understanding oxidation states not only helps in naming but also gives insight into the compound’s potential reactivity and stability within chemical reactions.