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

(b) tris-(ethylenediamine) chromium(III)trioxalatocobaltate (III)

Step 1: Identify the Ligands

In the given coordination compound, \([\mathrm{Cr}(\mathrm{en})_{3}]\), 'en' stands for ethylenediamine, which is a bidentate ligand, meaning it can attach at two places on the metal center. \([\mathrm{Co}(\mathrm{C}_{2}\mathrm{O}_{4})_{3}]\) contains oxalate \((\mathrm{C}_{2}\mathrm{O}_{4})^{2-}\), another bidentate ligand.

Step 2: Determine the Central Metal Ions and Their Oxidation States

The central metal in the complex \([\mathrm{Cr}(\mathrm{en})_{3}]\) is Cr, and in \([\mathrm{Co}(\mathrm{C}_{2}\mathrm{O}_{4})_{3}]\) is Co. Ethylenediamine is neutral, and each oxalate ligand contributes -2. Given the charge neutrality in the overall compound, Cr must be +3 in \([\mathrm{Cr}(\mathrm{en})_{3}]\) and Co must also be +3 in \([\mathrm{Co}(\mathrm{C}_{2}\mathrm{O}_{4})_{3}]\).

Step 3: Name the Positive Complex

The complex \([\mathrm{Cr}(\mathrm{en})_{3}]\) is named with the ligand ethylenediamine abbreviated as 'en'. Since there are three such ligands, we use the prefix 'tris-' to indicate this. Thus, the name is 'tris(ethylenediamine)chromium(III)'.

Step 4: Name the Negative Complex

For the complex \([\mathrm{Co}(\mathrm{C}_{2}\mathrm{O}_{4})_{3}]\), the oxalate ligand is named as 'oxalato'. Since the complex anion with cobalt is negative, we use 'cobaltate' instead of 'cobalt'. There are three oxalate ligands, so we use the prefix 'trioxalato-', and this portion is named 'trioxalatocobaltate(III)'.

Step 5: Combine the Names

Finally, combine the names of the positive and negative complexes to form the full name: 'tris(ethylenediamine)chromium(III) trioxalatocobaltate(III)'.

Key Concepts

Naming Coordination Compounds

Naming coordination compounds might sound daunting, but it's all about following some simple rules. Coordination chemistry uses specific terms and prefixes to correctly identify and describe a compound. In our example, the compound is \( \left[\mathrm{Cr}(\mathrm{en})_{3}\right]\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{3}\right] \).

1. **Identify Ligands and Metals**: Begin by identifying all ligands (molecules or ions attached to the central metal atom) and metal centers. Here, 'en' stands for ethylenediamine, a neutral ligand, and \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\), oxalate, an anionic ligand. The metals are chromium (Cr) and cobalt (Co).

2. **Use Appropriate Prefixes**: Ligands use prefixes indicating their number: 'di-', 'tri-', 'tetra-', etc. Metal names are next. Neutral ligands keep their name, while anionic ligands often end with '-o'.

3. **Consider the Metal's Oxidation State**: The oxidation state is shown with Roman numerals in parentheses. For complex ions, if the metal element ends the name and the ion is negatively charged, adjust the metal name to end with '-ate'.

In our example, 'tris-' indicates 'three' ethylenediamine ligands, 'chromium(III)' specifies the metal and its oxidation state, and 'trioxalatocobaltate(III)' combines the anion adjustment for cobalt and indicates the metal's oxidation state. The entire name is **tris(ethylenediamine)chromium(III) trioxalatocobaltate(III)**.

Bidentate Ligands

Bidentate ligands are a fascinating subset of ligands in coordination chemistry. The term 'bidentate' means 'two teeth' in Latin, which accurately describes how these ligands bind to a metal center.

How does a bidentate ligand work? Both ethylenediamine ('en') and oxalate \((\mathrm{C}_{2}\mathrm{O}_{4}^{2-})\) are bidentate ligands. This means each can form two bonds with a metal atom. Ethylenediamine has two nitrogen atoms that can attach to a metal. Oxalate has two oxygen atoms, both of which can form bonds.

There are several important roles for bidentate ligands:

• They increase the stability of coordination compounds due to the formation of chelate rings.
• The chelate effect provides an entropic advantage, making these compounds more thermodynamically favorable.
• Bidentate ligands can influence the geometric arrangement of the compound, commonly leading to complexes with specific and stable shapes.

Understanding bidentate ligands gives insight into how coordination compounds form and remain stable. Their ability to "bite" in multiple places ensures that the metal center is securely held within the structure.

Oxidation State Determination

Determining oxidation states in coordination compounds is key to properly naming and understanding these structures. It solves a critical puzzle piece of understanding how electrons are shared and transferred in metal complexes.

To find the oxidation state of the central metal in a complex like \( \left[\mathrm{Cr}(\mathrm{en})_{3}\right]\) and \(\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{3}\right]\),follow these steps:

1. **Calculate Overall Charge**: Realize that the compound's overall charge must be zero unless stated otherwise. This balance is achieved by the sum of the metal's oxidation state and the charges of all ligands.

2. **Determine Ligands' Contribution**: Calculate the total charge from ligands. Ethylenediamine is neutral, contributing zero to the charge. In contrast, each oxalate \((\mathrm{C}_{2}\mathrm{O}_{4}^{2-})\) contributes -2.

3. **Solve for the Metal's Charge**: With ligands' charges known and the overall charge zero, solve for the metal's oxidation state. For chromium in \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\), where ethylenediamine is neutral, Cr must be +3 to match zero. For cobalt in \(\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{3}\right]\), the negative charge from three oxalates totals -6; thus, Co is +3 to balance to zero.

Understanding oxidation states provides insights not only on the charge balance but also on the chemical behavior of the metal within the coordination compound. This step is fundamental to both naming conventions and deeper chemical analysis.