Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination number and the oxidation state of the element ' \(\mathrm{E}\) ' in the complex \(\left[\mathrm{E}(\mathrm{en})_{2}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\right] \mathrm{NO}_{2}\) (Here (en) is ethylene diamine) are, respectively, \([\mathbf{2 0 0 8}]\) (a) 6 and 2 (b) 4 and 2 (c) 4 and 3 (d) 6 and 3

Step-by-Step Solution

Answer

The coordination number is 6 and the oxidation state is 3. (d) 6 and 3

Step 1: Understanding the Complex

The complex is \([\mathrm{E}(\mathrm{en})_{2}(\mathrm{C}_{2}\mathrm{O}_{4})]\ \mathrm{NO}_{2}\). Here, (en) stands for ethylene diamine, which is a bidentate ligand, and \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) (oxalate) is also a bidentate ligand. NO₂ is the counter ion.

Step 2: Determining the Coordination Number

The coordination number is the number of ligand attachment points to the central metal atom. This complex contains two ethylene diamine ligands and one oxalate ion. Ethylene diamine has 2 donor sites and oxalate also has 2 donor sites, thus:\[ (\mathrm{en}) \rightarrow 2 \times 2 = 4 \text{ donor atoms} \]\[ (\mathrm{C}_{2}\mathrm{O}_{4}) \rightarrow 1 \times 2 = 2 \text{ donor atoms} \]Adding these gives a total coordination number of 6.

Step 3: Calculating the Oxidation State

Assume the oxidation state of 'E' is \(x\). The overall charge of the complex is neutral as given. Ethylene diamine is a neutral ligand, and oxalate contributes -2 due to its charge. Therefore, the relationship is:\[ x + 0 \times 2 + (-2) = +1 \] (The counter ion NO₂ adds a charge of +1)\[ x - 2 = 1 \]Solving for \(x\), you get: \[ x = 3 \]

Step 4: Matching the Findings with Options

We've found that the coordination number is 6 and the oxidation state of 'E' is 3. Checking the options provided, the correct choice is (d) 6 and 3.

Key Concepts

Coordination Number

The coordination number in a coordination compound is a crucial concept. It represents the total number of points at which ligands are attached to the central metal atom. In our complex, \( [\mathrm{E}(\mathrm{en})_{2}(\mathrm{C}_{2}\mathrm{O}_{4})] \mathrm{NO}_{2} \), we can determine the coordination number by counting these attachment sites. Each ligand bound to the central atom contributes specific donor atoms that link to the metal.

• Ethylene diamine (en) is a bidentate ligand, meaning it can attach to the metal at two points. Since there are two ethylene diamine molecules in the complex, it provides a total of \( 2 \times 2 = 4 \) donor sites.
• Oxalate \((\mathrm{C}_{2}\mathrm{O}_{4}^{2-})\) is also bidentate, contributing another 2 donor sites.

Adding together the donor sites from both types of ligands gives us a total coordination number of six. This means six locations on the central atom are occupied by the ligand attachment points, leading to a stable configuration in the compound.

Oxidation State

The oxidation state is a formalism used to describe how many electrons an atom effectively gains or loses when it forms a compound. Understanding the oxidation state of the metal in a coordination compound is pivotal. For our complex \([\mathrm{E}(\mathrm{en})_{2}(\mathrm{C}_{2}\mathrm{O}_{4})] \mathrm{NO}_{2}\), we need to solve for the oxidation state of the central atom assumed as \( x \).

• Ethylene diamine is a neutral ligand, carrying no charge, and thus contributes \( 0 \).
• Oxalate (\(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\)) carries a \(-2\) charge.
• The NO₂ counter ion carries a \(+1\) charge to balance the overall charge of the entire compound.

Substituting into the charge balance equation, we have:\[x + 0 \times 2 - 2 = -1\]Solving the equation yields \( x = 3 \). This means the metal E has an oxidation state of 3 in this coordination complex.

Ethylene Diamine

Ethylene diamine, often abbreviated as "en," is a common ligand in coordination chemistry. It serves as a bidentate ligand, indicating it has two donor atoms that can simultaneously attach to a central metal atom. Here are some highlights of ethylene diamine:

• It is a simple organic molecule with the formula \(\mathrm{NH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{NH}_2\) and acts as a neutral ligand, meaning it does not carry any charge.
• The nitrogen atoms each possess a lone pair of electrons, which make them excellent points of attachment to the metal center.
• As a bidentate ligand, it increases the stability of coordination compounds through chelation, which involves forming a ring-like structure in binding to the metal.

Ethylene diamine’s tendency to form stable chelate rings contributes to its frequent use in coordination chemistry, allowing for the formation of structurally robust and diverse complexes.

Bidentate Ligand

In coordination compounds, understanding the concept of a bidentate ligand is vital. A bidentate ligand, like ethylene diamine (en) or oxalate \((\mathrm{C}_{2}\mathrm{O}_{4}^{2-})\), connects to the central metal atom at two distinct sites. Here’s what makes them important:

• Bidentate ligands possess two donor atoms capable of forming two bonds with a metal center, which helps in the stabilization of the complex.
• They contribute to the chelation effect, which enhances the ligand's ability to "cling on" to the metal, forming a more stable and often inert complex structure.
• This double bonding prevents the dissociation of the ligands as easily as monodentate ligands might, thus providing greater durability to the metal-ligand interaction.

By utilizing bidentate ligands, chemists can craft more enduring and sophisticated coordination compounds that serve numerous applications in industrial and research fields.