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, [2008] (a) 6 and 2 (b) 4 and 2 (c) 4 and 3 (d) 6 and 3

Step-by-Step Solution

Answer

(d) 6 and 3

Step 1: Identify the Ligands

In the complex \([\mathrm{E}(\mathrm{en})_{2}(\mathrm{C}_{2}\mathrm{O}_{4})]\), 'en' represents ethylenediamine, which is a bidentate ligand, and \((\mathrm{C}_{2}\mathrm{O}_{4})^{2-}\) is an oxalate ion, also bidentate. Each bidentate ligand can form two coordinate bonds with the central atom.

Step 2: Calculate the Coordination Number

The coordination number is the total number of coordinate bonds formed between the central metal ion and the ligands. Ethylenediamine contributes 4 (since there are 2 en each forming 2 bonds) and oxalate contributes 2, leading to a total of 6 coordinate bonds. Thus, the coordination number is 6.

Step 3: Determine the Oxidation State

Let the oxidation state of the element 'E' be \(x\). The total charge of the complex ion \([\mathrm{E}(\mathrm{en})_{2}(\mathrm{C}_{2}\mathrm{O}_{4})]\) is neutral, but when combining with \(\mathrm{NO}_{2}^{-}\), the entire complex must balance out to a charge of -1. Knowing \(\mathrm{en}\) is neutral and \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) contributes \(-2\), set up the equation: \[ x + 0 + (-2) = -1 \]Solve for \(x\), so \( x = +3 \).

Step 4: Conclusion

The element 'E' in the complex has a coordination number of 6 and an oxidation state of +3. Therefore, the correct choice is (d) 6 and 3.

Key Concepts

Understanding Bidentate Ligands

In the fascinating world of coordination chemistry, ligands play a crucial role in defining the structure and function of complexes. A **bidentate ligand** is one that can bind to a central metal atom or ion through two donor atoms. This dual attachment allows for a more stable complex because two bonds are formed instead of one.

• Bidentate ligands have two atoms with lone pairs of electrons.
• They form a chelate ring structure by attaching at two points, which increases the stability of the complex.

In the example of ethylenediamine (abbreviated as 'en'), the nitrogen atoms act as the donor atoms, each donating an electron pair to form two coordinate covalent bonds with the metal center. Similarly, the oxalate ion, (\( \text{C}_2\text{O}_4^{2-} \)), can also act as a bidentate ligand, utilizing its oxygen atoms to bind in the same manner. This ability to form multiple bonds is a key feature you should remember, as it frequently appears in discussions on coordination compounds.

The Significance of Coordination Number

In coordination chemistry, determining the **coordination number** is essential for understanding the geometry and stability of a complex. The coordination number refers to the number of coordinate bonds between the central metal ion and its surrounding ligands.

• It tells you how many bonds the metal ion forms with its ligands.
• The value influences the shape and geometry of the complex.

In the given chemical species, the central metal ion forms six bonds: four from the two ethylenediamine ligands (each contributing two bonds) and two from the oxalate ion. Hence, the coordination number is 6. This number is key to defining the octahedral geometry often associated with six-coordinate complexes, which is prevalent in transition metals and offers maximal ligand interaction.

Determining the Oxidation State

The **oxidation state** in coordination chemistry helps us evaluate the electron count and the type of interactions within a complex. It indicates the charge of the central metal after accounting for all the electron exchanges with its ligands.

• This value allows chemists to deduce electronic configuration and predict reactivity.
• It is calculated by considering the charges of all individual components in the complex.

In the complex \([\text{E}(\text{en})_{2}(\text{C}_{2}\text{O}_{4})]\) paired with \(\text{NO}_2^-\), the ligands ethylenediamine are neutral, and oxalate contributes a charge of \(-2\). Knowing that the overall charge of the complex must balance to match with the anion, the oxidation state of \(\text{E}\) is determined to be +3. This information is paramount in characterizing the redox behavior and binding capacity of the element within the complex.