Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is a correct representation of tetraamminecopper(II) hexacyanoferrate(II)? (a) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{2}\) (b) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{2}\) (c) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right][\mathrm{Fe}(\mathrm{CN})]\) (d) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

(a) \(\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]_3\left[\mathrm{Fe}(\mathrm{CN})_6\right]_2\).

Step 1: Understand the Complex Ion

The problem requires identifying the correct representation of tetraamminecopper(II) hexacyanoferrate(II). Tetraamminecopper(II) refers to the complex ion \([ ext{Cu(NH}_3)_4]^{2+}\), which forms from copper with a +2 charge and 4 ammonia ligands.

Step 2: Analyze the Anion

Hexacyanoferrate(II) refers to the anion \([ ext{Fe(CN)}_6]^{4-}\), where iron is in a +2 oxidation state coordinated with six cyanide ions, each carrying a -1 charge. This results in an overall -4 charge.

Step 3: Determine the Charge Balance

To balance the charges in the compound, multiple tetraamminecopper(II) complexes will pair with hexacyanoferrate(II) ions. Each \([ ext{Cu(NH}_3)_4]^{2+}\) has a +2 charge and \([ ext{Fe(CN)}_6]^{4-}\) has a -4 charge. Therefore, a combination of 3 copper complexes (+6 charge total) with 2 iron complexes (-8 charge total) will not balance, neither will a 1:2 ratio.

Step 4: Select Correct Formula

The correct stoichiometric ratio to balance the charges would be 3 tetraamminecopper(II) complexes to each hexacyanoferrate(II), leading to the correct option being \(\left[ \text{Cu(NH}_3\right]_3 ight)[\text{Fe(CN)}_6\right]_2\). This matches with option (a) which balances the overall charges in a coordination compound.

Key Concepts

Complex Ions

Complex ions are foundational components of coordination chemistry. They consist of a central metal atom or ion surrounded by molecules or ions known as ligands. This forms a coordination complex. The central atom can be a transition metal such as copper or iron. These metals typically exhibit multiple oxidation states and have significant ability to bind with ligands due to their incomplete d-orbitals.

In the context of the exercise, tetraamminecopper(II) is a representative complex ion. It is written as \([\text{Cu(NH}_3)_4]^{2+}\). Here, copper (Cu) acts as the central metal ion. This copper ion forms coordinate bonds with ammonia (\(\text{NH}_3\)) molecules acting as ligands.

Charge Balance

In coordination chemistry, achieving a balanced charge is pivotal for identifying the correct chemical formula of a complex compound. The neutralization of charges between cations and anions in a coordination compound ensures stability.

In the exercise, the charge balance is between tetraamminecopper(II) complexes and hexacyanoferrate(II) ions. The tetraamminecopper(II) complexes each carry a charge of \(+2\), while every hexacyanoferrate(II) ion carries a \(-4\) charge. For the compound to be neutral, we need a certain number of these ions to balance each other out.

• This requires using three tetraamminecopper(II) ions to balance the overall charge with two hexacyanoferrate(II) ions.

The resulting balanced formula from the exercise for the entire compound is \([\text{Cu(NH}_3)_4]_3[\text{Fe(CN)}_6]_2\).

Ligands

Ligands are molecules or ions that donate at least one pair of electrons to the central metal ion in a complex ion. By doing so, they help stabilize the metal ion and allow the formation of coordination compounds.

Ligands can be monodentate, binding through a single atom, or polydentate, where several atoms can bond to the metal. In our example, ammonia (\(\text{NH}_3\)) and cyanide (\(\text{CN}^-\)) are examples of ligands:

• Ammonia is a neutral monodentate ligand, which binds through its nitrogen atom.
• Cyanide is an anionic monodentate ligand binding via the carbon atom with a negative charge.

These ligands, upon coordinating, can significantly influence properties such as the stability, color, and reactivity of the complex.

Oxidation States

Oxidation states in coordination chemistry refer to the charge of the central metal atom within a complex ion. This is crucial for understanding both the electronic structure and the reactivity of the compound.

In the exercise's example, copper in tetraamminecopper(II) has an oxidation state of \(+2\). This is indicated by the "II" in tetraamminecopper(II). It is responsible for attracting the neutral ammonia ligands. Similarly, in hexacyanoferrate(II), iron is also in a \(+2\) oxidation state, with the "II" denoting this. This affects the overall formation, charge, and interaction of the ions in the complex.

Knowing the oxidation states helps determine the charge the complex will carry, which is needed for balancing with other charged species, achieving neutrality in final compounds.