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) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (d) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\)

Step-by-Step Solution

Answer

The correct representation is (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 Compound Name

The compound tetraamminecopper(II) hexacyanoferrate(II) is a type of coordination compound. The name indicates the presence of two coordinated parts: tetraamminecopper(II) and hexacyanoferrate(II). The first complex, tetraamminecopper(II), contains copper coordinated to four ammonia molecules. The second complex, hexacyanoferrate(II), contains iron coordinated to six cyanide groups.

Step 2: Identify the Charges of the Complex Ions

The complex ion [Cu(NH₃)₄]²⁺ has a 2+ charge because copper(II) has a +2 oxidation state, and ammonia is neutral. The complex ion [Fe(CN)₆]⁴⁻ has a 4- charge because each cyanide ion (CN⁻) has a -1 charge, resulting in a 6 x -1 = -6 charge, added to iron(II)'s +2 charge, gives a total of -4.

Step 3: Determine the Formula by Balancing Charges

The overall charge of the compound should be neutral. To balance the charges between [Cu(NH₃)₄]²⁺ and [Fe(CN)₆]⁴⁻, three [Cu(NH₃)₄]²⁺ ions are needed to balance two [Fe(CN)₆]⁴⁻ ions. This creates a compound with a zero net charge: 3(+2) + 2(-4) = 0.

Step 4: Write the Correct Formula

After ensuring the charges are balanced, we can write the chemical formula of the compound. The formula with three tetraamminecopper(II) cations and two hexacyanoferrate(II) anions is \[ \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{2} \].

Key Concepts

Complex Ions

Complex ions are fascinating structures formed when metal atoms or ions bind with various ligands, such as ammonia or cyanide. These structures are integral to coordination compounds. A ligand is a molecule or ion that can donate a pair of electrons to the metal center, forming a coordinate covalent bond.
In the case of tetraamminecopper(II), the complex ion \( \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} \) the central metal ion is copper, which coordinates with four ammonia (NH₃) molecules. Despite copper's positive charge, the ammonia molecules are neutral, contributing no additional charge to the complex ion.
Similarly, in hexacyanoferrate(II) \( \left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-} \), the central metal ion is iron, surrounded by six cyanide ions (CN⁻), each contributing a negative charge due to their nature as ligands. The overall structure of these complex ions accounts for their stability and formation.
Complex ions often have distinct geometric shapes, such as octahedral or tetrahedral, which can affect their chemical properties, including reactivity and color. Understanding these ions is key to mastering the intricacies of coordination chemistry.

Charge Balancing

Charge balancing is essential when dealing with coordination compounds to ensure that the overall compound is electrically neutral. This involves understanding the charges on individual complex ions and combining them in such a way that their combined charge equals zero.
In this example, the \( \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} \) ion carries a 2+ charge, driven by the 2+ oxidation state of copper. On the other side, the \( \left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-} \) ion holds a 4- charge. Effective charge balancing requires pairing three of the 2+ charged copper complex ions with two of the 4- charged iron complex ions.
This results in a charge neutrality across the compound:

• Three \( \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} \) ions give a total charge of \((3 \times +2) = +6\).
• Two \( \left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-} \) ions offer a total charge of \((2 \times -4) = -8\).
• Combining these results in \((+6) + (-8) = 0\), achieving balance.

Charge balancing is crucial for constructing accurate formulas that reflect the actual chemical structure of coordination compounds.

Oxidation States

Oxidation states reveal how electrons are distributed in a compound and play a key role in forming complex ions. For any metal in a coordination compound, knowing its oxidation state helps determine its reactivity and its bonding profile with ligands.
Copper in the \( \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} \) complex ion shows a +2 oxidation state. This means copper has lost two electrons. Ammonia, which is neutral, doesn’t contribute any charge, maintaining the oxidation state solely on copper's contribution.
In \( \left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-} \), iron exists in a +2 oxidation state, indicated by the chemical name hexacyanoferrate(II). Cyanide ions, with a charge of -1 each, interact with iron by accepting its electron density, resulting in an overall charge of 4- when considering the collective charge of the six ligands and the central metal.
It’s crucial to understand how oxidation states influence the organization of electrons around the central metal ion, affecting both the structure and properties of the entire complex ion. Grasping these concepts empowers students to decipher and construct complex coordination compounds effectively.