Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

In which one of the following transition metal complexes, does the metal exhibit zero oxidation state? (a) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) (b) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{X}_{3}\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) (d) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

(a) \([\mathrm{Ni}(\mathrm{CO})_{4}]\) has nickel in zero oxidation state.

Step 1: Understand and Analyze the Question

The question asks us to identify which transition metal complex from the given options has the metal exhibiting zero oxidation state. The possible complexes provided are nickel carbonyl, iron water complex with chloride, cobalt ammine complex, and iron water complex with sulfate.

Step 2: Analyze Option (a) - Nickel Carbonyl

The complex given is \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\). In this complex, carbon monoxide (CO) is a neutral ligand. Therefore, the oxidation state of nickel in this complex is zero, as it balances out with the neutral carbonyl ligands.

Step 3: Evaluate Other Options

Let's quickly check other complexes to confirm they don't have a zero oxidation state: - (b) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]\mathrm{X}_{3}\) - Water is a neutral ligand, but X is likely a ligand with negative charge, indicating Fe is not zero.- (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) - NH₃ is neutral, but the 3 Cl⁻ ions indicate a positive Co charge.- (d) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]\mathrm{SO}_{4}\) - SO₄²⁻ indicates a positive Fe charge.

Step 4: Conclusion

The only complex where the metal is in a zero oxidation state is the nickel carbonyl \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\).

Key Concepts

Oxidation State

The concept of oxidation state is crucial when dealing with transition metal complexes. It refers to the charge that a metal atom would have if all ligands were removed, along with any electrons shared with it. Understanding oxidation states helps identify how metals interact with various ligands in a complex.

In transition metal complexes, metals often do not share electrons evenly with attached ligands. This creates a formal charge corresponding to their oxidation state. Identifying this state involves counting the charges exchanged between the metal and its ligands.

For example, if a metal forms a complex with negatively charged ligands, the metal oxidation state becomes positive to balance out the negative charges. Conversely, if a metal forms a complex with neutral ligands, its oxidation state might remain zero, as there are no charges to balance. This understanding is critical when evaluating which complexes have metals exhibiting specific oxidation states.

Nickel Carbonyl

Nickel carbonyl, \([\mathrm{Ni}(\mathrm{CO})_{4}]\), is a fascinating example of a transition metal complex where the metal exhibits a zero oxidation state. In the \([\mathrm{Ni}(\mathrm{CO})_{4}]\) complex, nickel is bonded with four carbon monoxide (CO) ligands.

CO is a neutral ligand, meaning it does not have any charge. Therefore, when nickel forms a complex with CO, it doesn't change its oxidation state from the elemental form. This leads to nickel having a zero oxidation state in nickel carbonyl:

• Nickel (Ni) begins with a neutral charge.
• CO ligands are neutral and do not carry charge.
• The sum of oxidation states in \([\mathrm{Ni}(\mathrm{CO})_{4}]\) is zero.

Understanding nickel carbonyl helps highlight how metals can maintain a zero oxidation state through bonding with neutral ligands.

Neutral Ligands

Neutral ligands are molecules or ions that form coordinate bonds with a metal atom without carrying an overall electric charge. They play a pivotal role in forming many transition metal complexes.

Here are some characteristics of neutral ligands:

• They do not alter the oxidation state of the metal since they are not charged.
• Examples include molecules like water (H₂O), ammonia (NH₃), and carbon monoxide (CO).
• These ligands help stabilize metal complexes by filling available coordination sites without influencing the metal's charge state.

In the context of transition metal complexes, neutral ligands like CO in nickel carbonyl allow the metal to retain its original oxidation state. This neutrality makes them excellent for studying different oxidation states and coordination environments in metal complexes.