On the basis of Sidgwick EAN rule, which of the following statement is not correct?

[Fe(CO)5] can act as both oxidizing agent and reducing agent

The correct answer is option 3. [Fe(CO)5] can act as both oxidizing agent and reducing agent.

The Sidgwick EAN rule, also known as the 18-electron rule, is a concept in coordination chemistry that helps us predict the stability of coordination complexes. It focuses on the idea that metal atoms in these complexes strive to achieve an effective atomic number (EAN) equal to the nearest noble gas in the same period of the periodic table.

Effective Atomic Number (EAN): Counting Electrons Effectively

Imagine the metal atom in a complex surrounded by its ligand molecules. The EAN considers all the electrons "felt" by the metal:

Electrons from the metal itself: This is the atomic number of the metal element.

Electrons donated by ligands: Each ligand donates a certain number of electrons when it forms a bond with the metal.

Adding these two electron counts gives us the EAN:

EAN = Atomic number of metal + Oxidation state electrons (usually 0) + Electrons from ligands

A Stable Configuration

Noble gas elements have a full valence electron shell, making them exceptionally stable and unreactive. The EAN rule suggests that metal atoms in coordination complexes also achieve stability by acquiring an electron count that mimics this noble gas configuration.

Applying the Rule: Understanding the Statements

Now, let us analyze the given statements based on the EAN rule:

1.\([Mn(CO)_6]\) can act as a reducing agent:

Here, Mn has an EAN of 36 (achieved through \(CO\) ligands).The EAN of 36 corresponds to Krypton (Kr), a stable configuration.However, Mn still possesses d-orbitals that aren't completely filled.These partially filled d-orbitals allow \(Mn(CO)_6\) to potentially donate electrons in certain reactions, acting as a reducing agent.

2. \([V(CO)_6]\) can act as an oxidizing agent:

Similar to \(Mn(CO)_6\), \(V(CO)_6\) also reaches an EAN of 36 with \(CO\) ligands, achieving a stable configuration. But, \(V(CO)_6\) also has partially filled d-orbitals. This allows \(V(CO)_6\) to potentially accept electrons under suitable conditions, behaving as an oxidizing agent.

3.\([Fe(CO)_5]\) can act as both oxidizing agent and reducing agent:

\(Fe\) in \(Fe(CO)_5\) has an EAN of 36, corresponding to the stable Krypton configuration. All available d-orbitals are likely filled by electrons from \(CO\) ligands. Due to this complete electron configuration, \(Fe(CO)_5\) is less likely to readily lose or gain electrons. Therefore, it's less favorable for \(Fe(CO)_5\) to act as both an oxidizing and reducing agent compared to the other options where d-orbitals play a role.

4. \([Mn(CO)_5]\) shows dimerization to gain stability:

\(Mn(CO)_5\) only has an EAN of 35 (one electron short of the stable Krypton configuration). This odd-electron situation makes \(Mn(CO)_5\) unstable according to the EAN rule. To achieve stability, \(Mn(CO)_5\) can take two possible routes:

Dimerization: Two \(Mn(CO)_5\) units combine to form \(Mn_2(CO)_{10}\), sharing electrons and achieving an EAN of 36 for each \(Mn\) atom.

Gaining another \(CO\) ligand: \(Mn(CO)_5\) can accept another \(CO\) ligand to become \([Mn(CO)_6]^+\), reaching the desired EAN of 36.

In essence, the Sidgwick EAN rule helps us understand how electron configurations influence the stability and reactivity of coordination complexes. While some complexes with a complete EAN might still exhibit redox behavior under specific circumstances, \(Fe(CO)_5\) with its filled d-orbitals is generally less likely to act as both an oxidizing and reducing agent compared to the other options where partially filled d-orbitals provide flexibility for electron movement.