Oxidation State of Fe in K₃[Fe(CN)₅NO] – Detailed Step-by-Step Solution

Correct Answer: (+1)

Understanding the Given Coordination Compound

To determine the oxidation state of iron (Fe) in K₃[Fe(CN)₅NO], we first need to identify the charge on the coordination entity and the charges carried by the ligands attached to the central metal atom. This is an important type of question in coordination chemistry because the oxidation state of the central metal can be calculated by balancing the total charge of the complex.

The compound contains three potassium ions, represented by K₃. Since each potassium ion has a charge of +1, the three potassium ions together contribute a total positive charge of +3. Therefore, the coordination entity [Fe(CN)₅NO] must carry a charge of −3 so that the complete compound remains electrically neutral.

Thus, we can write:

Charge on [Fe(CN)₅NO] = −3

Identifying the Charges on the Ligands

Charge on the Cyanide Ligand (CN⁻)

The complex contains five cyanide ligands. Each cyanide ligand, CN⁻, carries a charge of −1. Therefore, the total charge contributed by five cyanide ligands is:

5 × (−1) = −5

The cyanide ligand is an anionic ligand, and its −1 charge must always be included while calculating the oxidation state of the central metal ion.

Charge on the Nitrosyl Ligand (NO⁺)

The NO ligand is called the nitrosyl ligand. In this type of coordination complex, the nitrosyl group is treated formally as NO⁺. Therefore, the charge contributed by the nitrosyl ligand is +1.

This point is especially important because NO can show different bonding descriptions in coordination chemistry. For the oxidation-state calculation in the given complex, the formal ionic method treats the nitrosyl ligand as NO⁺.

Calculation of the Oxidation State of Fe

Let the oxidation state of iron be x. The total charge on the complex ion [Fe(CN)₅NO]³⁻ is −3. Therefore, the sum of the oxidation state of Fe and the charges of all the ligands must be equal to −3.

x + 5(−1) + (+1) = −3

Simplifying the equation:

x − 5 + 1 = −3

x − 4 = −3

Adding 4 to both sides gives:

x = +1

Therefore, the oxidation state of iron in K₃[Fe(CN)₅NO] is +1.

Why the Correct Answer Is (+1)

The correct answer is obtained by considering the overall charge of the coordination entity as −3, each of the five cyanide ligands as CN⁻, and the nitrosyl ligand as NO⁺. The five cyanide ligands contribute −5, while the nitrosyl ligand contributes +1. To make the total charge of the complex equal to −3, iron must have an oxidation state of +1.

Mathematically:

Oxidation state of Fe + Charge of five CN⁻ ligands + Charge of NO⁺ = Charge on the complex

x − 5 + 1 = −3

x = +1

Final Answer

The oxidation state of Fe in K₃[Fe(CN)₅NO] is (+1).