Problem 159 - A Complete Resource Book in Chemistry for JEE Main

Question

The IUPAC name of the coordination compound \(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) is (a) potassium hexacyanoferrate (II) (b) potassium hexacyanoferrate (III) (c) potassium hexacyanoiron (II) (d) ripotassium acyanoiron (II)

Step-by-Step Solution

Answer

The correct IUPAC name is (b) potassium hexacyanoferrate (III).

Step 1: Identify the Central Metal & Ligand

In the coordination compound \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), identify the central metal and the ligand. The central metal atom is Fe (Iron), and the ligand present is cyanide \((\mathrm{CN}^-)\).

Step 2: Determine the Oxidation State of the Metal

The oxidation state of iron must be calculated. Each potassium ion contributes \(+1\) charge, for a total of \(+3\) since there are three potassium ions. Cyanide has a \(-1\) charge per CN\(^-\) and there are six of them. Let the oxidation state of Fe be \(x\). Then, the equation is: \[3(+1) + x + 6(-1) = 0\] Solving: \[3 + x - 6 = 0\] \[x = +3\] Thus, the oxidation state of iron is \(+3\).

Step 3: Write the Name Using IUPAC's Nomenclature

The systematic IUPAC name is comprised of: the cation first, followed by the anion. For anionic complex species, as here, the naming ends with the suffix '-ate'. Affix the oxidation state of Fe in parentheses in Roman numerals. Therefore, the IUPAC name is "potassium hexacyanoferrate (III)".

Step 4: Confirm and Select the Correct Option

Among the given options, recognize that option (b), "potassium hexacyanoferrate (III)", is accurate by comparing it with the determined IUPAC name.

Key Concepts

Oxidation State Determination

Determining the oxidation state of the central metal in a coordination compound is crucial for IUPAC naming. In the compound \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), we first consider the charges of all components. Potassium \((\mathrm{K}^+)\) ions each carry a \(+1\) charge. With three \(\mathrm{K}^+\) ions, the total positive charge from potassium is \(+3\). Cyanide \((\mathrm{CN}^-)\) ligands, on the other hand, each have a \(-1\) charge, contributing a total of \(-6\) from the six ligands.
To find the oxidation state of iron \((\mathrm{Fe})\), we set up the equation \[3(+1) + x + 6(-1) = 0\] where \(x\) is the oxidation state of Fe. Solving this equation: \[3 + x - 6 = 0\] gives \(x = +3\). Therefore, the oxidation state of the iron in this compound is \(+3\). This step is essential as it guides us in the correct placement of its state in the compound's name.

Central Metal Identification

Identifying the central metal in a coordination compound helps determine both the chemical properties and the correct nomenclature. For the compound \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), the central metal is iron \((\mathrm{Fe})\).
In coordination compounds, the central metal is the atom to which ligands bind. These ligands form coordinate covalent bonds with the metal. This central role influences the electronic structure and stability of the entire complex. Additionally, the central metal can affect the color, magnetic properties, and reactivity of the compound.
When writing IUPAC names, determining the central metal is crucial because its identity, along with its oxidation state, must be correctly indicated using specific suffixes and Roman numerals.

IUPAC Nomenclature Rules

The IUPAC naming system follows specific rules to systematically name coordination compounds, ensuring clarity and consistency across chemical literature. Here’s a breakdown of naming guidelines based on the compound \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \).

• The cation (\(\mathrm{K}^+\) in this case) is named before the complex anion.
• The complex part, \([\mathrm{Fe}(\mathrm{CN})_{6}]^{3-}\), uses a specific order: ligands first, then the metal.
• Ligands like cyanide use a specific root ("cyano-"), followed by their count indicated as a prefix ("hexa-").
• For anionic complexes, modify the metal name with a suffix "-ate." In the presence of the anion, iron becomes "ferrate."
• The oxidation state of the metal, here \(+3\), is shown in Roman numerals within parentheses.

The IUPAC name then becomes "potassium hexacyanoferrate (III)," following these guidelines and reflecting the precise structure and composition of the molecule.