Problem 142 - 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 (III) (b) potassium hexacyanoferrate (II) (c) tripotassium hexacyaniron (II) (d) potassium hexacyanoiron (II)

Step-by-Step Solution

Answer

(a) potassium hexacyanoferrate (III)

Step 1: Identify the Central Metal

In the compound \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), the central metal atom in the coordination complex is iron, denoted as \( \mathrm{Fe} \).

Step 2: Determine the Oxidation State of the Metal

The compound contains three potassium cations \( \mathrm{K}^+ \) and six cyanide anions \( \mathrm{CN}^- \). The overall charge of the complex ion is \(-3\) due to three \( \mathrm{K}^+ \) balancing the entire compound. For \( \mathrm{Fe} \) to balance the \(-6\) charge from six \( \mathrm{CN}^- \), its oxidation state must be \(+3\).

Step 3: Name the Ligands and Central Metal

The cyanide ligand is represented as \( \mathrm{cyano} \). Because there are six cyanide ligands, we use the prefix **hexa** to denote this. The central metal, iron, is within an anionic complex, so we use **ferrate** instead of just iron. Thus, the partial name is hexacyanoferrate (III). The name includes (III) to indicate iron's oxidation state of +3.

Step 4: Include the Counter Ions

The compound contains three potassium ions \( \mathrm{K}^+ \), which are outside the coordination sphere, and they should be named at the beginning of the compound. Therefore, the full IUPAC name becomes potassium hexacyanoferrate (III).

Key Concepts

IUPAC Nomenclature

IUPAC nomenclature for coordination compounds is a systematic method to name chemical compounds using a set of defined rules. This helps in ensuring consistency and clarity in the identification of compounds globally. In the provided coordination compound, \(\mathrm{K}_{3}[\mathrm{Fe}(\mathrm{CN})_{6}]\), several key steps were followed to derive its IUPAC name.

The compound's cations and anions are identified first. Here, the central metal atom is iron (\(\mathrm{Fe}\)) within the anionic complex \([\mathrm{Fe}(\mathrm{CN})_{6}]^{3-}\). Potassium (\(\mathrm{K}^{+}\)) serves as the counter ion. Naming starts with the cation name followed by the anionic complex.

Next comes identification and naming of ligands. Cyanide (\(\mathrm{CN}^{-}\)) is termed "cyano" in IUPAC nomenclature. The prefix "hexa" indicates there are six cyanide ligands surrounding the metal.

Furthermore, the elemental group the metal belongs to affects naming. For anionic complexes, 'ate' suffix is added to the central metal, turning "iron" into “ferrate.” Consequently, the IUPAC name derived is "potassium hexacyanoferrate (III)", with "(III)" specifying the iron's oxidation state.

Oxidation States

Oxidation states are crucial in coordination chemistry as they convey the number of electrons that a central metal atom is using to bond with surrounding ligands. This particular metric helps derive the correct chemical formula.

To calculate the oxidation state of iron in the compound \(\mathrm{K}_{3}[\mathrm{Fe}(\mathrm{CN})_{6}]\), you start by considering all charges involved. Potassium ions \(\mathrm{K}^{+}\) each have a charge of \(+1\), contributing to a total charge of \(+3\) from the three potassium ions. Each cyanide ligand \(\mathrm{CN}^{-}\) contributes a \(-1\) charge, leading to an overall \(-6\) from the six cyanides. Therefore, the total charge of the complex ion must balance the entire compound at \(-3\).

For iron, the oxidation state must offset these \(-6\) charges from cyanides to ensure that the resultant complex anion's net charge fits with that of the potassium ions, which means iron must have an oxidation state of \(+3\). Thus, \(\mathrm{Fe}^{3+}\) allows for this balancing act, confirming the name "potassium hexacyanoferrate (III)" with the numeral reflecting iron's oxidation state.

Coordination Chemistry

Coordination chemistry revolves around understanding compounds featuring a central metal atom bonded to surrounding molecules or ions, known as ligands. Such bonding can significantly impact chemical, physical, and structural properties.

The compound \(\mathrm{K}_{3}[\mathrm{Fe}(\mathrm{CN})_{6}]\) is an excellent example of a coordination complex wherein iron serves as the central atom. Cyanide ligands, strong field ligands due to their negative charge and small size, surround it. This duration of bonding imparts a distinct geometry and influences properties such as color, magnetism, and stability.

The central concept here lies in the coordination number, representing the number of ligand bonds to the central metal. In this example, a coordination number of 6 emerges as six cyanide ions coordinate with the iron, creating an octahedral geometry common to many coordination complexes. This geometric arrangement significantly affects how the complex absorbs and transmits light, thus contributing to observed colors in solutions.

Coordination complexes are prevalent in biological and industrial applications. Understanding the basics of ligand behavior and geometry aids in tailoring materials for specific functions.