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 Coordination Compound

The given compound is \(\mathrm{K}_3[\mathrm{Fe}(\mathrm{CN})_6]\). It consists of 3 potassium ions \((\mathrm{K}^+)\) outside the coordination sphere and the complex ion \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\).

Step 2: Determine Oxidation State

To find the oxidation state of \(\mathrm{Fe}\), let's use the charge balance:The overall charge of the complex ion \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\) is \(-3\). Each cyanide ligand \((\mathrm{CN}^-)\) carries a \(-1\) charge. Since there are 6 cyanide ions, they contribute \(-6\) in total. Let the oxidation state of \(\mathrm{Fe}\) be \(x\). So,\[x + (-6) = -3\]\[x = +3\]

Step 3: Name the Anion Part

The anion part is \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\). According to IUPAC nomenclature, we first name the ligands. Here, the ligand 'cyano' is named as 'cyanido'. Since there are six \(\mathrm{CN}^-\) ligands, we use the prefix 'hexa'. The metal is named as 'ferrate' because the complex is an anion and we use its Latin root. Indicate the oxidation state of iron with Roman numerals: (III). Therefore, name the anion as 'hexacyanoferrate (III)'.

Step 4: Name the Cation Part

The compound has three potassium ions as the cation. The cation is named before the anion, without any change. So, the cation is simply named 'potassium'.

Step 5: Combine Names

Combine the names of the cation and anion to get the full IUPAC name of the compound. Potassium comes first followed by hexacyanoferrate (III). Thus, the IUPAC name is 'potassium hexacyanoferrate (III)'.

Key Concepts

Coordination Compounds

A coordination compound is a complex substance made up of a central atom or ion, often a metal, surrounded by a group of molecules or ions known as ligands. These ligands are attached to the central atom via coordinate bonds, which means both electrons in the bond come from the ligand. In the compound \(\mathrm{K}_3[\mathrm{Fe}(\mathrm{CN})_6]\) from the exercise, iron (Fe) is the central atom. The six cyanide ions \((\mathrm{CN}^- )\) act as ligands and encapsulate the iron atom to form the coordination sphere \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\).
Coordination compounds can vary in complexity and structure. These compounds play pivotal roles in many biological systems and industrial processes. For example, hemoglobin, the oxygen-carrying molecule in blood, is a complex of iron within a coordination compound structure.
Because of this central role, understanding coordination compounds is crucial for students exploring chemistry, particularly inorganic chemistry.

Oxidation State

The oxidation state describes the degree of oxidation of an atom within a compound. It is a theoretical charge that an atom would have if the compound was composed of ions. It's essential for determining the electron distribution in a molecule. In coordination compounds, the overall charge is the sum of the oxidation state of the central atom and the charges of the ligands.
For the coordination compound \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\), the exercise determines the oxidation state of iron (Fe) as +3. Here's how that works:

• The entire complex has a charge of -3.
• Each cyanide ion \((\mathrm{CN}^- )\) carries a -1 charge. With six cyanides, the total charge from ligands becomes -6.
• Let the oxidation state of iron be \(x\). The equation becomes: \(x + (-6) = -3\), leading to \(x = +3\).

This deduction shows how key oxidation states are in naming and understanding chemical behavior within compounds.

IUPAC Naming Conventions

IUPAC naming conventions provide a systematic approach to naming chemical compounds, ensuring every name reflects the compound's structure. This system increases clarity and reduces confusion among chemists globally. For coordination compounds, the naming approach involves specifying both the cation and the anion clearly.
In the example compound, \(\mathrm{K}_3[\mathrm{Fe}(\mathrm{CN})_6]\), we first name the cation, potassium, which doesn't change from its elemental name. The complex anion \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\) is named next, starting with the ligands and followed by the central metal with its oxidation state indicated in Roman numerals.
This rule ensures that anyone can precisely identify the composition and structure of the compound across various chemical studies and practices. It is an essential tool for students to master when learning about chemical nomenclature.

Ligand Naming Rules

Ligands are key components in coordination compounds, as they bind to the central atom, forming complex ions. Naming ligands adheres to specific rules so that everyone understands the ligand's identity and properties within the coordination compound.

• Ligands are named before the central metal in IUPAC names.
• Neutral ligands bear their common names, like water called "aqua" or ammonia called "ammine."
• Anionic ligands change their ending to 'o', as seen in cyanide (CN\(^-\)) which becomes "cyanido."
• Prefixes indicate the number of each type of ligand, such as "di-," "tri-," and so on. If six cyanide ligands are present, you would use "hexa-."

In our compound example, we use "hexacyanoferrate" for describing the ligands attaching to iron, with "ferrate" indicating the complex ion's anionic nature. Understanding these naming rules allows students to accurately identify and name intricate coordination compounds in their studies.