Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the coordination compounds (a) \(\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Fe}(\mathrm{CN})_{2} \cdot 4 \mathrm{KCN}\) (c) \(\mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CuSO}_{4} \cdot 4 \mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

The coordination compounds are (b) and (d).

Step 1: Understand Coordination Compounds

Coordination compounds consist of a central metal atom or ion that is bonded to surrounding molecules or ions, known as ligands, which can donate a pair of electrons to the metal. These complexes are generally between transition metals and neutral molecules or ions.

Step 2: Evaluate Compound (a)

The compound (a) \(\text{FeSO}_{4} \cdot (\text{NH}_{4})_{2} \text{SO}_{4} \cdot 6 \text{H}_{2}\text{O}\) does not exhibit characteristics of a coordination compound. It is more accurately a double salt known as Mohr's salt, where no coordinate covalent bonds are present.

Step 3: Evaluate Compound (b)

The compound (b) \(\text{Fe(CN)}_{2} \cdot 4 \text{KCN}\) represents the coordination compound K4[Fe(CN)6]. The Fe ion is surrounded by CN- ligands, forming complex bonds, making it a coordination compound.

Step 4: Evaluate Compound (c)

The compound (c) \(\text{KCl} \cdot \text{MgCl}_{2} \cdot 6 \text{H}_{2}\text{O}\) is simply a hydrated salt mixture without complex formation between metal ions and ligands, so it is not a coordination compound.

Step 5: Evaluate Compound (d)

The compound (d) \(\text{CuSO}_{4} \cdot 4 \text{NH}_{3}\) can be seen as Cu(NH3)4SO4, where the Cu ion is surrounded by NH3 ligands through coordination bonds, thus forming a coordination compound.

Key Concepts

Transition Metals

Transition metals are found in the d-block of the periodic table. They have an essential role in forming coordination compounds. These metals have unique properties that make them ideal central atoms in complexes.

• Variable Oxidation States: Transition metals can exhibit more than one oxidation state, allowing them to form different types of coordination complexes.
• Ability to form Complex Ions: Due to their partially filled d orbitals, they can easily accept electron pairs from ligands, enabling complex ion formation.
• Catalytic Properties: Many transition metal-containing coordination compounds show catalytic activities, playing crucial roles in industrial chemistry.

These metals can form colored compounds and possess magnetic properties, which are observed due to the arrangement of their d electrons.

Ligands

Ligands are ions or molecules that bind to the central metal atom in a coordination compound. They donate one or more pairs of electrons, forming coordinate covalent bonds.

• Types of Ligands: They can be classified based on the number of donor sites. Monodentate ligands have one, while polydentate ligands, like EDTA, can have multiple bonding sites.
• Examples: Common ligands include water (H2O), ammonia (NH3), and cyanide ion (CN-), all of which have lone pairs of electrons to donate to metal ions.

Ligands determine the geometry and stability of coordination compounds. The interaction between ligands and metal ions is a fascinating topic, as it highlights how a change in ligand can alter the properties of the entire complex.

Coordination Bonds

Coordination bonds, or coordinate covalent bonds, occur when ligands donate their lone electron pairs to a metal ion. This type of bonding is central to the formation of coordination compounds.

• Formation: In a coordination bond, a ligand with at least one lone pair of electrons forms a dative bond with the central metal ion, which has an empty orbital.
• Characteristics: Coordination bonds are often reversible and can be broken, allowing ligands to exchange places in solution, a property utilized in chemical reactions and catalysis.

These bonds differ from ordinary covalent bonds because the pair of electrons involved is supplied entirely by the ligand, offering a unique dynamic within the compound.

Double Salts

Double salts are composed of two different salts crystallized together but differing from coordination compounds. They only display their properties when dissolved in solvents like water.

• Example: Mohr's salt ( FeSO_4 •( NH_4)_2 SO_4 • 6 H_2O) dissociates into its respective ions in solution.
• Differences: Unlike coordination compounds, double salts do not maintain complex structures upon dissolution, returning to their simple constituent ions.

They are useful in various chemical processes where simple ions rather than intricate structures are required. Understanding the difference between double salts and coordination compounds is crucial for grasping the comprehensive nature of inorganic chemistry.