Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

The hypothetical complex chlorodiaquatriammineco-balt (III) chloride can be presented as (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\right] \mathrm{Cl}_{3}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Cl}\right]\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}_{3}\right]\) (d) \(\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

Option (d) is correct:  ext{[ ext{CoCl(NH}_3 ext{)}_3( ext{H}_2 ext{O})_2] ext{Cl}_2} .

Step 1: Understand the Composition

The complex name given is chlorodiaquatriamminecobalt (III) chloride. This indicates there are three ammonia (NH3) ligands, two water (H2O) ligands, and one chloride (Cl) ligand within the coordination sphere. Additionally, there are two chloride ions outside the coordination sphere.

Step 2: Identify Coordination Sphere

Based on the composition from Step 1, identify that the coordination sphere for the cobalt (III) complex must contain [Co(NH3)3(H2O)2Cl]. This means Co is surrounded by three NH3, two H2O molecules, and one Cl ligand.

Step 3: Calculate Total Charge

Calculate the overall charge of the coordination sphere. Cobalt (III) has a +3 charge. Since NH3 and H2O are neutral and Cl has -1 charge, the charge of the coordination sphere is (+3) + (-1) = +2.

Step 4: Determine Number of Chlorides Outside Coordination Sphere

Since the complex is presented as a chloride, and the coordination sphere is +2, there should be two chloride ions (each with charge -1) outside the sphere to balance the charge, providing an overall neutral compound.

Step 5: Match with Options

The correct molecular formula based on the composition within the coordination sphere and the charge balance is (d)  ext{[ ext{CoCl(NH}_3 ext{)}_3( ext{H}_2 ext{O})_2] ext{Cl}_2} .

Key Concepts

Coordination Sphere

In the fascinating realm of coordination chemistry, the coordination sphere plays a crucial role. It represents the central metal ion and the ligands directly attached to it.

• Within the coordination sphere, the cobalt ion binds with specific ligands.
• These ligands, such as ammonia (NH extsubscript{3}) and water (H extsubscript{2}O), occupy positions around the metal ion.
• The coordination sphere is typically enclosed in square brackets in chemical formulas. For example, in our exercise:
  • Coordinating ligands: Chlorodiaquatriamine.
  • Structure: Cl is within, and additional Cl ions are outside the coordination sphere.

The stability and properties of the entire coordination compound heavily depend on the composition of this sphere.

Charge Balance

Charge balance is a fundamental concept ensuring that coordination compounds remain electrically neutral.

• The metal ion's oxidation state and the ligands within the coordination sphere define its overall charge.
• In our example, cobalt (III) has a +3 charge, while neutral ligands like NH extsubscript{3} and H extsubscript{2}O do not affect the overall charge.
• However, ligands such as Cl- contribute negatively by -1.

Hence, the charge within the coordination sphere for the given compound calculates as +2. To maintain neutrality, two external chloride ions in the solution balance the +2 charge, effectively making the total charge of the coordination compound zero.

Cobalt Complexes

Cobalt complexes are a popular subject of study in coordination chemistry due to cobalt's versatile bonding capabilities. Features of Cobalt Complexes:

• They can adopt different geometrical shapes, often octahedral, depending on the number and type of ligands.
• Cobalt typically forms coordination bonds with six ligands, creating a stable complex structure.
• Properties such as color and magnetic behavior can vary based on electronic configuration.

In our complex, the cobalt (III) ion is coordinated with five ligands, resulting in a distinct coordination geometry that influences its chemical characteristics.

Ligands in Coordination Compounds

Ligands are molecules or ions that donate a pair of electrons to the metal to form a coordination bond. They are vital in defining the structure and properties of coordination compounds.

• Common Ligands: Include ammonia (NH extsubscript{3}), water (H extsubscript{2}O), and chloride (Cl). Each brings different electronic and steric properties.
• Types of Ligands:
  • Neutral: Like NH extsubscript{3} and H extsubscript{2}O, do not contribute to the charge.
  • Anionic: Such as Cl-, contribute to the overall negative charge.
• Role in Coordination Compounds: The choice and arrangement of ligands influence a compound's stability, reactivity, and magnetic properties.

Thus, understanding the nature of ligands is essential for predicting and explaining the behavior of coordination compounds like chlorodiaquatriamminecobalt (III) chloride.