Problem 108 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following matching is/are correct here? (a) Secondary valencies - oxidation number (b) Primary valencies - hold ionisable species (c) Primary valencies - oxidation state (d) Secondary valencies -ordination number

Step-by-Step Solution

Answer

Correct matchings are (c) Primary valencies - oxidation state, and (d) Secondary valencies - coordination number.

Step 1: Understand Primary Valencies

Primary valencies in coordination compounds refer to the oxidation state of the central metal atom. These are satisfied by anions like Cl⁻, SO₄²⁻, etc., that are dissociated in solution. Compare this to the given options, option (c) correctly describes primary valencies as related to the oxidation state.

Step 2: Understand Secondary Valencies

Secondary valencies refer to the coordination number, which is the number of ligand atoms that are bound directly to the central metal atom. Therefore, option (d) correctly matches secondary valencies to coordination number.

Step 3: Evaluate Incorrect Options

Evaluate the incorrect options: (a) secondary valencies do not refer to the oxidation number, and (b) primary valencies do not hold ionisable species.

Key Concepts

Primary Valencies

In coordination compounds, primary valencies are crucial as they define the oxidation state of the central metal atom. Think of the oxidation state as the charge left on the metal when all attached groups are removed along with their electron pairs. Primary valencies are satisfied by anions that can dissociate in a solution. This means ions like chloride ( Cl^{-} ), sulfate ( SO_{4}^{2-} ), and other such anions fulfill this role by balancing out the charge of the metal.

• Oxidation State: Determines the primary valency and it's crucial for the stability of the compound.
• Anions: Necessary to satisfy the primary valency, usually ionizable and free in solution.

Primary valencies are essentially the main link between the metal and the outer ionic environment, ensuring overall neutrality.

Secondary Valencies

Secondary valencies connect to the central metal atom through the coordination number. These are not related to the charge of the compound. Instead, secondary valencies focus on how the ligands—or molecules/ions—bond directly to the central metal.
The coordination number indicates how many ligands are bonded to the metal center. In simple terms, it counts the number of points of attachment.

• Coordination Number: Indicates the secondary valency, highlighting the direct bonds formed with the metal.
• Ligands: Molecules/ions such as water, ammonia, or chloride that form bonds with the central metal.

These valencies are crucial for the shape and spatial arrangement of the coordination compound, influencing its chemical behavior and properties.

Oxidation State

The oxidation state in a coordination compound is tied to its primary valency. It represents the net charge that a central metal atom would have if all its ligands and pairs of electrons were removed. This concept is essential for determining how the metal interacts with other ions or molecule species.
To find the oxidation state, consider the charge on the central atom and its surrounding ligands. The overall charge of the compound will guide your calculation.

• Calculation: Sum of charges on the metal and its ligands leads to the overall charge.
• Crucial Role: Helps predict reactivity and stability in the chemical environment.

Understanding the oxidation state is crucial for any discussion about primary valencies and understanding the overall behavior of the coordination compound.

Coordination Number

Coordination number reveals the number of ligand bonds directly formed with the central metal atom, distinctly different from the primary valency. It determines the secondary valency and is an important aspect of a coordination compound's classification.
The coordination number affects the geometry and symmetry of the metal-ligand bonding. A coordination number can range from 2 to 12, with 4 and 6 being most common.

• Direct Bonds: Each bond to a ligand contributes to the coordination number.
• Shapes and Geometry: Coordination numbers dictate spatial arrangements like tetrahedral, octahedral shapes, etc.

Coordination numbers are key to understanding the physical shape and interaction properties of coordination compounds in various chemical arenas.