The primary and secondary valencies of cobalt respectively in [Co(NH₃)₅Cl]Cl₂ are :

In the coordination complex [Co(NH₃)₅Cl]Cl₂, cobalt exhibits two types of valencies:

1. Primary Valency (Oxidation State):

• The primary valency corresponds to the oxidation state of the central metal ion.
• In this complex, ammonia (NH₃) is a neutral ligand, and chloride (Cl⁻) is a monodentate ligand.
• Let the oxidation state of cobalt be x. The complex has one chloride ion within the coordination sphere and two chloride ions outside.
• The overall charge of the complex is neutral, so:
  • x + (5 × 0) + (-1) + 2(-1) = 0
  • x - 1 - 2 = 0
  • x = +3
• Therefore, the primary valency (oxidation state) of cobalt is +3.

2. Secondary Valency (Coordination Number):

• The secondary valency refers to the coordination number, indicating the number of ligand donor atoms directly bonded to the central metal ion.
• In this complex, cobalt is bonded to five ammonia (NH₃) ligands and one chloride (Cl⁻) ligand within the coordination sphere.
• Thus, the coordination number is:
  • 5 + 1 = 6
• Therefore, the secondary valency (coordination number) of cobalt is 6.

Correct Answer:

Given the options:

• a) 3 and 5
• b) 2 and 6
• c) 2 and 8
• d) 3 and 6

The correct answer is d) 3 and 6.

Explanation:

In the complex [Co(NH₃)₅Cl]Cl₂, cobalt has a primary valency of 3 (oxidation state) and a secondary valency of 6 (coordination number). This explanation aligns with Werner's coordination theory.