Question

The primary and secondary valencies of cobalt respectively in \([ Co(NH_3)_5Cl]Cl_2\) are :

• A. 3 and 5
• B. 2 and 6
• C. 2 and 8
• D. 3 and 6

Answer 1

The Correct Option is D

To solve this question, we need to understand the concept of primary and secondary valencies in coordination compounds, as proposed by Alfred Werner. 

Werner's coordination theory explains that:

• The primary valency corresponds to the oxidation state of the central metal ion. It is satisfied by negative ions or other molecules outside the coordination sphere.
• The secondary valency is the coordination number, representing the total number of atoms directly bonded to the central metal ion, typically within the coordination sphere.

Let's examine the coordination compound \([ Co(NH_3)_5Cl]Cl_2\):

1. The central metal ion is Cobalt (Co).
2. The compound \([ Co(NH_3)_5Cl]Cl_2\) indicates that outside the coordination sphere, we have 2 Cl^- ions, which suggests Co is in the +3 oxidation state to balance the charge:
   • Cobalt primary valency = +3
3. In the complex ion \([Co(NH_3)_5Cl]^2+\), the secondary valency (coordination number) is determined by the number of ligands directly attached to Co:
   • There are 5 NH₃ (ammonia) molecules and 1 Cl^- ion bonded to cobalt, giving a total of 6 ligands.
   • Cobalt secondary valency = 6

Therefore, in the compound \([ Co(NH_3)_5Cl]Cl_2\), the primary and secondary valencies of cobalt are 3 and 6, respectively.

Hence, the correct answer is 3 and 6.