Recall Werner’s theory

• Primary valency = oxidation state of the metal ion.
• Secondary valency = coordination number of the metal ion.

Given complex $[Co(NH_3)_5Cl]Cl_2$

Here:

• Inside the coordination sphere: $[Co(NH_3)_5Cl]^{2+}$
• Outside the coordination sphere: $2Cl^-$

Find the oxidation state of cobalt Let oxidation state of Co be $x$.

In $[Co(NH_3)_5Cl]^{2+}:$

• $NH_3$ is a neutral ligand, so contribution = $0$
• coordinated $Cl$ contributes $-1$

Therefore, $x + 5(0) + (-1) = +2$ $x - 1 = 2$ $x = +3$

So, the primary valency of cobalt is 3.

Find the coordination number Ligands directly attached to cobalt are:

• $5$ molecules of $NH_3$
• $1$ ligand $Cl$

Total number of coordinate bonds: $5 + 1 = 6$

So, the secondary valency is 6.

Match with options

• Primary valency = $3$
• Secondary valency = $6$

Hence the correct option is: $\boxed{D}$

Comparison with stored answer Stored correct answer: D

This matches our derived answer.