Interpret the formula

The given complex is written as: $Co(en)_2Cl_3$ where $en$ = ethylenediamine, a bidentate ligand.

Use the silver nitrate test

We are told:

• $3$ moles of the complex give $3$ moles of $AgCl$ with excess $AgNO_3$.

Hence, 1 mole of the complex gives 1 mole of $AgCl$.

Therefore, there is 1 chloride ion outside the coordination sphere.

So the complex must actually be represented as: $[Co(en)_2Cl_2]Cl$

because only the outer-sphere $Cl^-$ reacts immediately with $AgNO_3$ to form $AgCl$.

Find the secondary valency

Secondary valency = coordination number of the metal.

In $[Co(en)_2Cl_2]Cl$:

• each $en$ is bidentate, so $2$ ligands of $en$ contribute: $2 \times 2 = 4$
• inside the coordination sphere, there are $2$ coordinated $Cl^-$ ligands, contributing: $2$

Therefore, total coordination number of Co is: $4 + 2 = 6$

Final answer

The secondary valency of Co is: $\boxed{6}$