3 moles of metal complex with formula `Co ( en) _(2) Cl _(3)` gives 3 moles of silver chloride on treatment with excess of silver nitrate . The secondary valency of Co in the complex is ___________.
(Round off to the nearest integer)

Answer

To determine the secondary valency of cobalt in the complex \( Co(en)_2Cl_3 \), we need to analyze the information given in the problem step by step.

### Step 1: Understand the Reaction
The problem states that 3 moles of the metal complex \( Co(en)_2Cl_3 \) yield 3 moles of silver chloride (AgCl) when treated with excess silver nitrate (AgNO3). This indicates that each mole of the complex releases one mole of chloride ions (Cl^-) into the solution.

**Hint:** The formation of AgCl indicates the presence of chloride ions in the complex.

### Step 2: Analyze the Complex
The formula of the complex is \( Co(en)_2Cl_3 \). Here, \( en \) (ethylenediamine) is a bidentate ligand, meaning each \( en \) can coordinate to the cobalt ion at two sites.

**Hint:** Identify the types of ligands in the complex and their coordination capabilities.

### Step 3: Count the Ligands
In the complex \( Co(en)_2Cl_3 \):
- There are 2 bidentate ligands \( en \), contributing \( 2 \times 2 = 4 \) coordination sites.
- There are 3 chloride ions (Cl^-) that can also coordinate to the cobalt ion.

**Hint:** Remember that bidentate ligands contribute more than one coordination site.

### Step 4: Calculate the Total Coordination Number
The total coordination number (or secondary valency) of cobalt can be calculated by adding the contributions from the ligands:
- From 2 \( en \): 4 coordination sites
- From 3 \( Cl \): 3 coordination sites

So, the total coordination number is:
\[ 4 \text{ (from } en \text{)} + 3 \text{ (from } Cl \text{)} = 7 \]

**Hint:** The coordination number is the total number of ligand attachment points around the central metal ion.

### Step 5: Round Off to the Nearest Integer
Since the question asks for the secondary valency rounded off to the nearest integer, we find that the coordination number of cobalt in the complex \( Co(en)_2Cl_3 \) is 7.

**Final Answer:** The secondary valency of cobalt in the complex is **7**.