Following sidwick.s rule of EAN, `Co(CO)_(x)` will be:

Options

A. `Co(CO)_(4)`
B. `Co(CO)_(3)`
C. `Co(CO)_(6)`
D. `Co(CO)_(10)`

Explanation

To solve the question regarding the effective atomic number (EAN) of the complex `Co(CO)_(x)` using Sidwick's rule, we will follow these steps:

### Step 1: Understand Sidwick's Rule
Sidwick's rule states that the effective atomic number (EAN) of a metal complex can be calculated using the formula:
\[ \text{EAN} = Z - O + 2C \]
where:
- \( Z \) is the atomic number of the central metal atom,
- \( O \) is the oxidation state of the metal,
- \( C \) is the coordination number (the number of ligands attached to the metal).

### Step 2: Identify the Components
For the complex `Co(CO)_(x)`:
- The central metal atom is cobalt (Co).
- The atomic number of cobalt (Co) is 27.
- The oxidation state of cobalt in this case is 0 (since CO is a neutral ligand).
- The coordination number \( C \) is represented by \( x \), which is what we need to find.

### Step 3: Set Up the Equation
According to Sidwick's rule, we want the EAN to equal the atomic number of the nearest noble gas. The nearest noble gas to cobalt is krypton, which has an atomic number of 36. Therefore, we set up the equation:
\[ 36 = 27 - 0 + 2x \]

### Step 4: Solve for \( x \)
Now, we can simplify and solve for \( x \):
1. Substitute the values into the equation:
\[ 36 = 27 + 2x \]
2. Rearranging gives:
\[ 36 - 27 = 2x \]
\[ 9 = 2x \]
3. Dividing both sides by 2:
\[ x = \frac{9}{2} \]
\[ x = 4.5 \]

### Step 5: Interpret the Result
Since \( x \) must be a whole number (as it represents the number of CO ligands), we round \( x \) to the nearest whole number. Therefore, we conclude that:
\[ x = 4 \]

### Final Answer
Thus, the complex can be represented as:
\[ Co(CO)_{4} \]