Given complex `[Co(NH_3)_4Cl_2]`. In it if `Cl - Co - Cl` bond angle is `90^@` then it is :

Options

A. Cis-isomer
B. Trans- isomers
C. Meridional and trans
D. Cis and trans both

Answer

To determine the type of isomer for the complex `[Co(NH₃)₄Cl₂]` given that the `Cl - Co - Cl` bond angle is `90°`, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Coordination Complex**: The complex given is `[Co(NH₃)₄Cl₂]`. Here, cobalt (Co) is the central metal ion, surrounded by four ammonia (NH₃) ligands and two chloride (Cl) ligands.

2. **Determine the Coordination Number**: The coordination number of cobalt in this complex is 6, as it is bonded to a total of 6 ligands (4 NH₃ and 2 Cl).

3. **Identify the Geometry**: For a coordination number of 6, the common geometries are octahedral. In an octahedral geometry, the ligands can be arranged in various ways leading to different isomers.

4. **Analyze the Given Bond Angle**: The problem states that the `Cl - Co - Cl` bond angle is `90°`. In octahedral complexes, if two identical ligands (in this case, Cl) are adjacent to each other, the arrangement is referred to as a **cis** configuration.

5. **Differentiate Between Isomers**:
- **Cis Isomer**: When the two identical ligands (Cl) are next to each other, the angle between them is `90°`.
- **Trans Isomer**: When the two identical ligands (Cl) are opposite each other, the angle between them is `180°`.

6. **Conclusion**: Since the bond angle given is `90°`, this indicates that the two chloride ligands are adjacent to each other, confirming that the isomer in question is a **cis isomer**.

### Final Answer:
The complex `[Co(NH₃)₄Cl₂]` with a `Cl - Co - Cl` bond angle of `90°` is a **cis isomer**.