An example of a sigma bonded organometallic compound is:

Options

A. Ruthenocene
B. Grignard's reagent
C. Ferrocene
D. Cobaltocene

Explanation

To determine an example of a sigma bonded organometallic compound, we need to understand what organometallic compounds are and identify which of the given options fits the criteria of having only sigma bonds.

### Step-by-Step Solution:

1. **Understanding Organometallic Compounds**:
Organometallic compounds are defined as compounds that contain a bond between a carbon atom of an organic group and a metal. The metal can be a transition metal, alkali metal, or alkaline earth metal.

2. **Identifying Sigma Bonds**:
Sigma bonds are the first bonds formed between two atoms and are characterized by head-on overlapping of atomic orbitals. In organometallic compounds, we are particularly interested in compounds that contain only sigma bonds between the metal and the carbon.

3. **Analyzing the Given Examples**:
- **Ruthenocene**: Contains a sigma bond between ruthenium and the cyclopentadienyl ring, but also has pi bonds.
- **Grignard Reagent**: This is represented as R-Mg-X, where R is an organic group and Mg is the metal. It contains a sigma bond between carbon and magnesium.
- **Ferrocene**: This compound has a sandwich structure with iron between two cyclopentadienyl rings, which includes both sigma and pi bonds.
- **Cobaltocene**: Similar to ferrocene, it has a sandwich structure with cobalt and contains both sigma and pi bonds.

4. **Selecting the Correct Example**:
Among the examples provided, the Grignard reagent (R-Mg-X) is the only one that contains a sigma bond between the carbon and the metal (magnesium) without any pi bonds.

5. **Conclusion**:
Therefore, the answer is the Grignard reagent, as it is an example of a sigma bonded organometallic compound.

### Final Answer:
The example of a sigma bonded organometallic compound is the **Grignard reagent**.

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