Definition of an organometallic compound

An organometallic compound is a compound that contains at least one direct bond between a carbon atom of an organic group and a metal atom.

Given compound

The compound is methyl magnesium bromide: $CH_3-Mg-Br$

Identify the important bonds

In this compound, the bonds present are:

• $C-Mg$
• $Mg-Br$
• $C-H$

Check which bond makes it organometallic

The defining feature of an organometallic compound is the carbon-metal bond.

Here, the bond is: $C-Mg$

Evaluate options

• A: $Mg-Br$ bond → Not the reason; this is metal-halogen bond.
• B: $C-Mg$ bond → Correct; this is a carbon-metal bond.
• C: $C-Br$ bond → Not present as the defining organometallic bond.
• D: $C-H$ bond → Common in organic compounds, not organometallic criterion.

Final answer

Therefore, $CH_3-Mg-Br$ is an organometallic compound due to the presence of the: $\boxed{C-Mg\text{ bond}}$