Organometallic compounds are classified based on the nature of the metal-carbon bond:

1. Sigma ($\sigma$) bonded organometallics: In these compounds, the metal atom and carbon atom are joined by a single covalent bond (sigma bond). Grignard's reagents ($R-Mg-X$) are a classic example where the alkyl group is bonded to the magnesium atom via a sigma bond . Other examples include $(CH_3)_4Sn$, $(C_2H_5)_2Zn$, etc.
2. Pi ($\pi$) bonded organometallics: These involve transition metals bonded to unsaturated ligands via their $\pi$-electron clouds. Ferrocene $[Fe(\eta^5-C_5H_5)_2]$, Cobaltocene $[Co(\eta^5-C_5H_5)_2]$, and Ruthenocene $[Ru(\eta^5-C_5H_5)_2]$ are all metallocenes (sandwich compounds) where the metal is $\pi$-bonded to two cyclopentadienyl rings.

Therefore, Grignard's reagent is the correct example of a sigma bonded organometallic compound.