1. Definition: Organometallic compounds are compounds containing carbon-metal bonds .
2. Grignard's Reagent: These are alkyl magnesium halides, represented as $R-Mg-X$. The bond between the carbon atom of the alkyl group ($R$) and the magnesium metal ($Mg$) is a sigma (${\sigma}$) bond formed by the sharing of electrons (though highly polar) .
3. Other Options: 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 metallocenes (sandwich compounds). In these complexes, the metal atom is bonded to the $\pi$-electron cloud of the cyclopentadienyl rings, making them $\pi$-bonded organometallic compounds rather than simple sigma-bonded ones.

Conclusion: Grignard's reagent is the correct example of a sigma-bonded organometallic compound among the choices.