To solve this problem, we need to identify which of the given options is a sigma bonded organometallic compound.$\\$Organometallic compounds are characterized by the presence of a metal-carbon bond. These bonds can be either sigma ($\sigma$) or pi ($\pi$) bonds.$\\$Let's analyze each option:$\\$• Grignard's reagent:$\\$Grignard's reagents are organometallic compounds with the general formula RMgX, where R is an alkyl or aryl group and X is a halogen.$\\$The bond between the carbon (R) and the magnesium (Mg) is a sigma ($\sigma$) bond.$\\$• Ferrocene, Cobaltocene, Ruthenocene:$\\$These compounds are examples of metallocenes, which are organometallic compounds consisting of a metal sandwiched between two cyclopentadienyl anions.$\\$The bonding in metallocenes involves delocalized pi ($\pi$) bonds between the metal and the cyclopentadienyl rings.$\\$Therefore, the only option that features a sigma ($\sigma$) bond is Grignard's reagent.$\\$Thus, the correct answer is Option 1: Grignard's reagent.