The juvenile hormone — a development regulator in mosquitoes — has 7 out of its 16 C–C bonds formed using organometallic compounds. Organometallic compounds contain a polar, covalent C–M bond. Carbon can be part of an alkyl, aryl, allyl, or a benzyl group. For metals, lithium, magnesium, aluminum, tin, copper, and zinc are commonly used. The greater electronegativity of carbon than the attached metal polarizes the C–M bond towards carbon, making the carbon atom electron-rich in organometallic compounds. As a visual comparison, examine the C–X bond in alkyl halides. The higher electronegativity of the halogen polarizes the bond towards it, making the carbon atom electron-deficient in haloalkanes. Organometallic compounds are carbanionic and are useful as strong bases and strong nucleophiles. As the electronegativity difference between the carbon atom and the metal in the C–M bond increases, the bond's percent ionic character increases. This means, the carbon becomes more nucleophilic and the compounds become more reactive.