In the insertion (reduction) method of preparing a main group organometallic reagent, a metallic main group element M reacts with an organohalide RX. The term reduction refers to the fact that the oxidation state of carbon in the organohalide decreases by two units. For example, MeCl → MeLi can be thought of as carbon(+1) → carbon(−1), or [H₃C⁺ Cl⁻] → [H₃C⁻ Li⁺]. In reality, MeCl and MeLi are much more covalent than this ionic formulation, but it highlights the change in formal oxidation state.

• Halogen remains in the −1 oxidation state throughout
• Oxidation state of carbon decreases by two units
• Oxidation state of metal increases by two units (or two metals atoms are both oxidised by one unit): M → M²⁺ + 2e⁻ or two lots of M → M⁺ + e⁻
• For a Group 1 metal: RX + 2M⁰ → RM^I + M^IX
• For a Group 2 metal: RX + M⁰ → RM^IIX