Multistage mass spectrometry experiments combined with density functional theory (DFT) calculations were used to examine the gas-phase synthesis and ion-molecule reactions of the organo-magnesates [CH3MgL2](-) (L = Cl and O2CCH3). Neutral species containing an acidic proton (HX) react with the [CH3MgL2](-) ions via addition with concomitant elimination of methane to form [XMgL2]- ions. Kinetic measurements combined with DFT calculations revealed reduced reactivity of [CH3Mg(O2CCH3)(2)](-) toward water, caused by the bidentate binding mode of acetate, which induces overcrowding of the Mg coordination sphere. The [CH3MgL2]- ions reacted with (i) aldehydes with enolizable protons via enolization rather than the Grignard reaction and (ii) CH3CO2H to complete a catalytic cycle for the decarboxylation of acetic acid. Other electrophilic reagents such as pivaldehyde, benzaldehyde, methyl iodide, and trimethylborate are unreactive. DFT calculations on the competition between enolization and the Grignard reaction for [CH3MgCl2](-) ions reacting with acetaldehyde suggest that while the latter has a smaller barrier, it is entropically disfavored.