Abstract: The allyl compounds of Mg, react with an olefin by inversion of the allyl group via a six center transition state. These precyclic reactions may be one of the most important classes of organic reactions. The reactions of propenyl magnesium halides with ethylene were studied using ab initio calculations. The methods used for calculations are RHF, B3LYP and MP2 with 6-31G* basis set. Researchers have investigated the structural properties, theoretical thermodynamic and kinetic data, i.e., rate constants of the reactions.