Mechanisms of platinum- and palladium-catalyzed nucleophilic substitutions of the allyl carbons of 2-chloro-2-propenyl ethyl carbonate have been studied by applying the ab initio molecular orbital method. By taking some model complexes, the nucleophilic substitution of the allyl carbon of (pi-allyl)platinum complex has been demonstrated to consist of three steps, the formation of a pi-allyl complex, conversion of the complex into a metallacyclic form, and the formation of an eta(3)-allyl product. The platinacyclo adduct and the eta(2)-complex are almost the same in stability. Replacement of the coordinated ligand in the eta(2)-product has been shown to be unfavorable from an energetic point of view. On the other hand, the palladium-catalyzed nucleophilic attack takes place at the terminal carbon of an allyl moiety to give an allylated product. It has been shown that the palladacyclobutane is less stable by similar to 15 kcal/mol than the eta(2)-complex. Thus, the formation of an eta(3)-allyl product via a metallacyclic adduct is unlikely in the palladium-catalyzed nucleophilic substitution. The replacement of the coordinated all;ene ligand with the starring material will take place in this case to promote the catalytic process. The relative stabilities of the eta(2)- and the metallacyclic forms in platinum and palladium complexes have been discussed in terms of orbital interactions.