Density functional theory (DFT) calculations have been conducted to gain insights into the mechanism and chemoselectivity of the Ni(0)-mediated carboxylation reactions of benzylidenecyclopropane with CO 2 . The experimentally observed selectivity of reaction products may be explained through the formation of a common π-complex Ni(L) 2 (η 2 -C 2 H 4 CCHPh), which can undergo a direct nucleophilic cyclic addition with CO 2 or an isomerization to four-membered nickelacycle complex followed by the CO 2 insertion. All possible pathways to afford the product precursor five/six-membered cyclic Ni–carboxylates species are examined, and their corresponding energetics are demonstrated. Among the various reaction pathways, we have found that the formation of five-membered cyclic Ni–carboxylate (2) via the bisligand route I A , leading to the target product cyclopropane derivative A, has a lower reaction barrier and is the most preferred in the polarity weaker solvent (toluene), which is very in good agreement with the experimental finding. As for the formation process of six-membered cyclic Ni–carboxylates (4 and 5), the rate-determining step is associated with the ring-opening of benzylidenecyclopropane to give a four-membered metallacyclic intermediate. In acetonitrile and using DBU as ligands, the monoligand route II B is competitive with the bisligand route II B because the activation barrier difference for the ring-opening benzylidenecyclopropane is small (1.34kcal/mol) and the energy barrier of CO 2 insertion into the NiC(sp 2 ) bond is lower. The monoligand cyclic Ni–carboxylate 4B′d, generated from the CO 2 insertion into the NiC(sp 2 ) bond of the proposed four-membered intermediate 2B′d, is predicted to be the most probable species leading to the branched α,β-unsaturated ester B.