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    The mechanism of cobalt-catalyzed methoxycarbonylation of bromoacetonitrile and methyl 2-halogen acetate was computationally investigated by comparing three different reaction pathways with the consideration of solvation and countercation effect. Pathway B, which goes through the addition of MeO− to (RCH2)Co(CO)4 and the elimination of [(RCH2)Co(CO)3(COOMe)]− to afford ester, is found to be the most kinetically favored. The previously proposed pathway A, which goes through twice SN2 reaction, and herein proposed pathway C, which undergoes the conventional migratory CO insertion of (RCH2)Co(CO)4 and further methanolysis of (RCH2CO)Co (CO)4 intermediate, are less favored due to their higher barrier. The thermodynamic contribution of countercation is distinct, but the mechanism preference still keeps.