Theoretical studies have been conducted in order to evaluate reaction mechanisms for CO₂ fixation processes catalyzed by transition metal complexes. Thus, detailed mechanistic steps (ring-opening, CO₂ insertion and ring-closing reactions) were described for the catalytic formation of cyclic carbonates from CO₂ and a series of epoxides based on Zn(salphen) complexes, in conjunction with NBu₄X(X=I, Br). Moreover, a highly active Al-based amino tris(phenolate) catalyst was considered to examine the same reaction as well as the catalytic formation of polycarbonates. The theoretical outcome was found to be fully in line with the experiments in a qualitative and quantitative manner.