The reactions between L=E donors [L: 1,3-dimethylimidazolyl framework; E: S (1), Se (2)] and halogens X2 (X: Cl, Br, I) to form either “T-shaped” hypervalent chalcogen (TY) or linear charge-transfer-type adducts (CT) have been studied in the density functional theory (DFT) context by considering both thermodynamic and kinetic aspects. Apart from the case of the adducts between sulfur donors and diiodine, hypervalent compounds are calculated to be always more stable than CT ones in the gas phase, in agreement with the experimental results based on spectroscopic and structural determinations. NBO and FMO analyses suggest explanations for this. Reaction mechanism studies reveal that CT adducts are always the first products of the reactions and no transition states connecting the reactants directly to TY adducts have been found. The present calculations indicate that, at least for 1 and 2, TY adducts are obtained from CT adducts and not directly from the reactants.