A theoretical study of the symmetry-concerted Diels-Alder analogue reactions of Mo2 with 1,3-butadiene and ethylene is presented. Structure optimizations indicated elongation of the Mo-Mo and C=C bonds in the complexes. The optimized bond distances were found to be close to ones known in organomolybdenum complexes. The presented potential surfaces indicated, in agreement with symmetry-concerted models, that the Mo2-Eth complex formation is thermally forbidden, as the reaction has to proceed in the excited state, but the Mo2-But complex may be formed in the ground state with addition of thermal energy.