The reaction mechanisms for the reactions of cobaltacyclopentadiene with isocyanate and isothiocyanate exclusively in singlet or triplet states, or involving transition between states, were studied by means of hybrid density functional theory (B3LYP) calculations. The results clearly showed that a two-state mechanism involving both singlet and triplet states is more favorable than the corresponding singlestate mechanism for all the reactions. We also found that the reactions of isocyanate and isothiocyanate to give pyridin-2(1H)-one and pyridine-2(1H)-thione complexes, respectively, are more favorable than those that give 2H-pyran-2-imine and 2H-thiopyran-2-imine complexes. This is mainly due to the greater stabilities of pyridin-2(1H)-one and pyridine-2(1H)-thione compared with 2H-pyran-2-imine and 2H-thiopyran-2-imine. A change in spin state plays a crucial role in the two-state mechanism; first, the C]N bond of the isocyanate or isothiocyanate inserts into the CoeCa bond of cobaltacyclopentadiene in the singlet state to give azacobaltacycloheptadienone or azacobaltacycloheptadienethione intermediates, and subsequent reductive elimination in the triplet state gives the pyridin-2(1H)-one or pyridine-2(1H)-thione complex, respectively.