The reactions of INCO to IOCN, INCS to ISCN, and INCSe to ISeCN have been studied at the MP2/6-311++G(2df)//B3LYP/6-31 1++G(2df) level. Geometries of reactants, transition states, and products have been optimized, and geometries of transition states are reported for the first time. The reasons that INCO, ISCN, and ISeCN are easily detected instead of IOCN, INCS, and INCSe have been explained successfully. The breakage and formation of chemical bonds along the reaction paths have been discussed by the topological analysis method of electronic density. The calculated results show that there are two kinds of structure transition states (STS) in the reactions studied.