The reactions of the first-row transition metal cations, Sc+ (D-3, D-1), Ni+ (D-2), Cu+ (S-1), with NH3 have been studied by the multiconfigurational and multireference-based theories, to clarify the similarities and differences in the reactivity of early (Sc+) and late (Ni+, Cu+) transition metal cations. In all the cases, the ion-dipole complex, MNH3+, is initially formed with a C-3v symmetry structure, which is the most stable complex in the respective potential energy surfaces except for Sc+ (D-1). The M+-NH3 binding energy was evaluated as 42.4, 37.8, 50.9, and 48.1 kcal/mol for Sc+ (D-3), Sc+ (D-1), Ni+, and Cu+, respectively. In the second step, M+ is expected to activate one N-H bond of NH3, leading to the insertion complex, HMNH2+. In Sc+ (D-3, D-1), three different stationary points of HScNH2+, i.e., C-s (in-plane), C-s (out-of-plane), and C-2v structures, were located, which correspond to a minimum point, a first-order saddle point, and a second-order saddle point, respectively. In these complexes, the singlet state originating from Sc+ (D-1) is largely stabilized compared to the triplets. The singlet HScNH2+ (in-plane) is calculated to be the most stable compound. There are three dissociation channels from HScNH2+, i.e., -->ScNH2+ + H, -->ScH+ + NH2, and -->ScNH+ + H-2. The third dissociation occurs through the transition state of a four-centered structure, with a small activation barrier of 23 kcal/mol, in both singlet and triplet surfaces. As to the late transition metal cations Ni+ and Cu+, there is no intermediate complex of HMNH2+, thus, all the dissociations occur through highly vibrational excitations of MNH3+. The calculated results are consistent with experimental observations.