Next, we analyzed the frontier molecular orbitals (FMOs) of 3a. The energy levels (eV) of the selected orbitals are given in Figure . The contribution of the lone pair on the N3 atom is confirmed in several of the FMOs, including the HOMO and HOMO-2. An in-phase overlap that forms a -bond between the N3 and P atoms is also found in the HOMO-2, which is consistent with the increased electron densities around the N3 atom found in the AIM analysis (Figure ). Molecular orbitals that include the lone pair of the phosphinimidoyl nitrogen (such as N3 in 3a) are also found in the FMOs of 3b (e.g. the HOMO and the molecular orbitals between the HOMO-2 and HOMO-5, Figure ) and the cationic species in 5 (denoting as 5′ in Figure ) (e.g. the HOMO and HOMO-3, Figure ). A comparison of the energy levels of the FMOs of 3a and 3b shows that the substituents on the B atom would influence the nucleophilicity/Lewis basicity of the N-borane-substituted phosphinimidoyl groups (Figure ). Moreover, the replacement of these N-borane moieties with an N-silyl substituent results in a decrease in the corresponding nucleophilicity/Lewis basicity. Thus, the nucleophilicity and/or Lewis basicity would decrease as follows: 3a > 3b >> 5.