It should be noted that H represents a total energy density given as the sum of G and V, where G is the kinetic energy density and V is the potential energy density. In general, a negative value of H is expected at a BCP included in a covalent bond. Moreover, increased covalency tends to give a value of G/ < 1 at a BCP, while a value of G/ ≥ 1 is expected for an ionic or a donoracceptor interaction. Thus, the C1-B bond in 3a should be classified as a covalent bond, albeit the partial contribution of a donor-acceptor interaction should be also involved (∇ 2  ≈ 0; H = -0.11; G/ = 0.99). This is also consistent with the fact that the C3-B bond has a higher covalency (∇ 2  = -0.15, H = -0.15; G/ = 0.71). Similarly, the N2-P and B-N3 bonds have substantial covalency, but they are rather polarized compared to the C1-B bond due to differences in the electronegativity of the B, N, and P atoms (for BCP2: ∇ 2  = 0.27, H = -0.12, G/ = 1.28; for BCP4: ∇ 2  = 0.12, H = -0.11, G/ = 1.05). In contrast, the increased ∇ 2  and G/ values of 0.90 and 1.89, respectively, at BCP3 clearly demonstrate the participation of both covalent and donor-acceptor interactions between the N3 and P atoms. This multiple-bond character can also be rationalized based on the (BCP) values which show the average number of electrons (based on the self-consistent field densities) shared at a BCP, and hence give an insight into the bond order. 24 In fact, the value of (BCP3) is 1.07 and is almost identical to values calculated for BCP5 (1.13) and BCP6 (1.09) involved in the C1-N2 and C2-N2 bonds, respectively, both of which are widely acknowledged to have multiple-bond character through delocalization of electrons over the imidazolium ring. Consequently, the (BCP) values calculated for BCP1 (0.39), BCP2 (0.44), BCP4 (0.54), and BCP8 (0.47) are consistent with single-bond character as the values are nearly half of those calculated for BCP3, BCP5, and BCP6. These results demonstrate that resonance structures shown in Figure would be taken in account for BCPIs synthesized in this work, which is also consistent with the results obtained from a natural bond orbital (NBO) analysis (Figures S19). Comparison of the energy levels (eV) for selected molecular orbitals in 3a, 3b and 5′ (the cationic species in 5). Energy levels for orbitals including the contribution of the lone pair of the phosphinimidoyl nitrogen atom are highlighted in blue. The HOMO and HOMO-2 of 3a are also shown.