For comparison, comparable chemical shifts were reported for Et3P=N(BCat)[B(C6F5)3] at B -6.4 and 28.0. 13b These results establish that the solvated structure of 10a is nearly identical with the structure corroborated by the SC-XRD analysis shown in Figure . The structural parameters of the 3a moieties in 9a and in 10a, with the exception of the B1-N3 bond lengths (1.608(5) in 9a vs 1.660(4) Å in 10a), are comparable. Notably, a partial multiple-bond character for the P-N3 bond is confirmed by the AIM analysis (see values of (BCP) in Figure ), which should be consistent with the contribution of the borenium form shown in Figure . Subsequently, we explored the reactivity of 10a toward an external Lewis acid. The reaction between 10a and CatBCl (1.1 equiv.) quantitatively afforded borenium species 11a with a [CatBCl2] -counterion (Figure ). Compound 11a could also be prepared via treatment of 3a with 2.0 equivalents of CatBCl. The molecular structure of 11a was confirmed by NMR and SC-XRD analyses (Figure ). We also confirmed the quantitative regeneration of 10a by mixing 11a and 3a (1.1 equiv.). Although we could not experimentally confirm the generation of a CLA species under the present reaction conditions, the molecular structure of 3a-B(Cat)Cl was optimized theoretically. Three isomers (CLA1 with a B1-N3-B2-Cl angle of 53.1°, CLA2 with a B1-N3-B2-Cl angle of 273.1°, and CLA3 with a B1-N3-B2-Cl angle of 144.3°) are predicted at the B97X-D/6-311+G(d,p)//B97X-D/6-31G(d)/PCM(CH2Cl2) level of theory. The structure of CLA1 is given in Figure , and those of CLA2 and CLA3 are given in Figure . The relative Gibbs energies of these CLAs are found to be -28.2 ~ -28.8 kcal mol -1 with respect to [3a + CatBCl] (+0.0 kcal mol -1 ), which shows that the formation of the CLAs are thermodynamically favorable. The heterolysis of the B-Cl bond in CLA1 and CLA2 was theoretically predicted to be essentially barrierless. For example, Int1 (-29.3 kcal mol -1 ) was furnished from CLA1 via TS1 (-26.7 kcal mol -1 ) and would yield 10a (-39.3 kcal mol -1 ) with ease (Figure ; for details of the reaction from CLA2, see Figure ). Importantly, these results suggest that borenium I (cf. 10a) is an intermediate in the formation of borenium II (cf. 11a) from a Nu (cf. 3a) and CatBCl (2.0 equiv.), demonstrating that path A shown in Figure would be plausible even for chloroboranes. Based on both experimental and theoretical analyses, the presented BCPIs are found to involve negative hyperconjugation between the phosphine imidoyl nitrogen and the phosphorus atoms.