It is well-known that borenium species II are furnished via the formal substitution of a leaving group (X -) with an external nucleophile (Nu) in the presence of a Lewis acid (LA) (Figure ). Two mechanisms have been commonly proposed for the formation of II following the generation of a classical Lewis acid-base adduct (CLA). Path A involves a stepwise mechanism that proceeds via B-X heterolysis to yield borenium species I followed by the subsequent capture of X -by LA. In path B, these steps proceed in a concerted manner. Previous studies have suggested that path A would only be plausible when the leaving groups X -are sufficiently stable and spontaneously dissociate from the CLA, such as X = I, OTf, and NTf2 (where Tf is trifluoromethanesulfonyl). Conversely, the reaction that proceeds via the cleavage of the relatively strong B-Cl bonds is believed to take place through path B (or an analogous associated transition state), given the addition of LA is predominantly required to abstract the Cl -from the boron atoms. To date, there has been no report on either the direct isolation of a borenium species including the Cl -counterion via the reaction between a chloroborane and Nu in the absence of LA, nor has there been one that provides direct evidence of the interconversion between borenium species with Cl -and [Cl-LA] -counterions. In the 1 H NMR spectrum of 10a, the resonance of H1 (the atomic labelling is shown in Figure ) is observed at H 10.4, which suggests the formation of a hydrogen bonding interaction with Cl -in CD2Cl2. This result is consistent with the results from the SC-XRD analysis that show the participation of a hydrogen bonding interaction between these atoms (H1•••Cl -= 2.47 Å, Figure ). In addition, the AIM analysis also showed the participation of several non-covalent interactions between the Cl -and P-t Bu groups (Figure ). In the 31 P NMR spectrum, the resonance is observed at P 106.5, demonstrating a substantial decrease in the electron density on the P atom via the formation of a -bonding interaction between the lone pair on the N3 atom and the empty p orbital on the B2 atom. In the 11 B NMR spectrum, two independent resonances for the tetracoordinate B1 and tricoordinate B2 atoms are observed at B -0.7 and 25.0, respectively.