All calculations were performed using the Amsterdam Density Functional (ADF) program (ADF2019.305). Geometries and energies were calculated at the BLYP level of the generalized gradient approximation (GGA). The DFT-D3(BJ) method developed by Grimme and coworkers was used to correct for dispersion interactions. Molecular orbitals (MOs) were expanded using a large, uncontracted set of Slater-type orbitals (STO): TZ2P. The TZ2P basis set is of triple- quality, augmented by two sets of polarization functions. All electrons were treated variationally. The accuracies of the fit scheme (Zlm fit) and the integration grid (Becke grid) were set to VERY GOOD. All Lewis pairs X3B-LB (X3 = F3, Cl3, Br3, I3; LB = NH3, MeCN, N2) were optimized with both C1 (i.e., without) and C3v symmetry constraints. All optimized structures were inspected to be true minima through vibrational analyses (no imaginary frequencies). Only for the long-bonded adducts I3B-LB (LB = MeCN, N2) in C3v symmetry we found two small imaginary frequencies (ca -10 cm -1 ) because the adducts are not entirely linear along the B-N-Y (Y = C and N, respectively) bond axis. Additionally, except for X3B-NCMe (X = Br, I), in which MeCN approaches the Lewis acid laterally by the CN bond, the geometrical and energetic differences between the C1 and C3v structures are minimal (less than 0.02 Å and 0.1 kcal mol -1 ). We have therefore used the Lewis pairs with C3v symmetry throughout this work, as it allows us to decompose the orbital interaction into its irreducible representation contributions and a more equitable comparison between all studied adducts (the Cartesian coordinates of all studied species are provided in the Supporting Information). The molecular structures were illustrated using CYLview. Activation strain and energy decomposition analyses Insight into the nature of Lewis acid-base pairs is obtained by applying the activation strain model (ASM) 29 in conjunction with quantitative Kohn-Sham molecular orbital (KS-MO) 30 theory and a matching energy decomposition analysis (EDA) 31 along the X3B-LB bond formation. The potential energy surface along the B-N bond formation was scanned in a stepwise manner from 1.00 to 4.00 Å with 0.005 Å per step, resulting in 121 steps in total. The overall bond energy ΔE is then decomposed into the respective total strain and interaction energy, ΔEstrain and ΔEint, and these values are projected onto the B-N bond distance [Eq.