The reaction path for the formation of the BX₃-NH₃ (X = H, F, Cl, Br) complexes, was divided into two major processes: 1 – rehybridization of the acid, adopting a pyramidal geometry, and; 2 – complex formation from the pyramidal geometries of the acid and base.  An Interacting Quantum Atoms approach was applied to investigate the Lewis acidity trend presented by these compounds.  Topological analysis suggests that the boron-halogen bond exhibits a considerable degree of ionicity. Relative Gradient Analysis associated with the IQA partition indicates the acid-base complex formation is highly dependent on classical electrostatic potential. With the increase of X electronegativity, a higher degree of ionicity of the B-X is observed, causing an increase in the absolute value of X and B charges, increasing not only the attractive potentials between acid and base, but the repulsive potentials are also enhanced. This increase in the repulsive potentials is the main factor behind the acidity trend exhibited by the trihalides.  Changes in geometry are relevant only for complexes where BH_­­­3 acts as an acid, where lower steric hindrance facilitates the adoption of the pyramidal geometry observed in the complex. The CCTDP analysis shows that infrared intensities of BX₃-NH₃ are determined mostly by the atomic charges, and not by the charge transfer or polarization. The opposite is observed in covalent analogs.