The Lewis acid-base adducts (P4E3). (BX3) (E = S, Se; X = Br, I) and (P4Se3). (NbCl5) have been prepared and characterized by Raman, IR, and solid-state P-31 MAS NMR spectroscopy. Hybrid density functional calculations (B3LYP) have been carried out for both the apical and the basal (P4E3). (BX3) (E = S, Se; X = Br, I) adducts. The thermodynamics of all considered species has been discussed. In accordance with solid-state P-31 MAS NMR and vibrational data, the X-ray powder diffraction structures Of (P4S3). (BBr3) [monoclinic, space group P2(1)/m (No. 11), a = 8.8854(l) Angstrom, b = 10.6164(2) Angstrom, c = 6.3682(l) Angstrom, beta = 108.912(1)degrees, V = 568.29(2) Angstrom (3), Z = 2] and (P4S3). (BI3) [orthorhombic, space group Pnma (No. 62), a = 12. 5039(5) Angstrom, b = 11.3388(5) Angstrom, c = 8.9298(4) Angstrom, V = 1266.09(9) Angstrom (3), Z = 4] indicate the formation of an apical P4S3 Complex in the reaction Of P4S3 with BX3 (X = Br, I). Basal adducts are formed when P4Se3 is used as the donor species. Vibrational assignment for the normal modes of these adducts has been made on the basis of comparison between theoretically obtained and experimentally observed vibrational data.