The structural parameters, nature of the bonding, and stability of H3YBX3 (X = H, F, and Cl; Y = N, P) complexes have been studied at the G2(MP2) level of theory. G2(MP2) results show that the ammonia complexes are more stable than the corresponding phosphine complexes. This stability varies in the same order as the acidity of BX3 Lewis acids. The NBO partitioning scheme shows that there is a stronger charge transfer from PH3 to BX3 than from NH3. It proves also that the shortening of the P-H bond length upon complexation is due to an increasing "s" character in this bond.