The conformational preference of the RCHO...BX3 complexes (Rdouble bondH, CH3, CH(2)double bondCH, F, OH, NH2, NMe2; Xdouble bondH, F, Cl) were studied. It was found that all of the RCHO...BH3 systems prefer the eclipsed conformation. Most, but not all, of the RCHO...BF3 systems prefer the eclipsed conformation. Most, but not all, of the RCHO...BCl3 systems prefer the staggered conformation. Three driving forces are responsible for the conformational preference of RCHO...BX3. The hyperconjugation interactions, including the sigma*(B-X)-Lp-(O), sigma*(B-X)-pi(Cdouble bondO), pi*(Cdouble bondO)-Lp(X), pi*(Cdouble bondO)-cr(B-X), sigma*(formyl C-H)-Lp(X), and sigma*(C-R)-Lp(X) interactions, favor the eclipsed conformation. The steric effect favors the staggered conformation. Furthermore, the geometry relaxation effect favors the eclipsed conformation. A balance among the hyperconjugation interactions, steric effect, and geometry relaxation effect is present in both the eclipsed and staggered conformations. If the hyperconjugation interactions and the geometry relaxation effect dominate, as in RCHO...BH3 and most RCHO...BF3, the eclipsed conformation is preferred. If the steric effect dominates, as in most RCHO...BCl3, the staggered conformation is preferred. In addition, all of the RCHdropNH...BX3 (Rdouble bondH, CH3, CHdouble bondCH(2); X = H, F, Cl) complexes are found to favor the eclipsed conformation because of the presence of the N-H bond. All of the RCHdouble bondO...AlX3 (Rdouble bondH, CH3, CH=CH2, MeO, NH2, Me2N; Xdouble bondH, F, Cl) complexes favor the eclipsed conformation because the O...Al distances are very long in these complexes, causing undersized steric effects.