Good quality ab initio calculations (MP2) show that the water adducts of BX3 and CX3+ have totally different structures (X = F-I). While all H2O-BX3 complexes have classical C-s symmetric structures with strong B-O bonds and additional H-bonding, the heavier CX3+ cations (X = CI-I) form weakly bonded "non-classical" water adducts that maximize C-X pi-bonding rather than C-O sigma-boncling. The delocalization of the positive charge as the driving force for pi-bond formation is absent in BX3, and therefore, pi-bonding is only weak and not structure determining in H2O-BX3. Since the PES of all H2O --> EX30/+1 particles (E = B, C) is very flat, flexible basis sets (like TZVPP) are required to rigorously characterize the adducts. In earlier calculations (J. Am. Chem. Soc. 1997, 119, 6648), classical structures were reported for all H2O --> EX30/+1 (E = B, C) complexes, likely resulting from the insufficient quality of the basis sets employed. By introducing a positive charge to three coordinate boron-halogen cations Do --> BX2+ (Do = NH3, OH2, X-H), also the B-X bonds shrink due to the stronger pi-bonding induced by the positive charge delocalization and if compared to the respective neutral compounds like H2N-BX2 or BX3. The "non-classical" water adducts also suggest that the mechanism of organic reactions involving carbenium ion intermediates with alpha-bromine or -iodine substituents and a nucleophile may proceed through halogen- rather than carbon coordination.