Good quality ab initio calcns. (MP2) show that the water adducts of BX3 and CX3+ have totally different structures (X = F-I). While all H2O-BX3 complexes have classical Cs sym. structures with strong B-O bonds and addnl. H-bonding, the heavier CX3+ cations (X = Cl-I) form weakly bonded \"non-classical\" water adducts that maximize C-X p-bonding rather than C-O s-bonding. The delocalization of the pos. charge as the driving force for p-bond formation is absent in BX3, and therefore, p-bonding is only weak and not structure detg. 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 calcns. (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 pos. charge to three coordinate boron-halogen cations Do -> BX2+ (Do = NH3, OH2, X-H), also the B-X bonds shrink due to the stronger p-bonding induced by the pos. charge delocalization and if compared to the resp. neutral compds. like H2N-BX2 or BX3. The \"non-classical\" water adducts also suggest that the mechanism of org. reactions involving carbenium ion intermediates with a-bromine or -iodine substituents and a nucleophile may proceed through halogen- rather than carbon coordination. [on SciFinder (R)]