Results of quantum chemical calculations of the title complexes show that they are strongly bonded. The bond dissociation enthalpies (298 K) of the H2C-H+...B bonds were calculated to be 85-88, 71-74 and 44-47 kJ mol(-1) for B = NH3, H2O, and HF, respectively. The periodic trend in the bond dissociation enthalpies, C-H vibrational frequencies, and C-H bond lengths is found to be in accord with a model of the interaction which in addition to the normal ion/dipole terms incorporates a variable amount of hydrogen bonding. The hydrogen bond character is most pronounced in the complex with ammonia and practically absent in the complex with hydrogen fluoride. The complex with water represents an intermediate situation. The role of this general type of complexes in gas-phase and condensed-phase reactions is discussed.