The nature and origin of the pi -H interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides (BH3, CH4, NH3, H2O, and HF) has been investigated by carrying out high level ab initio calculations. The results indicate that the strength of the pi -H interaction is enhanced as one progresses from CH4 to HF. Unlike conventional H-bonds, this enhancement cannot be simply explained by the increase in electrostatic interactions or the electronegativity of the atom bound to the it H-bonded proton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy are important. Thus, the inductive energy is highly correlated to the olefinic pi -H interaction as we progress from CH3 to HF. On the other hand, both electrostatic and inductive energies are important in the description of the aromatic pi -H interaction. In either case, the contribution of dispersion energies is vital to obtain an accurate estimate of the binding energy. We also elaborate on the correlation of various interaction energy components with changes in geometries and vibrational frequencies. The red-shift of the Yy-H mode is highly correlated to the inductive interaction. The dramatic increase in the exchange repulsion energies of these pi complexes as we progress from CH4 to HF can be correlated to the blue-shift of the highly IR active out-of-plane bending mode of the it system.