Morokuma's energy partition scheme has been used to calculate the electrostatic (ES), polarization (PL), exchange (EX), and charge transfer (CT) components of the total energy of interaction for various complexes between nucleophiles (vinylamines, vinylol) and electrophiles (BH 3, formaldehyde, Cl 2, methylchloride, CH 3 + and NH 4 +). Simple geometry variations have been made in order to determine their influence on the studied complexes. When dealing with vinylamine, the role of nitrogen pyramidalization complexes has been studied. A particular emphasis has been given to the case of formaldehyde complexes for the study of nucleophilic additions to carbonyl compounds. The results showed that the classical HSAB concepts were generally not verified by our analysis. Neither the ES component which can be related to the charge contribution, nor the component CT which can be treated in terms of frontier MO interactions were sufficient to account for the actually calculated features. The strong geometry dependence of the component EX, whose value is related to the overlap of the occupied molecular orbitals of the two interacting species, was generally dominant when the studied complex did not carry any formal charge.