Quantum chemical conformational analysis for electron donor-acceptor (EDA) systems, H3B-NH3, H3B-PH3, H3Al-NH3 and H3Al-PH3, has been performed. For H3B-NH3 and H3B-PH3, the rotational barrier is found to be invariant with an increase in the central bond (X-Y) length. For H3Al-NH3 and H3Al-PH3, however, the rotational barrier increases with an increase in the central bond length. Decomposition of the total energy into various components and their contributions to the frontier orbitals (HOMO, HOMO-1, HOMO-2 and HOMO-3) have been analyzed in detail to explain the origin of such anomalous changes in the rotational barrier. Charge transfer and favorable "back bonding" are found to be the crucial factors governing the variations in the rotational barrier for such systems.