The equilibrium structures and barriers for rotation around the C-X bond are calculated for the allyl cations and anions CH(2)CHXH(2)(+/-) (X = C, Si, Ge, Sn, Pb) using quantum mechanical ab initio methods. Effective core potentials are employed for the heavy atoms. The allyl cations are predicted with a planar geometry. All allyl cations are stabilized by a conjugative interactions. The strength of the resonance interactions as measured by the rotational barrier decreases from 37.8 kcal/mol (X = C) to 14.1 kcal/mol (X = Si), 12.0 kcal/mol (X = Ge), 7.2 kcal/mol (X = Sn), and 6.1 kcal/mol (X = Pb). The allyl cations are additionally stabilized by a bonding and through-space charge interactions, which have the same magnitude as the resonance stabilization. The equilibrium geometries of the heavy atom allyl anions have strongly pyramidal XH(2) groups. The planar forms are much higher in energy. The calculations suggest that there is no resonance stabilization in the allyl anions, except in the parent anion CH2CHCH2-. The electronic structure of the molecules is investigated using the Laplacian of the electron density distribution.