Relationship between the C = O-X+ (X = H, Li, Na, K, Al, Cu) angle and covalent characteristic of the X+-M (M = CH2O, CH3CHO, acetone, imidazol-2-one (C2H2N2O), cytosine, gamma-butyrolactone) was investigated, theoretically. The calculated electron densities rho at the bond critical points revealed that the covalency of the M-X+ interaction depended on the nature of the cation and varied as H+ > Cu+ > Al+ > Li+ > Na+ > K+. The alkali cations tended to participate in electrostatic interactions and aligned with the direction of the molecule dipole or local dipole of C = O group to form linear C = O-X geometries. Because of overlapping with lone-pair electrons of the sp(2) carbonyl oxygen, the H+ and Cu+ formed a bent C = O-X angle. Al+ displayed an intermediate behavior; the C = O-Al angle was 180 degrees in [CH2O/Al](+) (mainly electrostatic), but when the angle was bent (146 degrees) under the effect of local dipole of an adjacent imine group in cytosine, the covalency of the CO-Al+ interaction increased. The C = O-X angles in M/X+ adduct ions were scanned in different O-X bond lengths. It was found that the most favorable C = O-X angle depended on the O-X bond length. This dependency was attributed to variation of covalent and electrostatic contributions with O-X distance. In addition, the structures of [CH2S/X](+) and [CH2Se/X](+) were studied, and only bent C = S-X and C = Se-X angles were obtained for all cations, although the dipole vectors of CH2S and CH2Se coincide with the C = S and C = Se bonds. The bending of the C = S-X and C = Se-X angles was attributed to the covalent characteristic of S-X and Se-X interactions due to high polarizability of S and Se atoms.