The electronic and molecular structures of H(2)X=XH(2) and H2C=XH(2) systems for X = C, Si, Ge, Sn, Pb have been studied by theoretical calculations. The calculations were based on approximate density functional theory within the local density approximation and augmented by nonlocal exchange and correlation corrections. A detailed bond analysis was performed, from which values for several intrinsic bond strengths have been extracted. The singlet-triplet splitting energies for XH(2) systems have been calculated in order to obtain accurate values for the preparation energy. The structures and bond energies are compared with the results for H(3)X-XH(3) and H3C-XH(3) systems. The peculiarities for the heavier homologues of ethylene, the trans-bent geometries and the essentially weak bond energy, are shown to be the consequences of enhanced interatomic as well as intraatomic Pauli repulsion.