Torsion potentials about the X-(X) double over dot H bond in homosubstituted primary carbenes (X = C) and silylenes (X Si) have been investigated at the multireference averaged coupled pair functional (MRACPF) level of theory. For the triplet species. the potentials are quite flat, but large barriers of torsion have been observed fur the singlet states of all carbenes and silylenes whose carbon or silicon atom adjacent to the divalent atom formes small bond angles with two of its further substitutents; other geometry parameters, even the bond angle at the divalent atom, proved to be of little or no importance. The said kind of deformation encourages the formation of a weak dative pi-like bond between the two X atoms, which by its 2-fold symmetry with respect to torsion about the bond axis is responsible for the observed two-minima torsion potential.