The heats of formation of the three-membered ring compounds Y 2H4X (Y = C, Si; X = CH2, NH, 0, SiH2, PH, S) and the thermodynamics of the insertion reactions X+ YH2=YH2 -- c-Y2H4X are predicted by using MP2/6-31G(d) energies at the 6-31G(d) geometries. Bent bond lengths are calculated by tracing the path of maximum electron density connecting two nuclei, with the 6-31G(2d) basis set at the 6-31G(d) structures. The short Si-Si internuclear distances in Si2H4X (X = CH2, NH, 0, PH, S) apparently are the result of severe bond bending rather than significant 1r character in the Si-Si bonds.