.pi.-Bond strengths of H2X:YH2: X = Ge, Sn; Y = C, Si, Ge, Sn

The molecular structures and π-bond strengths are determined using both MP2 and MCSCF+CI energies for a series of H 2 X=YH 2 compounds, where X=Ge or Sn and Y=C, Si, Ge, or Sn. These strengths are estimated both by evaluating the rotation barriers and by investigating the appropriate thermochemical cycles. The results show that C>Si∼Ge>Sn in their ability to form π-bonds