GERMASILENE (H2GE=SIH2) AND ITS ISOMERS SILYLGERMYLENE AND GERMYLSILYLENE - BOND-DISSOCIATION ENERGIES, PI-BOND ENERGIES, AND PREDICTIONS OF ISOMERIC STABILITY

The prototypical Ge = Si doubly bonded molecule germasilene (H2Ge = SiH2) and its valence isomers silylgermylene (H3Si - GeH) and germylsilylene (H3Ge - SiH) have been investigated in both their ground-state singlet and lowest lying triplet states. All electron ab initio quantum mechanical techniques were employed, including the effects of electron correlation via configuration interaction and coupled cluster methods. Silylgermylene is found to be the lowest lying isomer, about 6 kcal/mol below the trans-bent germasilene minimum. The pi-bond energy of germasilene is predicted to be 25 kcal/mol, essentially identical with those in disilene (H2Si = SiH2) and digermene (H2Ge = GeH2). The bond dissociation energy, however, decreases in the order Si = Si > Si = Ge > Ge = Ge, and in each case is smaller than that required to break the corresponding single bonds in disilane, germylsilane, and digermane. This phenomena is rationalized by consideration of differences in first and second bond dissociation energies in the parent hydrides, Walsh's so-called divalent state stabilization energies (DSSE). Semiquantitative estimates of the relative energies of group 14 doubly bonded compounds and their corresponding divalent isomers can oftentimes be obtained by properly accounting for the DSSE.