Contrasting stabilities of classical and bridged pyramidal Si<SUB>3</SUB>H<SUB>3</SUB>X molecules (X = BH<SUP>-</SUP>, CH, N, NH<SUP>+</SUP>, NO, SiH, P, PH<SUP>+</SUP>, and PO)

Trigonal-pyramidal Si3H3X systems have been studied at HF/6-31G&#8727;, MP2(FC)/6-31G&#8727;, and Becke3LYP/6-31G&#8727; levels. The classical trigonal-pyramidal structure (5) is a higher order stationary point for X = BH-, CH, NO, SiH, P, PH+, and PO, whereas it is a minima for X = N and NH+, at the MP2(FC)/6-31G&#8727; level. An alternative pyramidal structure (6, C3v) with three SiHSi 3c-2e bonds is minima, lower in energy than 5 by 47.7 (X = BH-), 39.1 (X = CH), 31.7 (X = N), 25.0 (X = NH+), 20.6 (X = SiH), 20.7 (X = P), 16.1 (X = PH+), and 18.2 (X = PO) kcal/mol. Isolobal analogy connects 6 with various triply hydrogen bridged pyramidal structures in organometallics

H-bridged structures for tetrahedranes A<SUB>4</SUB>H<SUB>4</SUB> (A = C, Si, Ge, Sn, and Pb)

Ab initio MO studies at the HF, MP2, and Becke3LYP levels on H-bridged tetrahedranes of the group 14 elements (A4H4, A = C, Si, Ge, Sn, Pb) with the 6-31G&#8727; basis set for C and Si and LANL1DZ and quasirelativistic pseudopotential basis sets for Ge, Sn, and Pb are reported. As expected, the classical Td structure 1 is more stable than all the other tetrahedrane alternatives for C4H4. The triply hydrogen bridged structure 2 (C3v) is found to be more stable for Si, Ge, and Sn. Pb4H4 prefers the four H-bridged structure 8 (Cs). However, the calculations with the quasirelativistic pseudopotential basis set show the quadruply H-bridged D2d structure 7 to be the most stable structure for Ge, Sn, and Pb. Thus the structures derived from the transition metal organometallic chemistry are competitive for heavier elements (Si to Pb). The periodic behavior begins only with the second period; Li to Ne, are the exceptions. Suggestions for the realization of these H-bridged structures for Pb from NaPb are discussed

Ab initio MO study of diverse Si<SUB>3</SUB>H<SUB>3</SUB><SUP>+</SUP> isomers

Structures and energies of many Si3H3+ isomers were investigated theoretically at the MP2/6-31G&#8727; level. The global minimum was the classical aromatic planar D3h structure (5). Isodesmic equations indicate the resonance stabilization energy to be half that of the analogous cyclopropenyl cation. The next lowest energy minimum, with a divalent silicon and a bridging hydrogen, also exhibits the 2&#960; aromaticity. Five planar Si3H3+ isomers display cyclic three-center-two-electron (3c-2e) delocalization, and eight minima have 3c-2e Si-H-Si bridged bonds. The planar tetracoordinated silicon and five-coordinated silicon also are represented. Eleven other minima were found within a 46 kcal/mol range. An H-bridged C3v structure, derived from B3H6+, is 42.1 kcal/mol above the global minimum. However, for Ge, Sn, and Pb these A3H3+ forms are more stable than the classical structures (5, J. Am. Chem. Soc. 1995, 117, 11361). In contrast to Si3H3+, C3H3+ has only four isomers in the 189 kcal/mol range. The silicon analogues of the C3H3+ acyclic structures, the prop-2-en-1-yl-3-ylidene cation and the 1-propynyl cation, are not favorable